AN ABSTRACT OF THE THESIS OF

Joel C. Gutierrez for the degree of Master of Science in Geology presented on December 17, 2020

Title: Exploring Volcano-Tectonic Connections in Cascadia – Temporal Linkages between Tephra Deposition and Megathrust Earthquakes

Abstract approved: ______Adam Kent

Explosively erupting volcanoes and megathrust earthquakes (Mw 8+ magnitude) occur at subduction zones and adjacent volcanic arcs. Volcanic eruptions are observed occurring close in time to megathrust earthquakes in the historical record from at least the 18th century CE to present in locations globally, including Japan in 1707 CE (Chesley et al., 2012) and 2011 CE (JAXA Space Technology Directorate); Chile in 1960 CE (Lara et al., 2004) and 2010 CE (Swanson et al., 2016); Kamchatka in 1952 CE (Walter, 2007); Alaska in 1964 CE (Walter et al., 2009); Sumatra 2005 CE (Walter et al., 2009); and Indonesia 2018 CE (Kim Hjelmgaard, 2018). Additional timing linkages between megathrust earthquakes and volcanic eruptions are also identified in Kamchatka, Alaska, and Central America (Walter et al., 2009). Previous research has been conducted exploring possible triggering relationships in other subduction systems; however, the Cascadia Subduction Zone (CSZ) has yet to receive attention from researchers regarding CSZ and Cascadia Volcanic Arc (CVA) triggering potential. The CSZ poses a significant hazard to the North American Pacific Northwest. If synchronicity can be established between CSZ megathrust seismic earthquakes and CVA volcanism, further research is required to establish the physical mechanisms linking these phenomena. The first step in this process is to establish that such correlations exist, which is the central focus of the present work. Thus, refining scientific understanding of these dual geohazards provides improved geohazard assessments and better informs emergency planners. This analysis of the volcano-tectonic connections in Cascadia and temporal linkages tephra deposition and megathrust earthquakes includes the refinement of the

Holocene paleoseismic and tephrochronology records. These records are found within both marine, and lacustrine sediment cores contain seismoturbidite sequences, and tephra deposits may provide detailed stratigraphic relationships and ages of seismic and volcanic events. The purpose of this research is to achieve a higher temporal resolution of regional CSZ megathrust earthquakes and eruptive histories of the CVA by correlating the 14C ages and stratigraphic occurrence of tephra and turbidite deposits between lake and marine sediment cores collected in the U.S. Pacific Northwest. Geophysical data collected from Lake Wapato and Rogue Apron sediment cores are stratigraphically correlated with previously established event bed data with sediment cores collected at inland Washington lakes and marine sediment cores collected along the CSZ margin. 14C age data collected within Lake Wapato provide stratigraphic control and correlates Lake Wapato and Rogue Apron records with established CSZ turbidite records. An analysis of the stratigraphic relationship between primary volcanic tephra beds and seismoturbidites in lake and marine cores in this study suggests that some CSZ and CVA event beds are observed in close stratigraphic proximity. Mazama tephra beds appear to occur immediately before the CSZ T14 event bed, observable in both lake and marine cores assessed in this study. Mount Saint Helens tephra bed Wn is observed in the Wapato core record and appears to occupy the CSZ T2 event bed's stratigraphic position. Additionally, the Mount Saint Helens tephra bed Yn is observed at the same depth as the CSZ T8 event bed in the Wapato core record. In the Bull Run Lake core record, the Mount Hood Timberline tephra bed is observed where the CSZ T5 event should be located. In assessing the extant CVA Holocene eruptive data compared against the CSZ Holocene turbidite record, numerous potential timing linkages are suggested.

©Copyright by Joel C. Gutierrez December 17, 2020 All Rights Reserved

Exploring Volcano-Tectonic Connections in Cascadia – Temporal Linkages between Tephra Deposition and Megathrust Earthquakes

by Joel C. Gutierrez

A THESIS

submitted to

Oregon State University

in partial fulfillment of the requirements for the degree of

Master of Science

Presented December 17, 2020 Commencement June 2021

Master of Science thesis of Joel C. Gutierrez presented on December 17, 2020

APPROVED:

Major Professor, representing Geology

Dean of the College of Earth, Ocean, and Atmospheric Sciences

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request.

Joel C. Gutierrez, Author

ACKNOWLEDGEMENTS

I could not have completed this research and manuscript without the support of collaborators, peers, family, and friends. Thank you Bran Black, Ann Morey-Ross, and Chris Romsos for all technical instruction and moral support. Additional thanks to Maziet Chesby, Val Stanley, and Cara Fritz for assistance provided at the Oregon State University Marine & Geology Repository. I am also thankful for the efforts of my major advisor and graduate committee members Professors Adam Kent, Adam Schultz, Frank Tepley, James Liburdy, Chris Goldfinger and Dr. Randall Milstein as well as Graduate Director Robert Allan for facilitating the completion of this research and manuscript. Additional thanks to Chris Harpel at the USGS Cascade Volcano Observatory. Most importantly, thank you to my family, Irish Perry and Meredyth Gutierrez for their support and love throughout my studies, research, and writing. Additional thanks to Mark & Sue Perry, Lezly & Clayton Terao, Cecil, Kay, and Ginia Gutierrez.

TABLE OF CONTENTS

Page

1 Introduction...……………………………………………………………………… 1

2 Geologic Setting.…………………………………………………………………... 3

2.1 North American Pacific Northwest………………………………………. 3

2.2 Tectonic Setting: Cascadia Subduction Zone (CSZ)…………………….. 3

2.3 Volcanic Setting: Cascade Volcanic Arc (CVA)………………………... 6

3 Previous Work.…………………………………………………………………... 14

3.1 The offshore paleoseismic record…………………….………………… 14

3.2 Paleoseismic record in Cascade lakes …………………………….….… 16

3.3 Tephrochronology………………………………………………………. 17

3.4 Triggering Relations between Subduction Zones & Volcanic Arcs……. 18

4 Methods …………………………………………………………………………... 21

4.1 Magnetic Susceptibility and Gamma Density……………………………23

4.2 Computed Tomography (CT)……………………………………………23

4.3 Lithologic Description ………….……………………………………… 24

4.4 Particle Size Analysis ……………………………………………………24

4.5 X-ray Fluorescence (XRF) ………………………………………………25

4.6 Radiocarbon Dating …………….……………………………………… 25

4.7 OxCal Age Modelling ……………………...……………………………26

4.8 Tephrochronology ………………………...…………………………… 26

TABLE OF CONTENTS (Continued) Page 5 Results ……………………………………………..………………………………27

5.1 Lake Wapato Results ………………………………….………………...27

5.1.1 Wapato Lake Bathymetry……………………………………...28

5.1.2 Wapato Lake Sub-bottom Profiling……………………………29

5.2 Wapato Core Stratigraphy & Petrophysics …………………….………..32

5.2.1 Wapato CT & Magnetic Susceptibility...... …...37

5.2.2 Wapato Tephra & XRF ……………………………….…….... 39

5.2.3 Wapato Tephra Geochemical Biplots……………….………... 42

5.3 Wapato Core Samples & Descriptions ………………………...………..45

5.4 Wapato Lake OxCal Age Model………………………...………..……..46

5.5 Wapato Intra-lake Well-log Correlation………………………………... 49

5.6 Leland Lake Results ……………………………...……………..……… 52

5.6.1 Leland Age Model……………………………………………..56

5.7 Bull Run Lake Results………………………………………………...... 59

5.7.1 Bull Run Lake Tephra Results……………………..…………..59

5.8 Inter-lake Well-log Correlation ……………………………..……...…... 63

5.8.1 Wapato-Bull Run Inter-lake Correlations……….……….…….63

5.8.2 Wapato-Leland Inter-lake Correlations……….……….…...….67

5.9 Rogue Apron Tephra Results…………………………………….………69

5.9.1 Rogue Apron Tephra Geochemical Biplots……………………72

TABLE OF CONTENTS (Continued) Page

6 Discussion ………………………………………………………………………....73

6.1 Origin of Event beds…………..………………………………………... 74

6.1.1 Fire……………………………………………………………..75

6.1.2 Storms………………………………………………………….76

6.1.3 Crustal Earthquakes……………………………………………77

6.1.4 Slab Earthquakes…………………………………………….…77

6.1.5 Plate Boundary Earthquakes…………………………………...78

6.2 Wapato Lake & Cascadia Marine OxCal Age Model Comparison…..….79

6.3 Bull Run Discussion…………………………………...…………...... … 83

6.3.1 Bull Run-Wapato Inter-lake Correlation………………………86

6.3.2 Bull Run OxCal Age Model………………………...…………87

6.4 Leland Lake Discussion……………………..……………………..….... 88

6.4.1 Leland OxCal Age Model……………………………………..88

6.5 Regional Well-log Correlations…….…………………………...... …..91

6.5.1 Inland Inter-lake Correlations…………………………...……..91

6.5.1.1 Wapato-Bull Run Correlation………………………..91

6.5.1.2 Wapato-Leland Correlation……………………...…..93

6.5.2 Offshore-Onshore Correlation………………………………....95

6.6 Rogue Apron Discussion…………………………………………..….....97

6.6.1 Rogue River Tephra Transportation…………………………..97

6.7 Tectono-Volcano Triggering in Cascadia ……………….…………...... 101

6.8 Evidence for Triggering Linkages in Cascadia ……….……………..... 119

TABLE OF CONTENTS (Continued) Page

7 Conclusion ……………………………………………………………………… 120

References Cited ………………..………………………………………………… 130

Appendices …………………………………………………………………………143

LIST OF FIGURES

Figure Page

2.1. Map of the Cascadia Subduction Zone & Cascade Volcanic Arc ………….…...4

2.2. Cascadia Subduction Zone Profile Diagram ……...……………..……………....5

2.3. Select Pacific Northwest Tephra Distribution …………………………..………9

3.1. Cascadia tectonic system energy state through the Holocene …………..……. 15

4.1. Cascadia Subduction Zone and Holocene-active Cascade Volcanic Arc….….. 23

5.1. Wapato Lake backscatter sonar bathymetric map with line-track of seismic survey and sediment core locations ………..………………………………………. 28

5.2 Wapato Lake backscatter sonar bathymetric map with scalloped lake bed features……………………………………………………………………………… 29

5.3. Wapato Lake Seismic Profile at WLC-01 coring site ………………………… 30

5.4. Wapato Lake Seismic Profile at WLC-02 coring site ………………………… 30

5.5. Wapato Lake Seismic Profile at WLC-03 coring site ………………………… 31

5.6. Wapato Lake Seismic Profile at WLC-04 coring site ………………………… 32

5.7. Example of a fining-upward event bed and background sediments in the Wapato Lake cores...……………………………………………………………………...…. 33

5.8. Example of a tephra bed and event bed load structures at Wapato Lake.…….. 34

5.9. Example of the upper boundary of the lapilli deposit at Wapato Lake…….…. 34

5.10. Example of successive, thin lamellae found within the Wapato Lake cores…. 35

5.11. Overall stratigraphy, lithology, and geophysics at Wapato Lake ……………. 36

5.12. X-ray Fluorescence (XRF) silicic tephra detection in WLC-04 at Wapato Lake……………………………………………………………………………….…41

5.13. X-ray Fluorescence (XRF) mafic tephra detection in WLC-04 at Wapato Lake……………………………………………………………………………….…42

LIST OF FIGURES (Continued)

Figure Page

5.14. Geochemical Biplots of tephra sample A4, A5, A6, and A7 from Wapato Lake compared to Mount Saint Helens tephra sets Y, W, and P…………………………. 43

5.15. Geochemical Biplots of tephra sample A8, A9, A10 and A11 from Wapato Lake compared to Glacier Peak tephras GP-G, GP-B, and GP-M…………………….…. 44

5.16. OxCal P-Sequence Age Model for WLC-04 at Wapato Lake ………..….…... 48

5.17. Well-log Correlation between at Wapato Lake…………………………….…. 50

5.18. Well-log Correlation between WLC-01 & WLC-04 at Wapato Lake……...… 51

5.19. Well-log Correlation between LLJ-07 & LLJ-01 at Leland Lake ……...……. 53

5.20. Smear Slide & XRF Results in LLJ-07 at Leland Lake ……...... ……. 55

5.21. Smear Slide of base of Mazama tephra bed in LLJ-07 at Leland Lake ...…..... 56

5.22. Smear Slide of Mazama tephra bed in LLJ-07 at Leland Lake ...... ……… 56

5.23. New OxCal P-Sequence Age Model for LLJ-07 at Leland Lake ...... …… 58

5.24. Bathymetric map of Bull Run Lake ...... …………………………………. 59

5.25. Smear Slide Results in Bull Run Lake ...... …………………..……..….…. 61

5.26. Well-log correlation between Bull Run Lake & Offshore Cores ....……….… 62

5.27. Inter-lake correlation between Wapato Lake and Bull Run Lake ………..….. 64

5.28. Flattened inter-lake correlation between Wapato Lake and Bull Run Lake.… 66

5.29. Inter-lake correlation between Wapato Lake and Leland Lake ………..…….. 67

5.30. Flattened inter-lake correlation between Wapato Lake and Leland Lake ….....68

5.31. Mount Mazama (Crater Lake), Rogue River, and Rogue Apron sites ………. 69

LIST OF FIGURES (Continued)

Figure Page

5.31. Mount Mazama (Crater Lake), Rogue River, and Rogue Apron sites ………. 69

5.32. Marine Sediment Core TN0909-1JC at the Rogue Apron site ..………....…... 70

5.33. Mazama Smear Slide Results at the Rogue Apron site ..………………..….... 71

5.34. Sample M1, M2, and M3 Tephra biplot at the Rogue Apron site compared to climactic Mazama and Llao Rock eruption tephra ……...... ……………..…... 73

6.1. Wapato Lake Event & Hemipelagic bed thickness ………….………..……….. 80

6.2. Wapato Lake & Marine CSZ OxCal P-Sequence Age Models Compared .……82

6.3. A profile of the Timberline tephra doublet on the southern flank of Mount Hood, Elk Meadows ……………..……………………….……………………………….. 83

6.4. Bull Run core with geophysical data plots and smear slide point count ratios between tephra, diatoms, and other lithics …………………………………………. 85

6.5. Wapato & Bull Run inter-lake correlation diagram illustrates a comparison between WLC-04 in Wapato Lake and a BRL-08 & -09 composite core …………. 86

6.6. Bull Run Lake & Marine CSZ OxCal P-Sequence Age Models Compared ..… 87

6.7. Leland Lake & Marine CSZ OxCal P-Sequence Age Models Compared …...... 90

6.8. Inter-lake correlation between Wapato and Bull Run Lakes ..…...... …….. 92

6.9. Inter-lake correlation between Wapato and Leland Lakes ..…...... …….. 94

6.10. Offshore-inland lake correlation ..…………………………………….....…… 96

6.11. Map of Rogue River watershed & Crater Lake ..………………….….....…… 98

6.12. Holocene Cascadia Megathrust Earthquakes & Cascade Volcanic Arc Eruption Timing …………………………………………………………………………….. 106

6.13. Holocene Cascadia Megathrust Earthquakes & Southern Cascade Volcanic Arc Eruption Timing .………………………………………………………………….. 108

LIST OF FIGURES (Continued) Figure Page

6.14. Holocene Cascadia Megathrust Earthquakes & Central Cascade Volcanic Arc Eruption Timing .………………………………………………………………….. 110

6.15. Holocene Cascadia Megathrust Earthquakes & Northern Cascade Volcanic Arc Eruption Timing .………………………………………………………………….. 112

6.16. Holocene Cascadia Megathrust Earthquakes & VEI 3-7 Northern Cascade Volcanic Arc Eruption Timing .……… ………………………………………….. 114

6.17. Holocene Cascadia Megathrust Earthquakes & VEI 4-7 Northern Cascade Volcanic Arc Eruption Timing .……… ………………………………………….. 116

6.18. Holocene Cascadia Megathrust Earthquakes & VEI 5-7 Northern Cascade Volcanic Arc Eruption Timing .……… ………………………………………….. 118

LIST OF TABLES Table Page

5.1. Geophysical peaks for event bed detection in WLC-04 at Wapato Lake ….…38

5.2. X-ray Fluorescence (XRF) peaks for tephra detection in Wapato Lake ….…. 40

5.3. WLC-01 & -04 C14 Sample Summary at Wapato Lake………………..……. 45

5.4. WLC-02 C14 Geophysical Peak Summary at Wapato Lake…………………. 46

5.5. Mazama Smear Slide Summary at Rogue Apron……………………….……. 72

6.1. Global Historically Observed Megathrust Earthquakes followed by Volcano Eruptions …………………………………………………………………..……… 103

7.1. Summary by coring site of well-log correlated CSZ event beds and volcanic tephra deposits.………………… …………………………………..……..……… 124

7.2. Southern CVA eruption timing compared to CSZ megathrust earthquake timing…………………………………………………………………………….... 126

7.3. Central CVA eruption timing compared to CSZ megathrust earthquake timing………………………………………………………………………….……127

7.4. Northern CVA eruption timing compared to CSZ megathrust earthquake timing………………………………………………………………………….……127

LIST OF APPENDICES Appendix Page

A. OxCal P-Sequence Age Model Input ……………………………….…….…... 144

B. OxCal P-Sequence Age Model Output Tables……………..…………….…..... 153

C. Holocene Cascadia Subduction Zone and Cascade Volcanic Arc Event Ages…156

D. Smear Slides ………………………………………………..……………..….... 165

LIST OF APPENDIX FIGURES

Figure Page

D1. Bull Run core BRL-08 Smear Slides …………………………………….……155

D2. Bull Run core BRL-09 Smear Slides ………………………………….………166

D3. Leland core LLJ-07 Smear Slides …………….…………………………….…167

D4. Rogue Apron core TN0909-1JC Section I Smear Slides ...…………....…...... 168

LIST OF APPENDIX TABLES

Table Page

B1. OxCal Output Table (Wapato) …………..………………………..………….. 153

B2. OxCal Output Table (Bull Run) ………………………..…………….………..154

B3. OxCal Output Table (Leland) ………………………...…………..…………...155

C1. Holocene Cascadia Subduction Zone and Cascade Volcanic Arc Event Ages...156

DEDICATION

I dedicate this work to my family Irish and Meredyth.

1

1. Introduction

Megathrust earthquakes (Mw 8+ magnitude) and explosively erupting volcanoes are commonly located along subduction zones and adjacent volcanic arcs. Recent research into potential timing linkages and triggering mechanisms at many subduction zones show that these earthquakes and eruptions threaten local and regional communities (Marzocchi and Piersanti, 2002; Walter & Amelung, 2007; Walter, 2007; Watt et al., 2009; De la Cruz-Reyna et al., 2010; Bebbington et al., 2011; Bonali et al., 2012; Ozawa et al., 2016). Prior research has considered both spatial and temporal aspects of subduction zones and volcanic arcs as disparate systems; however, considering event linkages in both systems necessitates framing them as a combined system. Additionally, recent research datasets are restricted to historically recorded megathrust earthquakes and volcanic eruptions which leaves prehistoric events out of the assessment. Despite these data limitations, volcanic eruptions are observed occurring close in time to megathrust earthquakes in the historical record from at least the 18th century CE to present including Japan 1707 CE (Chesley et al., 2012) and 2011 CE (JAXA Space Technology Directorate, 2012); Chile 1960 CE and 2010 CE (Swanson et al., 2016) ; Kamchatka 1952 CE (Walter et al., 2009); Alaska 1964 CE (Walter et al., 2009); Sumatra 2005 CE (Walter & Amelung, 2007); and Indonesia in 2018 CE (Kim Hjelmgaard, USA Today, 2018). There is an ongoing exploration of possible triggering relationships in other subduction systems; however, the Cascadia Subduction Zone (CSZ) not only poses a significant hazard to the North American Pacific Northwest (PNW) but also remains largely unknown and requires further research into the triggering relationships in the combined CSZ and Cascadia Volcanic Arc (CVA) system. Thus, refining scientific understanding of these dual geohazards provides improved geohazard assessments and better informs emergency planners. This study aims to improve understanding of potential timing linkages between megathrust earthquakes and volcanic eruptions in the PNW with a specific aim to inform updated geohazard assessments. Therefore, it is valid to look at the most recent geologic epoch, the Holocene (10000 years BP to present), to elucidate patterns of behavior and interactions between the CSZ and CVA. The Holocene CSZ paleoseismic record provides a well-established seismic event bed sequence. Meanwhile, the CVA tephrochronology record offers insight into eruptive activity during the most recent 10000 years when the regional tectonic and volcanic structures were similar to the present day. Therefore, combining paleoseismic and tephrochronology 2

methods provides the most current pattern of tectonic and volcanic behaviors. Thus, rather than rely on a smaller historical dataset, a comparison of the megathrust earthquakes and volcanic eruptions spanning 10000 years BP to present, provides an improved, generalizable analysis of these timing linkages and triggering relationships. In essence, this conflated method, thus used in this study, treats the tectonic and volcanic systems as one to provide a greater understanding of the interactions among the connected systems. For that purpose, this study compares the CSZ event bed record and volcanic tephra bed record in sampled PNW lake and marine sediment cores by constructing a regional stratigraphic well log correlation model connecting inland lakes and offshore marine coring sites. Additionally, chronostratigraphic proximity between CSZ event beds and primary tephra beds provide improved insight into how close in time CSZ megathrust earthquakes have occurred to CVA volcanic eruptions within the last 10000 years. While the purpose of this research is to determine linkages in the timing of CSZ megathrust earthquakes and CVA eruptions, it is noteworthy that recent research has worked on the parallel question of underlying triggering mechanisms and causation (Nostro et al., 1998; Marczocchi & Piersanti, 2002; Yokohama et al., 2002; Juppe et al., 2004; Walter, 2007; Walter & Amelung, 2007; Walter et al., 2007; Eggert & Walter, 2009; Walter et al., 2009; De la Cruz et al., 2010; Donne et al., 2010; Bebbington et al., 2011; Bonali et al., 2012; Lupi & Miller, 2014; Ozawa et al., 2016). These companion discussions of the timing of events and causality are crucial in understanding the combined tectono-volcanic system. Chapter 2 describes the geologic setting includes the tectonic and volcanic history and defining characteristics of the PNW. Chapter 3 provides a synopsis of extant research on the CSZ paleoseismic record and potential tectono-volcanic triggering relationships, while Chapter 4 describes the analytical methodologies that are the basis of this project. Chapter 5 provides paleoseismic and tephrochronological results from the lake and marine study areas. Chapter 6 provides a detailed discussion of the results and the implications for potential triggering relationships between tectonic megathrust earthquakes and increased volcanic eruptive activity. Chapter 7 provides conclusions as well as suggestions for future work.

3

2. Geologic Setting 2.1 North American Pacific Northwest

The Pacific Northwest (PNW) is a geologically diverse region, including northern California, Oregon, Idaho, Washington, and British Columbia. The dominant processes in the region during the Quaternary include the eastward subduction of the Explorer, Juan de Fuca, Gorda oceanic plates under the North American continental plate with resulting accretion of material onto the continental plate and volcanism along of the Cascade Volcanic Arc. The Cascadia Subduction Zone (CSZ) and the San Andreas transform fault (SATF) bounds the western edge of the North American Pacific Northwest. The CSZ is situated approximately 80 km offshore. Simultaneously, the SATF exhibits a general SE-NW orientation running from the Gulf of California, south of California to the offshore triple junction west of Mendocino, California, where the northernmost SATF meets the southernmost CSZ. The Cascade Volcanic Arc (CVA), situated east of the Willamette Valley, is generally in a north-south orientation and includes over 80 volcanic vents, calderas, craters, cinder cones, and composite volcanoes (Harris 2005; Hildreth, 2007). The Holocene-active volcanoes extend from Lassen Peak in northern California to Silverthrone Caldera, British Columbia (Harris, 2005).

2.2 Tectonic Setting: Cascadia Subduction Zone (CSZ)

The CSZ is a convergent plate boundary where two tectonic plates collide, with the Juan de Fuca, Gorda, and Explorer oceanic plates subducting northeastward under the North American continental plate. The CSZ is an active convergent zone prone to seismicity, with the Holocene seismic record indicating 18 distinct megathrust earthquakes (Mw 8+ magnitude) and a total of 43 either sectional or full ruptures along the subduction zone (Goldfinger et al., 2014, 2017, 2017a). With the CSZ, the denser, cooler oceanic plate moves roughly from west to east under the less dense North American continent plate (NACP). As the oceanic plates subduct under the NACP, the densest material moves under the NACP. In contrast, the least dense material is accreted onto the continental shelf, which, over time, pushes tectonically inland. The subduction of the oceanic plates exhibits an average dip of ~30o in the uppermost 100 km of mantle and extends to a depth of between 200 and 600 km (Cheng et al., 2017). The CVA, situated ~100 km inland, represents the inland extent of the oceanic plates dipping at ~30o under the NACP. East 4

of the Cascades, the oceanic plate slab increases the steepness of dip of subduction and is the approximate point in the subduction zone where melting or partial melting of the oceanic plates occurs (Hildreth, 2007; Schmandt & Humphreys, 2011; Goldfinger, 2017). The hydrated oceanic plates are subducting under the NACP, contributing to both the subduction rate as well as the depth and location where melt occurs under the NACP.

Figure 2.1 A map of the Cascadia Subduction Zone with Cascade Volcanic Arc. This map provides a geographic representation of positional relationships between the CSZ and volcanoes in the CVA. This map also includes non- Holocene-active volcanoes in Oregon (Mounts McLoughlin and Jefferson) as well as three Holocene-active CVA volcanoes in southern British Columbia that are not included in the present project. From figure 1 of Clynne, (2017).

The Cascadia subduction zone also exhibits a number of seismic phenomena related to ongoing subduction. Large earthquakes are recognizable in the geologic record through tsunami deposits, evidence of severe coastal subsidence and rebound cycles, soft-sediment deformation during liquefaction, and seismoturbidite deposits within marine and sediment cores (Goldfinger, 2017). Episodic tremor and slip (ETS) along the CSZ is also observable through a regional network of geodetic sensors suggesting slow slippage along the plate interface. These slow slip events suggest apparent crustal motion reversal, while fault motions maintain the prevailing subduction direction (Rogers et al. 2003). Cascadia's ETS zone is a major focus in continued scientific research due to peculiar seismic and geodetic behaviors of the ETS zone and the manner that it 5

interacts with and impacts on the CSZ strain rate—and may be a factor in recurrence, magnitude, and rupture lengths of megathrust tectonic CSZ earthquakes (Rogers et al. 2003).

Figure 2.2: Cascadia Subduction Zone (CSZ) Profile Map by USGS. This diagram provides a profile view of the CSZ system to illustrate the surface and subsurface interactions between the subducting oceanic plates under the North American Continental plate and generally where Cascade Volcanic Arc volcanic activity is observed from at least 10 ka to present.

While geophysical modeling has provided insight into underlying tectonic mechanisms found in the CSZ, those techniques have not conclusively determined the disposition of the oceanic plates beyond the hydrous melting point (Schmandt & Humphreys, 2011; Gao, 2018). For instance, geophysical modeling alternatively suggests subducting plate may be subducting flat, against the Precambrian NACP at nearly 90o (Schmandt & Humphreys, 2011) or that once the subducting slab weakens near the Moho transition, it detaches and subsequently interacts with a possible mantle plume under the Oregon-Idaho border (Gao, 2018). Although the mechanics and disposition of the CSZ tectonic plates are beyond the scope of this study; the interactions between the tectonic plates set the stage for the volcanic and geomorphic processes within this region and warrant mutual consideration of the tectonic and volcanic systems for geologic study. Assessing the coincidence in the timing of megathrust earthquakes and volcanic eruptions necessitate considering the tectonic and volcanic systems as a larger, combined system and contribute to the larger conversation about triggering mechanisms and causality. 6

2.3 Volcanic Setting: Cascade Volcanic Arc (CVA)

The CVA consists of at least twelve volcanoes active in the Holocene (i.e., Ankney & Johnson, 2013; Bacon et al., 2017; Beget, 1982; Christiansen et al., 2017; Clynne & Muffler, 2017; Donnelly-Nolan et al., 2017; Egan et al., 2015; Foit et al., 2016; Harris, 2005; Hildreth et al., 1997; Pallister et al., 2017). Hazards associated with the eruptions of these volcanoes threaten communities directly adjacent to the volcanoes and regional and global communities. The explosivity and volume of material of any particular eruption dictate impacts to populations, infrastructure, and natural habitats (i.e., Ankney & Johnson, 2013; Bacon et al., 2017; Beget, 1982; Christiansen et al., 2017; Clynne & Muffler, 2017; Donnelly-Nolan et al., 2017; Egan et al., 2015; Foit et al., 2016; Harris, 2005; Hildreth et al., 1997; Pallister et al., 2017). Locations adjacent to an eruption may experience pyroclastic and lava flows, proximal tephra falls, and lahars. Downslope, lahars may impact drainage for dozens of miles, containing a fast-moving liquified mixture of ash and mud with trees and other debris caught in the flows. Lahars are particularly relevant to the PNW because of the significant amounts of water stored as ice and snow on the volcano. Regionally and globally, distal volcanic tephra can travel for hundreds or thousands of miles from the parent volcano, resulting in global cooling events and ground air travel (i.e., Ankney & Johnson, 2013; Bacon et al., 2017; Beget, 1982; Christiansen et al., 2017; Clynne & Muffler, 2017; Donnelly-Nolan et al., 2017; Egan et al., 2015; Foit et al., 2016; Harris, 2005; Hildreth et al., 1997; Pallister et al., 2017). A brief summary of Holocene-active CVA volcanoes provides a context for the subsequent discussion on the timing of volcanic eruptions and tectonic CSZ earthquakes and are detailed in Appendix C.

Lassen Peak (40.4881° N, 121.5049° W)

The Lassen area (in the northern California Cascade Arc) includes several volcanic centers, including the Yuna, Lantour, Maidu, Dittimar, and Lassen volcanic centers as well as Brokeoff Volcano, Lassen Domefield, and Caribou volcanic field, representing disparate and concurrent eruptive eras of the area (reference). Lassen Peak, an active volcanic center erupted regularly for ~27000 years and can be described as a basaltic-andesitic lava dome stratovolcano; Lassen Peak also erupted during the years 1914-1917 CE, as well as at least two additional times in 847 CE and 1666 CE, all of which were VEI 4 eruptions—0.25 km3 ejecta material volumes (Lanphere 7

et al., 1999 & 2004; Clynne et al., 2002). Lava, pyroclastic, and lahar flows, and ashfalls are established geohazards emanating from Lassen Peak.

Mount Shasta (41.3099° N, 122.3106° W)

The Mount Shasta volcanic area (in the northern California Cascade Arc) includes several extinct, dormant, and active cones, peaks, and lava fields. Mount Shasta is located directly southwest of the High Cascade Arc Axis, just south of the Oregon-California border. Mount Shasta is considered a composite cone that has produced plinian eruptions with the oldest exposed material dating back to ~500000 years BP. There have been at least two VEI 4 eruptions of ~4.4 km3 ejecta volume during the Holocene, with the most recent occurring ~750 years BP, with the oldest Holocene eruption occurring 10690 years BP. Pyroclastic flows, ashfalls, lahars, and lava flows are established geohazards associated with Mount Shasta (Christiansen et al., 2017).

Medicine Lake (44.9991° N, 93.4192° W)

Medicine Lake volcano and associated lava flows (in the northern California Cascade Arc) encompass ~2200 km2 and are estimated to be as large as 600 km3 in total volume. Medicine Lake is a back-arc volcano similar to Newberry Crater (in the central Oregon Cascade Arc). Medicine Lake and Newberry Crater's suggest peculiarities of their eruptive history and behavior owing to the impact of the compressive CSZ and extensional Basin & Range tectonics (Clynne & Muffler, 2017). At least 22 Medicine Lake eruptions have occurred over the last 12500 years, ranging from VEI 1 (0.0001 km3 volume) to VEI 5 (4.4 km3). As with any volcano, Medicine Lake has experienced quiescent periods as well as bursts of activity, with several eruptions occurring in quick succession. In the early Holocene, Medicine Lake eruptions were more basaltic than its current basaltic-andesite to silicic output and classified as a shield volcano containing a 7x12 km wide caldera. Established Medicine Lake geohazards are lava flows, pyroclastic flows, and ashfall (Donnelly-Nolan & Grove, 2017).

8

Mount Mazama (42.9446° N, 122.1090° W)

Mount Mazama (in the southern Oregon Cascade Arc) is estimated to have reached an elevation of 3700 meters before the cataclysmic collapse forming Crater Lake and has an approximately 500 ka eruptive history (refernce). Mazama is situated atop a deep, likely mantle-derived magma source with connecting plumbing around which the continental crust is melted, resulting in accumulation of magma chambers containing dominantly silica-rich rhyodacitic magmas (Ankney et al., 2013; Karlstom et al., 2015). Two large Plinian rhyodacite eruptions occurred between 7850–7950 years BP (Llao Rock and Cleetwood eruptions) and have been associated with pumice and tephra falls, as well as lava flow deposits. These eruptions may have been a build-up to the climactic VEI 7 (Volcanic Explosivity Index) caldera-forming eruption 7630 ± 150 cal. years BP (Bacon & Wright, 2017).

The climactic Mazama eruption heavily impacted the region dozens of kilometers from the volcano, producing large pyroclastic flows traveling down drainages as far as 70 km (Bacon & Wright, 2017) and tephra falls across the North American Northwest. The eruption underwent a single-vent phase and culminated in a ring-vent phase directly preceding the caldera collapse, causing renewed ring-venting as the descending caldera's mass pressed downward on the magma chamber. The entirety of the eruption is estimated to have spanned only a few days, while tephra ejected into the atmosphere is thought to have remained suspended for at least three years, some of which is observed in Greenland ice sheet cores, providing the most recently updated age dating of the eruption (Zdanowicz et al., 1999; Bacon, 1983; Bacon & Lowenstern, 2005; Bacon & Wright, 2017).

The Mazama eruption left a wide variety of ejected volcanic materials, including fused granodiorite and accessory volcanoclastic blocks originally part of the 5 km-deep magma chamber walls associated with the climactic eruption (Bacon & Wright, 2017). Direct impacts from pyroclastic flows are observed particularly along the Rogue and North Umpqua river drainages and other low-lying areas, with massive localized airfall of large cooling ejecta, as well as thick deposits of tephra fall. The local impact on forests and wildlife would have been similar to the 1980 Mount St. Helens (MSH) eruption, but with a much larger impacted area (Bacon & Wright, 2017). 9

The approximate volume of material expelled in the caldera-forming Mazama eruption is between 50 and 58 km3 compared to the VEI 5 1980 MSH eruption that expelled 2.5 km3. For decades after the climactic Mazama eruption, lahars were retriggered when large precipitation events or seasonal snowpack melting saturated the thick tephra deposits on unstable, deforested slopes. Regional tephra fall deposition was distributed throughout what is now identified as Northern California, Oregon, Washington, western Idaho, western Montana, NW Nevada, British Columbia, and Alberta. Furthermore, Mazama cryptotephra has been located in Lake Superior cores, Nova Scotia, and the Greenland Ice Sheet (Kuehn et al., 2010; Bacon & Wright, 2017).

Figure 2.3: Distribution of visible deposits of the four volcanic ash beds. This map represents the published, observed extents of tephra fall deposits of Mount Mazama, Mount Saint Helens, Glacier peak, and Mount Meager. This distribution map provides insight into the distance tephra airfall can travel as well as the prevailing direction of tephra deposition due to the generally prevailing west-east winds in the region. (Jensen, 2018 modified from Jensen and Beaudoin 2016).

Newberry Crater (43.7221° N, 121.2345° W)

Newberry Crater (in the central Oregon Cascade Arc) is an active shield volcano covering an area of ~3100 km2 and a total volume of 500 km3 with a caldera of 6.4 x 8 km wide. Newberry produces andesitic to rhyolitic lavas and has been active for at least 600 ka, with the most recent eruption occurring 1260 years BP with a 4 VEI and an ejecta volume of at least 2.5 km3 As with many active Cascade volcanoes, historic Native American populations have been impacted by 10

Newberry eruptions, with resulting obsidian flows providing tool-making material. Several volcanic centers are adjacent to Newberry Crater, including Bachelor Peak and Three Sisters. Recent research has proposes that Newberry Crater may share, or have shared, a hotspot track with Craters of the Moon (Idaho) and Yellowstone Caldera (Wyoming) (Xue & Allen, 2006). Established Newberry Crater geohazards include lava, pyroclastic, lahar flows, and ashfall (Jensen et al., 2009).

South Sister (44.1034° N, 121.7692° W)

The Three Sisters (in the central Oregon Cascade Arc) are a chain of composite volcanoes— South Sister being the Holocene-active peak. South Sister produced andesite-rhyodacite lavas in the Pleistocene and, most recently, rhyolitic lavas (Fink & Anderson, 2017). The most recent South Sister eruptions are dated to ~1500 years BP, resulting in VEI 2 (0.001 km3 eject volume) eruptions. Established South Sister geohazards include lava, pyroclastic, lahar flows, and ashfall (Fink & Anderson, 2017).

Mount Hood (45.3736° N, 121.6960° W)

Mount Hood, located southeast of Portland, Oregon (in the northern Oregon Cascade Arc), presents this population with many geohazards, including lahars, edifice collapse, block and ash flows, and lava flows. While Mount Hood's eruptive history extends as far back as 500 ka BP (late-Pleistocene through late-Holocene), the volcano experienced a long quiet period. Awakening ~1500 years BP during the Timberline eruptive period and the Old Maid eruptive period is dated to ~1790 CE (Scott & Gardener, 2017). Adjacent to Mount Hood is a series of vents, cones, and lava fields, including the Boring Lava Field situated within Portland city limits.

Mount Adams (46.2024° N, 121.4910° W)

Mount Adams is situated directly east of Mount St. Helens (MSH) and may have a shared magma source with MSH (Hill et al., 2009; Bedrosian et al., 2018). Mount Adams is located far enough from population centers that impact from this particular volcano geohazard are minimal (Hansen et al., 2016), but include lahars and debris flows from the most recent eruption and date back to 1000 years BP. There have been at least 15 Holocene eruptions from Mount Adams, 11

ranging from VEI 2 (0.002 km3 ejecta volume) to VEI 5 (1.0 km3 ejecta volume) (Hill et al., 2009; Bedrosian et al., 2018). Nearly two dozen tephra deposit layers on the slopes of Mount Adams have been identified and dated (Hildreth & Fierstein, 1997; Hill et al., 2009; Bedrosian et al., 2018).

Mount Saint Helens (46.1914° N, 122.1956° W)

Through the Holocene, Mount Saint Helens (MSH) has proven to be one of the most active volcanoes in the Cascade Volcanic Arc. Due to the high level of MSH activity, coupled with the volcano's impact and subsequent scientific interest spawned from its 1980 eruption, MSH is the most thoroughly studied volcano in the continental United States. MSH eruptive phases are identified by time periods and characterized by eruption type, location within the MSH system, and eruptive material geochemistry (Pallister et al., 2017).

At the beginning of the Holocene (10000 years BP), MSH experienced approximately 6000 years of dormancy, becoming active again during the Spirit Lake eruptive phase (4000 years BP) when several eruptive periods were defined by tephra deposition and other deposits within the geologic record. The Smith Creek eruptive period 3300-4000 years BP), represented by tephra layer set Y, dacite lava flows, pyroclastic flows, block and ash-flows, as well as pumaceous dacite tephra. The Yn tephra layer (3660 ± 145 years BP) is the largest single eruptive event in MSH's established geologic history (likely a VEI 6) and was shortly followed by a deposition of the Ye tephra layer (3550 years BP) (Pallister et al., 2017; Kuehn, 2013).

The MSH Pine Creek Period (1600-3000 years BP) eruptive activity represents in the younger tephra layer set B (1600-2500 years BP) and is associated with basaltic andesite, andesitic lava flows and lahars, while the older tephra set P (2500-3000 years BP) is associated with andesitic lava flows, extensive dacitic dome-building, pyroclastic flows, and lahars (Pallister et al., 2017; Kuehn, 2017). The MSH Castle Creek eruptive period (1600-2500 years BP) had three prominent eruptions and is represented by tephra set B with eruptions compositionally producing basalt through dacite material lahars (Pallister et al., 2017; Kuehn, 2017). The earliest of the eruptions is represented by tephra layer Bi (1990-2025 years BP) and is associated with dacite dome building, pumaceous to lithic pyroclastic flows, and andesite to dacite lava flows. Tephra Bu1 through Bu8 (~1895 years BP) includes at least eight mafic to 12

andesitic lava deposits, each impacting a different slope of the volcano lahars (Pallister et al., 2017; Kuehn, 2017). The Castle Creek period includes several andesite deposits along the Sasquatch Steps dating back to 1700 and 1800 yrs BP (Pallister et al., 2017; Kuehn, 2013 & 2017).

The short duration eruptive MSH Sugar Bowl Period (850-900 CE) is represented by tephra layer D associated with a lateral blast producing minimal lithic ejecta, including the much older Ape Canyon stage (>160 ka-240 ka BP) material. The Kalama Period (1479-1725 CE) is represented by tephra sets W (1479-1482 CE) and X (1525 CE) lahars (Pallister et al., 2017; Kuehn, 2017). Many dates in this eruptive period are well-constrained due to an exhaustive list of paleomagnetic and dendrochronological studies (Pallister et al., 2017; Kuehn, 2013 & 2017).

A major MSH dacitic eruption is represented by tephra layer Wn (1479 CE) followed by tephra layer MSH We deposited in 1482 CE. Set W tephras are associated with several pyroclastic and tephra-producing eruptions (Pallister et al., 2017; Kuehn, 2013 & 2017). Set X tephras were deposited around 1505 CE and are associated with lahars and dacitic lava flows. Additional deposits are linked to eruptions occurring sometime between 1545 and 1550 CE. In 1650 CE, additional dome-building eruptions continued and were coupled with pyroclastic and lava flows, and additional flank-building lava flows continuing intermittently until approximately 1750 CE (Pallister et al., 2017; Kuehn, 2013).

The MSH Goat Rocks Period (1800–1857 CE) was represented by tephra layer T and associated with multiple small flank collapse eruptions linked to dacite pyroclastic and lava flows (Pallister et al., 2017; Kuehn, 2013 & 2017). The Modern MSH eruptive period includes eruptions occurring since the cataclysmic VEI five lateral eruption in 1980 (Pallister et al., 2017; Kuehn, 2013 & 2017). The Modern tephra does not factor into this study, but MSH is a continuing producer of eruptions and tephra (Pallister et al., 2017; Kuehn, 2013 & 2017).

Mount Rainier (46.8523° N, 121.7603° W)

Mount Rainier is a stratovolcano located roughly 97 km southeast of Seattle. The oldest known extrusive volcanic deposits are ~840 ka old and the most recent recorded eruptions dating to the late 19th century (Mullineaux, 1974; Sisson & Vallance, 2009). Mount Rainier's peak is heavily 13

glaciated and composed primarily of andesite (Mullineaux, 1974; Sisson & Vallance, 2009). The established geohazards associated with Mount Rainier are lava, pyroclastic flows, massive lahars, and ashfall. Mount Rainier has undergone at least nine tephra-producing eruptions of VEI 1 through 4, with ejecta volumes ranging between ~0.005 km3 and 0.30 km3, respectively (Mullineaux, 1974; Sisson & Vallance, 2009). Approximately 5 ka ago, the Osceola Mudflow, with a volume of ~2.5 km3, was triggered after an edifice collapse on Mount Rainier, whereby the mudflow traveled ~100 km down the White River and into Puget Sound, ultimately covering an area of ~550 km2. This mudflow was followed by successive eruptions rebuilding the summit (Mullineaux, 1974; Sisson & Vallance, 2009). The accepted notion that especially heavily glacier-capped volcanoes are most susceptible to massive lahars makes Mount Rainier among the most dangerous lahar-producing volcanoes near a large population center (Mullineaux, 1974; Sisson & Vallance, 2009).

Glacier Peak (48.1119° N, 121.1132° W)

Glacier Peak is a dacite stratovolcano located east of Darrington, Washington (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). In the late Pleistocene (~12500 yr BP), Glacier Peak produced the second largest volcanic eruption in the Cascades behind Mazama (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). This large Glacier Peak eruption is represented by tephra G deposits, followed by tephra set M sharing at least five separate tephra subsets with the younger tephra set B (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). The B-M-G tephra series is characterized by large quantities of lapilli and tephra-fall deposits distributed at least dozens of km away from Glacier Peak's summit (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). A dormant period between 6700 and 11250 years BP passed before the Dusty Creek eruptive period produced renewed activity, represented by tephra set D (5100-6700 yr BP) that produced pyroclastic flows; lava flows, lahars flows; and lapilli/tephra falls with an overall volume of 10 million m3 (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017).

The next Glacier Peak eruptive period is represented by tephra set A (3400 years BP) and is characterized as a series of smaller tephra and lapilli producing events followed by lahars (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). An eruption around 1800 years BP does not yet have representative tephra identified but is associated with multiple edifice collapse events 14

triggering pyroclastic flows and large lahars, some traveling at least 70 km to Puget Sound (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017). Between 500- and 1000-years BP, another succession of Glacier Peak eruptions occurred and have thus far not been linked to specific tephra sets (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017), but are associated with multiple edifice collapse events triggering pyroclastic flows and lahars. The most recent eruption at Glacier Peak is represented by tephra layer X (316 years BP ± 90). Tephra X has thus far been identified only on Glacier Peak slopes and has proven difficult to study (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017) through tephrochronology and dendrochronology and has been described as pumaceous lapilli 10 cm in diameter or less with total tephra fall volumes estimated at 100000 m3 (Beget, 1982). Prevailing winds appear to generally distribute tephra from Glacier Peak to the ESE towards Lake Chelan and the area of this thesis's study, at Wapato Lake, Washington (Beget, 1982; Foit et al., 2004; Kuehn et al., 2017).

Mount Baker (48.7767° N, 121.8144° W)

Mount Baker is one of the most isolated of the Holocene active Cascade volcanoes, and as such, has not enjoyed the amount of scientific inquiry many other US Cascade volcanoes have, though an eruptive history has been established with tephrochronology (Tucker & Scott, 2006; Tucker et al., 2007; Scott et al., 2020). The most recent Mount Baker eruptive period began in the late Pleistocene with multiple eruptions through the historical period (Tucker & Scott, 2006; Tucker et al., 2007; Scott et al., 2020). The last significant eruptive activity at Mount Baker was heat fluxes experienced at Sherman Crater in the mid-1970s (CE). Mount Baker is predominantly andesitic and has produced large pyroclastic flows, lava and lahar flows, as well as tephra (Tucker & Scott, 2006; Tucker et al., 2007; Scott et al., 2020).

3. Previous Work 3.1 The offshore paleoseismic record The offshore Holocene CSZ turbidite record's uniformity provides the best-constrained megathrust earthquake recurrence model globally and fertile ground for further inland studies. Goldfinger (2017) suggests that Washington's inland lake stratigraphies are correlated with well- constrained marine sediment core records where radiometric dates, relative ages, and geophysical properties are used as correlative data. The result is a general stratigraphic 15

agreement of the recurrence of event beds between sediment cores in marine and lake environments. The 10000-year turbidite paleoseismology record found in marine sediment cores similarly provides a view of the CSZ region's seismic past. Goldfinger et al. (2012; 2017), along with other researchers (Atwater 1987; Rogers & Dragert 2003; Nelson et al., 2006; Higgins 2009; Morey 2013; Atwater et al., 2014; Liethold et al., 2018) have constructed a solid foundation from which new research can delve further into tantalizing questions about the causal relationship between megathrust earthquakes and volcanic eruptions, as well as physical mechanisms propagating that causality.

Figure 3.1. Plot illustrating long-term seismic activity and recurrence intervals of the Cascadia Subduction Zone and relative energy cycle over time. Offshore-onshore paleoseismic data is represented in this diagram, illustrating the likelihood that the timing of CSZ events has come in four distinct event clusters throughout the Holocene. From Figure 4 of Goldfinger et al. (2013). As stated previously, research has established the synchroneity of the turbidite paleoseismic records found in marine and lacustrine sediment cores (Morey et al., 2013; Goldfinger et al., 2017). In Figure 3.1, Goldfinger et al. (2013) illustrate the recurrence interval of CSZ megathrust earthquakes for the past 10 ka with a relative magnitude of 18 distinct megathrust earthquakes shown on the y-axis, represented in a cycle of potential energy gain between Cascadia events and kinetic energy loss at the time of a megathrust earthquake event.

Many subduction zones, including systems in the Caribbean, Central American, and South American, have had similar methods applied to marine and lacustrine sediment cores, offering insight into seismic recurrence and magnitude potential. Two primary techniques are often used to help distinguish seismic from non-seismic turbidites: 1) sedimentological determination of individual event origin; and 2) regional correlations requiring synchronous and widespread (i.e., earthquake) triggering. 16

3.2 Paleoseismic record in Cascade lakes Lake sediment core records illuminate correlatable stratigraphic and geophysical signals inland and across the Cascade at Lake Wapato, Washington. In addition, Moernaut et al. (2019) look to lacustrine turbidite deposits to study the paleoseismological record in Chile at Lakes Villarrica and Calafquen, where they employ tephrochronology that better constrain ages of tephra-laden seismoturbidites. Coastal marshes and lakes in Oregon and Washington provide a 3000-year paleoseismic record displaying cyclical intervals of sudden coastal subsidence and seismogenic tsunami inundation sediment deposits.

In previous work in Cascadia and elsewhere, seismogenic turbidites have been correlated to lake basins and fjords using offshore techniques similar to those used by Goldfinger (2017); Karlin et al., (2004); St. Onge et al. (2004); Guyard et al., (2007); Waldmann et al., (2008). For example, Howarth et al. (2012) develop a lacustrine record of earthquakes along the Alpine Fault in New Zealand: and found that the frequency and timing of the lake turbidites are very similar to the New Zealand offshore turbidite record. Goldfinger (2017) suggests that megathrust earthquakes are most likely to have generated lake deposits with similarity in physical properties, event frequency, and chronology. Intending to determine whether inland Cascade lakes contain seismically-induced turbidite sequences corresponding with previously determined marine CSZ turbidite sequences, Goldfinger (2017) sampled lakes in the Puget Sound, Olympic Peninsula, Mount Hood, and eastern Cascade regions. The lakes include Lake Leland, Washington on the Hood Canal; Lake Sawyer, 80 km south of Seattle, Washington; Bull Run Lake near Mount Hood, Oregon; and Wapato Lake, in the eastern Cascades of Washington. Lakes sampled are located in western regions and contain post-glacial stratigraphy containing Mazama tephra and numerous turbidite event beds. Goldfinger (2017) interpret that the stratigraphic units positively correlate between Leland and Sawyer lakes which are as 90 km apart on opposite sides of Puget Sound. There are nineteen Cascadia-induced turbidite beds above the Mazama ash in the two lakes, compared with sixteen events in the offshore paleoseismic record at the same latitude. Goldfinger (2017) suggests a tentative correlation between all sixteen events, along with correlations between Leland and Sawyer lakes. 17

3.3 Tephrochronology Tephra are fine-grained volcanic glass shards generated during extrusive volcanic eruptions. Tephra and other volcanic ejecta become airborne, are transported and distributed by prevailing winds. Tephrochronology offers insight into the geochemistry of tephra deposits elucidating the tephra's volcano of origin. Individual tephra deposits become well documented in literature due to their ubiquitous nature in regional stratigraphies; examples of such regions include Thera, Greece; Campi Flegrei, Italy (e.g., Tomlinson, et al., 2012; Fitzsimmons et al., 2011; Nowaczyk et al., 2012); Yellowstone (e.g., Morgan et al., 2017; Mastin et al., 2014) and Mount Mazama (e.g., Spano, et al., 2017; Bacon et al., 2017), United States, among many others. In the North American Pacific Northwest, Mount Mazama is one of the most identifiable tephra deposits in the region (Bacon, 1983 and 2017; Zdanowicz et al., 1999; Egan et al., 2015; Spano et al., 2017; Jensen et al., 2019). Tephrochronology also involves the chemical analysis of tephra deposits and leads to matching specific tephra to a parent volcano (Foit et al., 2004 & 2016; Kuhn et al., 2009; Peterson et al., 2012; Pyne-O’Donnell et al., 2012 & 2016; Balascio et al., 2015; Egan et al., 2015; Ponomareva et al., 2017; Jensen et al., 2019). This analysis allows for determining spatial distributions of tephra from each volcano in the CVA region and trace chemicals and the evolution of regional volcanic activity.

Primarily, tephrochronology involves the use of tephra bed stratigraphy to develop a chronology and may also include geochemical and geochronology methods. Chemical analysis of tephra deposits that may be matched to a parent volcano, however identification of origin of a tephra is not necessary to provide stratigraphic correlative insight (Foit et al., 2004 & 2016; Kuhn et al., 2009; Peterson et al., 2012; Pyne-O’Donnell et al., 2012 & 2016; Balascio et al., 2015; Egan et al., 2015; Ponomareva et al., 2017). Each Holocene-active CVA volcano has an eruptive timeline, where each eruption has an age range established through the calibrated 14C dating of organic materials associated with tephra deposition or with other eruptive styles. These products offer insight into CVA volcanic activity and provide the timing of volcanic events with other geological events and depositions.

Since the 1960s, the study of tephra has become a significant source for establishing geologic dates, stratigraphy, and provenance of tephra. In recent years, tephra studies have expanded into the analyses of cryptotephra—tephra visible only with a microscope or other 18

scanning procedures. Cryptotephra tends to distribute much further afield than macro-tephras. While the study of cryptotephrochronology is a quickly expanding subfield with the potential to inform this study is limited to tephra analysis of macrotephras with the hope cryptotephra analyses will be the next logical progression from this study (Foit et al., 2004; Egan, 2015; Balascio et al., 2015; Davies, 2015; Foit & Mehringer, 2016; MacKay et al., 2016). In Balascio et al., 2015, an X-ray fluorescence (XRF) analysis of lake sediment cores provide elemental data with the potential to detect tephra in core sections—for instance, plotting levels of Si, K, Ti/Ca, Sr, and Mn/Ca may be a rhyolitic tephra signal while Mn, K, Fe, K, Cu/K, Ti/K, and Zr/Rb may indicate the presence of mafic tephra. Tephrochronology plays a pivotal role in refining the timing and dates of important geologic, ecological, archaeological, and climatic events, dovetailing with other analytical techniques such as radiocarbon dating, stratigraphy, volcanology, and paleoseismology (Foit et al., 2004; Kuhn et al., 2009; Pyne-O’Donnell et al., 2012; Ponomareva et al., 2017).

3.4 Triggering Relations between Subduction Zones & Volcanic Arcs Volcanic and tectonic systems interact with one another and interact with their environments at different temporal and spatial scales (Eggert & Walter, 2009). It is likely that triggering relationships between tectonic and volcanic events are spread across a wide range of time and geography, depending on the size of any given tectonic or volcanic event. The literature suggests that energy gains and losses are related to the amount of strain present within a subduction zone system (Walter, 2007). Changes in strain due to subduction zone earthquakes can propagate to adjacent crustal faults, ultimately affecting magma reservoirs and conduits residing below the Cascade Volcanic Arc (Walter, 2007). Statistical modeling and surveying of either global or disparate regional megathrust volcano linkages have been limited to historical events, thus providing researchers with insignificant temporal samplings of events (Manga & Brosky, 2006; Walter 2007; Walter & Amelung, 2007; Watt et al., 2009; Higgins, 2009; De la Cruz-Reyna et al., 2010; Bebbington & Marzocchi, 2011; Bonali et al., 2012; Ozawa et al., 2016). A variety of megathrust earthquakes, including the 1952 Kamchatka (Mw 9.0); 1960 Chile (Mw 9.5); 1964

Alaska (Mw 9.2); the 2004/2005 Sumatra-Andaman (Mw 9.3 and Mw 8.7); the 2018 Ambrym-

Vanuatu (Mw 6.4), all had proximal volcanoes erupt explosively following these seismic events. Walter & Amelung (2007) suggest post-seismic eruptions occur with volcanoes located within a 19

certain distance from earthquake rupture areas, and rarely erupting volcanoes are more susceptible to tectonic triggering.

Avouris et al. (2017) studied the relationship between open volcanic systems globally— volcanoes with magma-atmosphere interface during the time of the study (2004-2010 CE), such

as lava lake or vent at or near the surface and Mw ≥7 tectonic earthquakes. They monitored

geodetic and degassing changes at the volcanoes, creating a time series of SO2 loading,

comparing this data with 69 Mw ≥7 earthquake events that occurred from 2004-2010 in the

region (Avouris et al., 2017). The results suggest that open-vent basaltic volcanoes increase SO2 degassing following earthquakes with elevated PDS (peak dynamic stress) values. Positive responses are observed in the data from Ambrym (63% increase in emissions with a PDS of 5 kPa) and Villarrica (Avouris et al., 2017). Negative responses are observed at volcanoes experiencing cyclical active such as conduit plugging from viscous magmas that is seemingly producing reduced SO2 degassing from Merapi. Guego, and Bagana (Avouris et al., 2017). The solo volcano that did not show an observed degassing response from an earthquake was at Rabaul-Ulawun (Avouris et al., 2017). While the Avouris et al. (2017) study provides a snapshot into a short 6-year modern dataset, the results suggest likely some volcanoes react within days of a large magnitude earthquake and specifically, open system volcanoes react more forcefully in

SO2 output in the short term than closed volcanic systems. These results point to at least some volcanoes reacting to large earthquakes, however the term triggering may be interpreted as both direct and indirect causal relationships. Direct triggering is observable with events that occur near in time while indirect triggering is part of a chain-reaction of processes that may produce longer gaps in time between the triggering event and the triggered.

Sulpizio and Massaro (2007) analyzed first-order volcanic triggering mechanisms which include short- and long-term unloading, seismic energy effects, and changes in far field stress due to geodynamic processes (Sulpizio and Massaro, 2007). They remind the reader that short term and long term processes as do tectono-volcanic processes near and distant to volcanoes impact volcanic eruptive behavior. Macro processes overlay and interact with localized, internal processes specific to an individual volcano. Of interest is the discussion of the impact of seismic energy on magmatic systems through static and dynamic stresses. Both types of stress decay at different rates, with static energy decreasing more rapidly with distance than dynamic stress 20

(Sulpizio and Massaro, 2007). While there seems to be some statistical agreement between the occurrence of a megathrust earthquake and volcanic eruption, this seems to vary by region and study. A statistically significant response immediately after the earthquake (Linde and Sacks, 1998) has been observed for volcanoes ≥ 750 km from the epicenters. This observation suggests that volcanic eruptions are triggered by dynamic deformation (Brodsky et al., 1998; Manga and Brodsky, 2006). Dynamic crustal deformation is induced by seismic body and surface waves while displacement occurs across a fault with consequent viscoelastic relaxation of the crust account for permanent static deformation (Sulpizio and Massaro, 2007). Unclamping of magma conduits following the release of tectonic energy from a subduction earthquake can introduce new magma into magma chambers. Cailleau et al. (2007) studied possible triggering relationships in the Central American volcanic arc and subduction earthquakes of the Cocos Plate. Specifically, they assess the changes in Coulomb failure stress between arc volcanoes in Nicaragua regionally that preceded

the 1972 Ms 6.2 Managua earthquake. The interpretation in Cailleau et al. (2007) pointed to megathrust ruptures occurring due to weakening of crustal fault coupling near volcanic arcs. An extension of this interpretation is that volcanoes are a surface expression of a weak point in the crust with connecting faults susceptible to rupture triggered by adjacent tectonic megathrust earthquakes and subsequent changes in magmatic plumbing, thus triggering volcanic eruptions. Unloading of the crust from a particularly large VEI eruption could presumably destabilize the crustal-tectonic regime, thus triggering a megathrust earthquake. In many tectonic triggering studies, several promising theories are set forth supporting or refuting potential causal relationships between megathrust earthquakes and volcanic events (Manga & Brosky, 2006; Walter 2007; Walter & Amelung, 2007; Watt et al., 2009; Higgins, 2009; De la Cruz-Reyna et al., 2010; Bebbington & Marzocchi, 2011; Bonali et al., 2012; Ozawa et al., 2016). Researchers interested in exploring possible timing and triggering mechanisms between megathrust earthquakes and adjacent volcanic arc eruptions have used a wide variety of methods (Manga & Brosky, 2006; Walter 2007; Walter & Amelung, 2007; Watt et al., 2009; Higgins, 2009; De la Cruz-Reyna et al., 2010; Bebbington & Marzocchi, 2011; Bonali et al., 2012; Ozawa et al., 2016). However, previous work has neglected Cascadia, and there is no published work utilizing either paleoseismology or tephrochronology to explore timing and trigger mechanisms. 21

4. Methods Over the 2015 field season, lake sediment cores were collected at numerous North American Pacific Northwest sites by students and faculty in the Oregon State University Active Tectonics and Seafloor Mapping lab, led by Chris Goldfinger (see USGS NEHRP Final Report G13AP00066 for full discussion). The focus of this research has been to study sediment cores taken from lake sites (from West to East) in Lake Leland, Sawyer, and Wapato, all of which are located in Washington state. The general geologic settings of these lakes are essential when providing stratigraphic spatial-temporal context.

In this study, the methodology is based on analyses of existing sediment cores (Figure 4.1) from Wapato Lake, Washington (47.9167° N, 120.1601° W); Lake Leland, Washington (47.8964° N, 122.8825° W); and Bull Run Lake, OR (45.4557° N, 121.8334° W); as well as the offshore Rogue Apron (42.2942° N, 125.08.38° W). Analyses for lakes Wapato, Leland (Goldfinger, 2017), and Bull Run (Goldfinger et al., 2020), and Rogue Apron (Goldfinger et al., 2012) contribute to previously published data collected from sediment cores at marine and other regional lake sites. Wapato and Leland lake sediment cores were collected by deploying a push corer, while Bull Run lake cores were collected using a gravity corer. Each coring operation was conducted via either a coring platform on either a USGS barge or two canoes with aftermarket modifications. All cores were transported to the Oregon State University Marine Core Repository in Corvallis, Oregon, cataloged, and subsequently stored in a climate-controlled refrigerated structure.

Event bed correlation is achieved by applying well log correlation methods used widely within the oil, gas, and mining industries (McCubbin, 1982; Morton-Thompson & Woods, 1992; Lovlie and van Veen, 1995; Chen et al., 2009) and commonly used in academic and ocean drilling program/integrated ocean drilling program (ODP/IODP) cores (Fukuma, 1998) and applied to paleoseismic studies (i.e., Schnellman et al., 2002; Abdelayem et al., 2004; Goldfinger, Nelson, Johnson, Arsenault, et al., 2004; Hagstrum et al., 2004; Iwaki et al., 2004; Karlin et al., 2004; Stone et al., 2004; Goldfinger et al., 2007; Goldfinger et al., 2008). Both turbidite and tephra bed deposits produce high CT density and magnetic susceptibility values presenting peaks in data plots and as bright features in CT greyscale images. In contrast, the 22

RGB images may present as bright or dark features depending on the geochemical content of deposits.

When multiple sediment cores are collected within the same lake or marine locale, local site correlation can be achieved by matching peaks and troughs of each unique geophysical signature in data plots between cores. Similarly, regional stratigraphic correlations can be constructed by matching geophysical peaks and troughs between geophysical plots. Visual correlations are informative by observing changes in RGB and greyscale images and correlating color changes and bright features to data plot peaks. Each data plot peak and trough provides a fingerprint representing a particular event bed informing intra- and inter-lake correlations (Goldfinger et al., 2007, 2008, 2012, 2017, 2020). Once candidate sampling sites were identified, 14C samples were taken at the base of tephra bed deposits combined with geochemical analyses to ascertain their provenance and age.

Figure 4.1: Cascadia Subduction Zone and Holocene-active Cascade Volcanic Arc (CVA). Canadian CVA volcanoes are not included in this map or present study. 23

4.1 Magnetic Susceptibility & Gamma Density For this study, a Geotek XYZ Multi-Sensor Core Logger (MSCL-S) splitcore logger using a Bartington MS2E point sensor was employed to determine the magnetic susceptibility of core materials. Each halved core section was placed on the instrument bed and scanned at a 1 cm resolution, with roughly one core section taking 45-60 minutes to scan. The instrument also collected digital color reflectance RBG (Red Blue Green) high-resolution color images. Magnetic Susceptibility MS2E point sensor is paired with the MS3 meter, measuring at a 26 SI range (GEOTEK.co.uk, 2020) and used for high resolution (1 cm) scans of the split sediment core surface. The sensor was calibrated to measure the presence of standardized iron oxide, converting the scanned values into SI units with positive values representing high concentrations of ferrous, magnetic sediment, and negative values representing a dearth of such material. Once each core section scan was completed, ASCII text files were produced. The data was converted to *.csv files, then plotted with Adobe Illustrator and placed vertically next to CT and RBG sediment core images. Gamma density data was also collected on the Rogue Apron marine cores due to the availability of the GEOTEK Multi-Sensor Core Logger full sensor array while at sea. The GEOTEK sensor can detect full spectral natural gamma data at high resolution and is configured with 3"x 3" crystal NaI (Tl) detectors housed within 6” diameter lead shields (GEOTEK.co.uk, 2020). The gamma density detectors have a 6-8% resolution at the 0.662 MeV peak of 137Cs as elemental yields (concentrations of K, U, and Th) data are collected over a range of 0-3 MeV (GEOTEK.co.uk, 2020). The data produced were ASCII text files that are, in turn, converted to *.csv files and plotted in Adobe Illustrator for comparison to the Magnetic Susceptibility and CT plots. This geophysical data coupled with ground truth data provides an illustrative representation of sediment core material (Wetzel & Balson, 1992; Weber et al., 1997; Rothwell and Rack, 2006; Goldfinger et al., 2012 and 2017).

4.2 Computed Tomography Analysis of the sediment cores began with non-destructive Computed Tomography (CT) imaging (Toshiba Aquillon 64 slice system with 0.4 mm voxel resolution) at the Oregon State University College of Veterinary Medicine to provide greyscale images for use as proxies for core material densities. The CT system is operated by a credentialed imaging technician. Raw data were processed using SanteCT Viewer software, followed by ImageJ software. Within ImageJ, a profile was drawn across the CT image resulting in a plot saved into a *.csv file. The CSV data is 24

plotted with Adobe Illustrator to produce 0.5 mm voxel resolution data points placed vertically next to the CT and Red, Blue, Green (RBG) sediment core images and correlated to the corresponding core depths to provide accurate distributions of core material density within the sediment cores. The variations in CT density with depth throughout a sediment core are a correlatable geophysical attribute between sediment cores within a lake and between lakes. Increases in the density of core material may reflect the presence of tephra, carbonates, and organic material and are therefore a useful, non-destructive tool providing this research with numerous data to determine the usefulness of a sediment core, as well as a data source for future research.

4.3 Lithologic Description Collected core sections were split into halves with a handheld corded splitter, prepared, and a lithological log and facies descriptions for each core section completed, including initial soil/sediment grain size, Munsell color, and depositional information. Tephra and event beds were then recorded, with descriptions and locations within the core noted for future sampling. Similarly, potential radiocarbon sample sites were labeled based on quantity and type of organic material present at a given core depth. Initial determination through visual inspection offered potential locations of turbidites, bioturbation, or other disturbance features, providing a foundational assessment of core contents. Such contents suggested how best to proceed with future geophysical analyses, sampling, and overall core conditions, along with the potential quality of future analyses.

4.4 Particle Size Analysis For this study, sediment samples were collected at approximately 5 cm intervals for each core section to determine sediment particle size and analyzed by laser diffraction methods using a particle size analyzer (PSA) Beckman-Coulter LS 13-320 laser counter after digesting organics using a 25% hydrogen peroxide treatment. No samples from Wapato WLC-04 were processed from 260 centimeters to 457 centimeters due to the presence of a well-rounded and well-sorted pumaceous lapilli bed with grains (on average 1.5 cm in diameter) being too large to process with the PSA. These laboratory techniques' overall goal was to determine the physical characteristics of core materials at Lake Wapato. This project is specifically interested in physical traits indicating seismoturbidite sequences, such as fining-upward grain size 25

distribution. PSA data from the particle size analyzer were placed into an Excel spreadsheet, saved as a *.csv file, plotted in Adobe Illustrator to produce 5 cm resolution data points placed vertically next to the CT and RBG sediment core image, then correlated to corresponding core depths provide accurate descriptions of grain size distributions in sediment cores.

4.5 XRF (X-ray fluorescence) This study includes a lithological log from cores scanned by a Cox Analytical Systems ITRAX X-ray fluorescence (XRF) corescanner at 0.5 cm resolution. However, the veracity of the XRF elemental data can be discerned by matching peaks in XRF element ratio plots with visible tephra deposits, therefore informing the likely presence of cryptotephra in Lake Wapato cores. Cores 1, 2, 3, and 4 XRF elemental data are plotted and specific elemental ratios are used to establish visible tephra bed(s) as controls and illustrate where potential cryptotephra may be found for future work. Peaks in Si, K, Ti/Ca, and Sr plots suggest the presence of rhyolitic tephra, while peaks in Mn/K, Fe/K, Cu/K, Ti/K, and Zr/Rb plots suggest the presence of mafic tephra (Balascio et al., 2015). Peaks in K, Zr, and Ca/K may illustrate the presence of non- weather derived turbidite sequences (Corella et al., 2014).

4.6 Radiocarbon Dating In this study, samples collected from sediment cores that were either plant (root, leaf, branch, bark, or charcoal) or animal (calcium carbonate shell) were placed in sealed and marked bags, freeze-dried, and shipped to the W. M. Keck Carbon Cycle Accelerator Mass Spectrometer (AMS) at the University of California, Irvine (UC-Irvine). The UC-Irvine facility operates a 500 kV compact AMS unit produced by the National Electrostatics Corporation, Model # NEC 05MV 1.5SDH-2 (Southon, 2005). The UC-Irvine lab produced uncalibrated sample ages that included the 14C age in years before present (yrs BP) with an error notated as ± yrs BP. An example notation would be 9850 ± 25 yrs BP would indicate the median age of the sample is 9850 years before present with a standard deviation of 25 radiocarbon years. These ages are reported as years before 1950 CE (Common Era) AMS dates such as those provided by UC- Irvine include uncertainty ranges and are notated as 1σ and 2σ. 1σ and 2σ are the resultant uncertainty from measuring isometric radiocarbon ratios and represents a measurement of uncertainty with 2σ representing ~95% confidence that a particular sample is within a given age range and 1σ representing ~68% confidence the sample is within a given age range. Radiometric 26

age 2σ uncertainty tends to be reported in the literature, though it provides a less precise age of a sample than the 1σ uncertainty (Bowman, 1990; Southon et al., 2005). In this study, OxCal software was used to calibrate the sample ages.

4.7 OxCal Age Modeling OxCal is a radiocarbon calibration software that considers multiple external age constraints, including sedimentation rates and multiple 14C ages, by applying Bayesian statistics to consider overlapping probabilities (Goldfinger et al., 2012; Ramsey, 1995, 2001). Where radiocarbon dates are missing due to lack of suitable material or a gap in sampling, sedimentation rate alone can inform OxCal of event dates. Measurement of only the event-free, hemipelagic in situ core material can provide sedimentation rates. Basal erosion in event beds is a source of uncertainty in determining sedimentation rates and must be considered while constructing an event-free model by considering each event bed and the surrounding stratigraphy. 14C samples are collected from directly below the extant base of the event bed to mitigate this source of bias and provides an approximate minimum and maximum date for the event and mitigates, to some extent, the error produced from basal erosion (Ramsey, 1995 and 2001; Goldfinger et al., 2012 and 2017). OxCal is a software based on a Bayesian Markov Chain Monte Carlo calibration and age model (Ramsey, 1995 and 2001; Goldfinger et al., 2012 and 2017). Within OxCal, P-sequence age modeling is employed in which event-free depth values are input along with the radiocarbon ages. By subtracting observed event bed thicknesses from the full core stratigraphy, event-free depth values are achieved (Goldfinger et al., 2012, 2017, and 2020). Matching coeval deposits between cores provides further detail to the correlation of geophysical plot peaks and troughs and between greyscale and RGB images of the cores (Goldfinger et al., 2020).

4.8 Tephrochronology For this study, fifteen visible tephra samples were taken from eleven lake sediment core sections, four terrigenous, and two Rogue Apron marine core sections where tephra samples were wet sieved through a 20-micron sieve; treated with a 24-hour 30% H2O2 digestion and Calgon rinse; placed in a sediment heat dryer for 96 hours; and finally shipped to the Electron Microprobe Laboratory at the University of Alberta where Dr. Britta J. L. Jensen conducted tephra identification. An additional eleven visible tephra samples were collected from Lake Wapato. In addition, USGS Cascade Volcano Observatory scientists James Vallance and Alexa Van Eaton 27

collected four terrigenous tephra samples from areas between Glacier Peak and Lake Chelan. All these samples were shipped to the University of Alberta for identification.

Tephra deposits in this study are identified by morphology, mineralogy, and major element compositions of sampled glass shards and stratigraphic contexts of the deposits. Once samples arrived at the University of Alberta, established laboratory practices were instituted. Assessment of size fractions was conducted via a stereoscopic microscope to determine whether the sample contained a sufficient fraction of glass for an Electron Microprobe (EMP) analysis (Jensen et al., 2010). Glass was separated from heavy mineral fractions with a heavy liquid tetra bromoethane followed by the use of a hand magnet to remove magnetite from heavy mineral fractions. The remaining material was processed through a Frantz Magnetic Separator (LB-1) to isolate Fe-Ti oxides for ilmenite (Jensen et al., 2010). Each glass shard was then mounted, polished, coated with carbon, then analyzed by EMP. Fe-Ti oxides were further analyzed by wavelength dispersive spectrometry in a JEOL superprobe. In the superprobe, a defocused 10 micron-diameter beam at a 15 keV accelerating voltage and 6 nA beam current was applied to ensure Na and K mid-analysis migration was minimized (Jensen et al., 2010). All samples were compared to an unpublished, non-public database of previously identified reference tephra samples to identify the tephra beds used in this research (Jensen et al., 2010).

5. Results In the subsequent sections, the bathymetric and seismic sub-bottom survey results are described (5.1), followed by a description of the Wapato Lake sediment core stratigraphy and geophysical results (5.2). In addition, initial tephra XRF and geochemical results from Wapato are presented (5.3), followed by and initial Wapato OxCal age model and intra-lake well-log correlation (5.4 & 5.5). XRF, smear slide, geochemical, and an updated OxCal age model are presented for Leland Lake and Bull Run Lake (5.6 & 5.7, respectively), with a tentative regional inter-lake well-log correlation presented (5.8). Finally, the Rogue Apron tephra results are presented (5.9).

5.1 Lake Wapato Results Lake Wapato is located at an elevation of 114 meters on a bench above Lake Chelan's southeastern shore (Goldfinger et al., 2017). This bench consists of glacial till material striated sub-parallel to glacial movement along the Lake Chelan valley (Goldfinger et al., 2017). Lake Wapato trends to the northwest as a relatively flat-bottomed lake basin with a gradual lake bed 28

slope on the northwest end of the lake while the rest of the lake bed perimeter slope is relatively steep. Wapato is fed by a minor, intermittent drainage entering from the northeast while a small, intermittent stream drains the lake in the southwest. The maximum depth of the Wapato Lake is 21 m (Goldfinger et al., 2017) and is 0.805 km2 or 199 acres in size. Bathymetry, backscatter, and seismic profiles were additionally collected and processed for this study.

5.1.1 Wapato Lake Bathymetry

Figure 5.1: Wapato Lake back scatter imagery bathymetric map illustrating the line-track of the seismic survey. Wapato sediment core locations (WLC- 01 to WLC-04) are presented. Seismic profiles in Figures 5.1-5.5 are located at each of the coring sites.

The Wapato Lake bathymetry is morphologically consistent with a glacially scoured or plucked depression. The northwestern Wapato shoreline is bounded by Quaternary alluvium, while the southern shore is bounded by Quaternary glacial drift deposits, identified as a moraine comprised of unstratified ice-worked till material. Scalloped slopes along the northwestern section of the lake (Figure 5.2) may be indicative of a variety of processes, including cyclical periglacial freezing-thawing (Ulrich et al., 2010); cyclical shoreline water level changes (Chen & Malloof, 2017); and various types of slope failure, including seismogenic (Badger & Watters, 2004). The Wapato shoreline is dominated by fluvial and glacial alluvium, with the bathymetry reflecting 29

slope failure susceptibility in the northwest sector of the lake. These slope failures appear evident in planar features trending sub-parallel to the overall inclination of the lake. The northeast and southwest bathymetric slopes range from 0.00182o-47.4o—slopes are steepest near the central northeastern section of the shore and a lesser extent along the southwestern shoreline.

0 km 0.25 Figure 5.2 Scalloped lake bed structures observed in the bathymetric data believed to represent lake bed slope failures.

5.1.2 Wapato Lake Sub-bottom Profiling Fifteen survey transects of seismic data collected during sediment coring at Lake Wapato, provide a profile of the lake bottom and subsurface structures that offer insight into lakebed slopes, subsurface deformation, and structures. These identifiable structures include lake basins, faulting, sediment accumulation, and evidence of slope failure within the lake. CHIRP seismic survey profiles (Figures 5.3-5.6) were produced from data collected at Wapato Lake along track lines (Figures 5.1). Wapato WLC-01 sample site is approximately 85 meters from the west shore and is separated by the next nearest core site (WLC-04) by approximately 347 meters to the ESE. WLC-01 was collected at a depth of approximately 15 meters, on the west slope of the southern half of the lake. Approximate depth conversion from TWT (two-way time depth) to meters is circumvented by using the measured, observed coring depths noted in the field. 30

Figure 5.3: WLC-01 Core Site Seismic profile. An approximate depth is in meters based on a combination of on- site core location depth measurements and TWT backscatter data. The core sample site for WLC-01 is located approximately 85 meters off the west shore and ~15 meters deep.

The seismic profile for WLC-02 (Figure 5.4) shows a shallow bench at the southern-most section of the lake The WLC-02 seismic profile reveals a succession of sediment deposits at the base of the lake’s western slope. This coring site is approximately 6.8 meters in depth.

Figure 5.4: WLC-02 Core Site Seismic profile. An approximate depth is in meters based on a combination of on- site core location depth measurements and TWT backscatter data. The core sample site for WLC-02 is located approximately 85 meters off the west shore and ~6.8 meters deep. WLC- 02 is situated on a shallow bench at the southern-most corner of the lake and further reveals a succession of sediment deposits at the base of the lake’s western slope.

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The seismic profile for WLC-03 (Figure 5.5) is situated in the southwest section of Lake Wapato (351 meters to the SW of WLC-01, and 461 meters to the WNW of WLC-02) and shows WLC- 03 is located in the middle of a likely slope failure. This coring site is approximately 13-15 meters in depth.

Figure 5.5: WLC-03 Core Site Seismic profile. An approximate depth is in meters based on a combination of on-site core location depth measurements and TWT backscatter data. The core sample site for WLC-03 is located approximately 85 meters off the west shore and ~15 meters deep. WLC-03 is situated in the southwest section of Lake Wapato and shows the coring site is located in the middle of a likely slope failure. The seismic profile for WLC-04 (Figure 5.6) is located at the southeast section of the lake and shows the coring site situated at the base of the lake’s bottom slope, but atop a localized rise, possibly due to an old slope failure deposit. Based on the core’s physical properties, the top of the core may be disturbed due to coring activity but does not appear to include a slope failure layer at the top of the core. This coring site is approximately 12.5-15.68 meters in depth. The lake's bathymetry reveals gradual slopes on the NW and SE extents and steep slopes on the NE 32

and SW shores, with the deepest part of the lake at the base of the NE shore and the narrow center of the lake with a depth of 21.3 meters.

Figure 5.6: WLC-04 Core Site Seismic profile. An approximate depth is in meters based on a combination of on-site core location depth measurements and TWT backscatter data.WLC-04 is situated at the base of the lake’s bottom slope, but atop a localized rise, possibly due to an old slope failure deposit. This coring site is approximately 15.68 meters in depth.

5.2 Wapato Core Stratigraphy & Petrophysics Primary Facies

The primary stratigraphy of Lake Wapato cores is comprised of at least five facies:

1) Hemipelagic background lake sediment; 2) Sharp based, fining-upward event beds; 3) Tephras; 4.) Lapilli; and 5.) Thin Lamellae. 33

Background Sedimentation

The background stratigraphy of the Wapato cores primarily consists of clay to fine silt grained hemipelagic sediment. A near-constant tephra presence within the background detected by smear slide and geophysical data could be from small eruptive events or post-eruptive aeolian and fluvial transportation into the lake. The majority of the core material consists of these sediments, making it relatively easy to observe facies changes. Wapato cores include potential turbidites and tephra beds with load features (Figure 5.8). These structures appear in CT images directly below the base of the event beds and are can be reflected in CT density and magnetic susceptibility plots. The Wapato cores all contain successive fining-upward, silty to sandy beds with abrupt bases and load structures. These beds are also diatom and tephra rich, in which CT density and magnetic susceptibility peaks attribute to the concentration of diatoms and tephra.

Fining Upward Event Beds

Background Sediments

Figure 5.7 Example of a fining-upward event bed and background sediments found within the Wapato Lake cores.

Tephra

Tephra is observed in background sedimentation at Wapato as well as separate, primary airfall tephra beds. In addition to this, tephra beds possess multiple geochemical signatures found in lithic event beds suggesting reactivated tephra deposits of multiple origins during slope failure events. 34

MSH Wn Tephra Bed

Load features

Figure 5.8 Example of a tephra bed and event bed load structures. In this example, the MSH Wn tephra bed appears to have an event bed at its base, along with tell-tale load features. Lapilli

A thick deposit of lapilli dominates the lower depths of WLC-04 (Figure 5.9). Lapilli grain size range from 0.20-6.4 cm and is deposited with initially appears to reflect little or no sorting. On closer inspection of the CT density greyscale and Osirix images, discernable layering is observable represented by defined, bright (high density) signals. The angles of lapilli grain positions also reflect some flow patterns similarly seen in slope failures. Successive layers reflect slightly offset grain angle positions.

Late Pleistocene Glacier Peak Lapilli (Tephra B & G)

Figure 5.9 Example of the lapilli deposit found within the Wapato Lake cores.

Thin Lamellae

The thin, wispy lamellae deposits (Figure 5.10) represent distinct changes in color, grain size, and lack structure at the 0.5 mm voxel resolution of the CT imagery. The thickness of these 35

lamella range from 0.5 mm to 12 mm—some correlating with small peaks in density when compared to the background CT data.

Thin Lamellae

Figure 5.10 Example of successive, thin lamellae found

within the Wapato Lake cores. These deposits may represent brief changes in surficial or biogenic processes.

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Figure 5.11: The stratigraphy, lithology, geophysical properties, and sample data of Wapato Lake WLC-04. WLC-04 is the representative core for Wapato Lake. CT Osirix, Magnetic Susceptibility RGB, Lithologic Column, and CT greyscale images are flanked by CT density and Magnetic Susceptibility plots. Radiocarbon and Tephra samples are marked.

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5.2.1 Wapato CT & Magnetic Susceptibility All Wapato cores show CT density and magnetic peaks associated with event bed and tephra facies (Figure 5.7 & 5.8, respectively). In WLC-04, at least 17 distinct, sharp peaks in the CT density plot represent increases in sediment density from the background (Figure 5.12 & Table 5.1, respectively). A 12 cm thick plateau of CT plot peaks from 28-40 cm core depth correlates to a deposit in the sediment core that is light grey (approximate Munsell color of 2.5 YR 5/1). A Magnetic Susceptibility data plot peak corresponds to the bottom 5 cm of this deposit. Other notable CT and Magnetic Susceptibility data plot peaks include a cluster of 7 relatively large peaks and four slightly smaller peaks starting at ~145 cm and extending downwards to ~195 cm.

The CT and Magnetic Susceptibility peaks correspond with one another and correspond to bright laminae in the CT image and either light grey or black colored laminae in the RGB image. Between 215 and 223 cm, a doublet of CT and Magnetic Susceptibility peaks are observable and correspond to a pair of bright laminae in the CT image and a pair of dark black laminae in the RGB image. Beginning at a depth of 243 cm and extending to the bottom of the core, the CT and Magnetic Susceptibility data plots exhibit a chaotic character. Between 243 and 257 cm, there is an increased concentration of small gastropod shells and tephra. An abrupt stratigraphic boundary is present at 260 cm, extending to the bottom of the core. The material consists of beige and grey pumaceous lapilli (0.25-1.25 cm diameter in size) with a general background of grey and black silt and sand-sized sediment, including a mixture of tephras. Overall the general background sediment in the core ranges from Munsell color of 2.5Y 4/2 to 2.5Y 5/2 with the abruptly occurring deposits ranging from 2.5Y 3/1 to 8/1 and 5Y 3/1 to 6/1.

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CT Magnetic Event or Tephra bed Tephra ID/Radiocarbon Depths Density Susceptibility (cm) Peaks Peaks cal. years BP

3-5 X X N/A Top of Core disturbed 12-16 X X Fining-upward Event Bed

Tephra-fall with distinct fining MSH Tephra Layer Wn upward. 471 ± 0.5 (A4 & A5) 28-40 X X

44-52 X Fining-upward Event Bed

56-64 X Fining-upward Event Bed 72-76 X Fining-upward Event Bed

84-91 X Fining-upward Event Bed

95-101 X X Fining-upward Event Bed

Fining-upward Event Bed R8 870 ± 80 106-114 X X R16 1095 ± 50

118-126 X X Fining-upward Event Bed R14 1215 ± 90

126-136 X X Fining-upward Event Bed

141-146 X X Fining-upward Event Bed

152-157 X X Fining-upward Event Bed

At least 6 events beds or tephra beds A6 (Mixed Tephra) A7 3660 ± 145 159-178 X X

220-226 X X Doublet fining-upward Event Bed 229-235 X X Fining-upward Event Bed R15 4750 ±110

237-256 X X At least 3 events beds or tephra beds

Fining-upward Event Bed R9 6285 ± 110 A8 Tephra Layer B Glacier Peak (Late 257-260 X X Pleistocene) Lapilli layer intermixed with coarse A9, A10, A11 Tephra sand grain sized tephra and sediment Layer B or G Glacier 260-454 X X Peak (Late Pleistocene) Table 5.1 WLC-04 Geophysical Peaks for detection event beds. 39

5.2.2 Wapato Tephra & XRF Fifteen visible tephra samples were collected from 11 lake sediment core sections, four terrigenous, and two Rogue Apron marine sediment cores and shipped to the Electron Microprobe Laboratory at the University of Alberta, where Dr. Britta J.L. Jensen conducted tephra identification. Cores WLC-01 and -04 XRF elemental data for Lake Wapato are plotted (Figure 5.13 & 5.14) as certain elemental ratios to achieve the following results: 1.) use visible tephra bed(s) as controls for XRF detection of tephra and 2) illustrate where potential cryptotephra may be found for trial tephra detection and identification. Peaks seen in the correlation diagram (Figure 5.12) plots in the Si, K, Ti/Ca, and Sr tend to indicate the presence of silicic tephra. Peaks in the Mn/K, Fe/K, Cu/K, Ti/K and Zr/Rb plots (Figure 5.13) may suggest the presence of mafic tephra (Balascio et al., 2015). The K, Zr, and Ca/K data may illustrate the presence of non-weather derived turbidite sequences (Corella et al., 2014). XRF data, along with peaks in the magnetic susceptibility data plots, may provide insight into the presence of seismic and volcanic event beds. A trial set of samples collected from Wapato WLC-01 at core depths of 87 cm (A12), 180 cm (A13), and 253 cm (A1). These collection sites were selected based upon the XRF elemental ratio data plot peaks.

This study includes a total of nineteen tephra and cryptotephra samples—glass shards of <125 μm (MacKay et al., 2016). The resulting identifications indicate a mix of numerous tephra geochemical profiles—in these samples, separate tephra populations are identified, though this is not always possible. Geochemically identifying tephra or a population within a mixed tephra sample presents a set of unique challenges, including limitations in available tephra geochemical databases. Figure 5.13, tephra identification(s) are notated; if the parent volcano and date of the eruption are established in the literature of if the tephra identifier and date are provided. Tephra geochemical biplots are available below to compare the geochemical profiles of the sample. 40

Core 4 XRF Detection of Silicic and Mafic Tephras

Silicic Detection (Peaks) Table 5.2 WLC-04 WLC-04 Depth (cm) Si K Ti/Ca Sr Ca/K XRF Silicic and Mafic Tephra 15 X X X X Detection & 28-40 X X X X Confirmation. Plot peaks in the Si, K, 85 X X X X X Ti/Ca, and Sr plots 100-110 X X X X X tend to indicate the presence of rhyolitic 148-157 X X X X tephra, peaks in the 180 X X X X X Mn/K, Fe/K, Cu/K, Ti/K, while Zr/Rb 195-200 X X X X plots tend to indicate 212-220 X X X X the presence of mafic tephra. K, Zr, and 230-240 X X X X Ca/K have the Mafic Detection (Peaks) potential to illustrate the presence of non- WLC-04 Depth (cm) Mn/K Fe/K Cu/K Zr/Rb Ti/K weather derived 18-21 X X X X X turbidite sequences. See Figures 5.12 and 47-53 X X X X X 5.13 for matching 88-90 X X X X X core diagrams. 110-113 X X X X X 126-132 X X X X X 145-158 X X X X X 161-167 X X X X X 220-230 X X X X X 248-256 X X X X X 41

Figure 5.12. Core 4 Wapato Lake, Washington sediment Core data. From left to right, CT density and magnetic susceptibility with XRF data plots and correlating peaks in data suggest possible silicic tephra material depths.

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Figure 5.13. Core 4 Wapato Lake, Washington sediment Core data. From left to right, CT density and magnetic susceptibility with XRF data plots and correlating peaks in data suggest possible mafic tephra material depths. 5.2.3 Wapato Tephra Geochemical Biplots Geochemical analyses conducted on the tephra samples collected from Wapato Lake provide the data to produce the following biplots. The majority of Holocene Cascades eruptions produce 43

tephra that is rhyodacitic in composition of (Harris, 2005). Following the practices set in (Foit et al., 2004; Kuehn et al., 2002; 2009; 2013; and Jensen et al., 2016; 2017), samples were compared to representative geochemical biplots and known stratigraphic positions of previously identified tephra. Sample A4 is from 45 cm in WLC-01 and sample A5 (Figure 5.14) is from 38 cm in WLC-04, was compared to late Holocene MSH eruptions and matched the MSH tephra Wn (471 ± 0.5 cal. yrs BP) geochemical signature.

In Figure 5.15, most of the plotted points of samples correspond to the VEI 5 MSH Wn tephra signature and fits into the chronostratigraphy of the WLC-04 (Pallister et al. 2017; Kuehn 2017). In Sample A6 (Figure 5.14), the geochemistry profile is also compared to late Holocene MSH eruptions, but does not match any one individual MSH tephra and is likely a reactivated deposit with multiple MSH populations. Stratigraphically, sample A6 was collected between radiocarbon samples with ages of 1215 ± 90 and well-dated tephra (MSH Yn) dating to 3660 ± 145 cal. yrs BP. This outcome and other tephra samples presented in this mixed deposit suggest previously deposited, distinct tephra deposits were reactivated and redeposited in some process. In the case of sample A7 (Figure 5.15), it was clear the tephra sample is MSH Yn (3660 ± 145 cal. yrs BP). A stratigraphic and radiocarbon sample supports this identification.

Figure 5.14 Geochemical biplot of tephra sample A4, A5, A6, and A7 from WLC-04 compared to published MSH Yn, Ye, Wn, We, and Pu/Py tephra geochemical profile.

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Samples A8, A9, A10, and A11 (Figure 5.15) are all tephra from deposits near the Holocene- Pleistocene boundary. The prevalence of lapilli in the deposits suggests the source volcano is close enough to deliver a large volume of lapilli to Wapato. Glacier Peak is approximately 77 km NW of Wapato Lake, while MSH is located ~245 km to the WSW of the lake and is known to have delivered tephrafall to Wapato. In this case, the lapilli geochemistry suggests the samples in the deeper section of the Wapato lake cores likely came from Glacier Peak.

Figure 5.15 Geochemical biplots illustrates the tephra samples A8, A9, A10, and A11 from WLC- 04 compared with the published reference geochemical profile of Glacier Peak tephras B (GP-B) and G (GP-G). A8 and A11 are good matches for GP-B while A9 and A10 are good matches for GP-G. A tentative GP-M profile is plotted, but there is currently not enough data available to differentiate GP-M from GP-B and GP-G tephra.

5.3 Wapato Core Samples & Descriptions WLC-01 is located just upslope from what appears to be a small slope failure (Figure 5.3). One leaf collected at 13 cm of core depth, dating to a modern 14C age (1950 CE-present). Five additional 14C samples were collected from WLC-01 (Table 5.3). The top of WLC-01 contains less consolidated material with the top 8 cm of the core containing disturbed material.

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Core Core Core C-14 OxCal Model Age (Median) Section Depth Description Raw Age # Sample # cal. yrs BP # (cm) wood WLC1B R12 68.5 fragment 645 ± 50 610 ± 40 wood WLC1C R1 129 fragment 3345 ± 25 3580 ± 40

Core WLC1C R2 152.5 charcoal 3740 ± 15 4100 ± 50 1 WLC1C R13 202.5 charcoal 5240 ± 80 6030 ± 110 Calcium Carbonate gastropod WLC1D R3 250 shells 5400 ± 20 6230 ± 30 wood WLC4A R8 43 fragments 920 ± 20 870 ± 40 Calcium Carbonate gastropod WLC4B R16 114 shells 1165 ± 15 1090 ± 40

Small calcium carbonate bivalve Core WLC4B R14 135.5 shells 1240 ± 15 1210 ± 40 4 Small calcium carbonate bivalve WLC4B R15 206.5 shells 4210 ± 35 4740 ± 70

Small calcium carbonate bivalve WLC4C R9 253 shells 5475 ± 15 6290 ± 20 WLC4D R11 343 wood 7985 ± 25 8880 ± 100 Table 5.3: Wapato Lake cores WLC-01 & WLC-04 C14 Sample Summary.

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WLC-02 (47o 54’47.86” N 120 o 09’24.11” W)

WLC-02 is 74 cm in total length and contains a large amount of disturbed material includes reactivated temporally disparate tephra originating from Mazama, MSH, and Glacier Peak eruptions, large quantities of gastropod shell-laden material (~10-45%), as well as charcoal- and organic-rich materials. Due to the short length of WLC-02 and proximity to the lake bed surface, this core also represents a high amount of disturbance. The compactness of material decreases up-core towards the lake bed surface and is therefore easily disturbed by currents, turbidity, and lacustrine biota. The top 16 cm may represent a slope failure at approximately 130 cal. years BP and is dominated by MSH Wn, dated to 471 ± 0.5 cal. years BP, the overall disturbance in this core, lessens the importance WLC-02 holds in the overall stratigraphic and event bed record for Lake Wapato. Similarly, the 14C sample sites are not of a high quality due to its level of disturbance. Gastropod shells prevalent throughout this core contain peaks in CT and magnetic susceptibility, ranging from 0-16 cm.

Event Type CT Density Magnetic Tephra or Depths (cm) Peaks Susceptibility Peaks Seismoturbidite 0-16 X X Disturbed 24-31 X Disturbed

48-64 X Disturbed

Table 5.4: WLC-02 Geophysical Peaks for detection event beds WLC-03 (47o 54’51.03” N 120 o 09’28.59” W)

WLC-03 is 52 cm and was collected 84 meters WSW of WLC-04. WLC-03 has no identifiable 14C or tephra sample sites, and due to the relatively small length and proximity to the lake bed surface, WLC-03 provides limited stratigraphic and event bed records.

5.4 Wapato Lake Age Model A preliminary OxCal P-Sequence age model was processed (Figure 5.17), encompassing most of the Holocene (~0-10 ka), with samples yielding model ages from 470-8880 cal. yrs BP. The calibrated and modeled ages are provided in the OxCal output table in Appendix 10.2. The model includes 22 beds, with two identified tephra beds (MSH tephra Wn 470 ± 0.5 calibrated 47

by dendrochronology and Mazama 7630 ± 150). Interevent and event bed thicknesses were measured, and since event beds represent zero time, these thicknesses are subtracted to construct an event-free stratigraphy (Ramsey, 2001; Goldfinger et al., 2020). Radiocarbon and tephra samples collected inform the construction of an event-free age model. As listed in the OxCal model output table, the agreement indices value between 60 and 100 are acceptable (Ramsey, 2009)—the majority of these values within the Wapato age model are 90 or above, with only two event beds (WLC18 & WLC19) modeled ages yielding 68 and 70, respectively at the bottom of the core. With the uncertainty interval indicating WLC18 & WLC19 event beds event-free depths, these may be incorrect. WLC18 & WLC19 event beds are approximations located amid the chaotic lapilli deposit and are considered the best estimates given the nature of the data available within that deposit. The overall model index is 101%. OxCal model input is given in Appendix 10.1, with the output in Appendix 10.2 (Goldfinger et al., 2012; 2017; 2020). 48

Figure 5.16 A new Wapato Lake event-free age model with local Wapato event bed numbers.

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5.5 Wapato Lake Intralake Well-log Correlation One of the first steps in assessing potential regional stratigraphic correlations is to evaluate each coring site by comparing cores from the same site to construct an intra-site correlation. In Figure 5.17, the images and data plots are placed next to each other, constructing an unflattened well log correlation reflecting true depth. Due to core break disturbance, some correlation has to be extrapolated. In comparing WLC-01 and -04, general stratigraphic and model age agreement are indicated. Cores 2 and 3 are too short of providing enough data to correlate; however, they may represent the shallowest part of the lake bed lost at the tops of WLC-01 and -04. WLC-04 is used for the correlation between separate lakes because it appears to represent the best event bed record in Wapato Lake. The next step in well-log correlation is to vertically flattened core diagram images and data of WLC-01 and -04 (Figure 5.17). Flattening (Figure 5.18) was achieved in Adobe Illustrator using the mesh tool, with OxCal model ages, data plots, and data images informing flattening decisions. Placing the CT density plot of WLC-04 next to the plot of WLC-01 provides insight into the level of similarity of the well log data between cores. Similar to intra-lake correlation, inter-lake and regional correlations are achieved by constructing unflattened and flattened well-log diagrams. 50

Figure 5.17 A correlation diagram of WLC-01 & WLC-04 in Wapato Lake. Each core has their own vertical depth scale. Due to core break disturbance in WLC-01, WLC-04 is selected as the representative core for the remainder of this manuscript. 51

Figure 5.18 A diagram to illustrate a comparison of Core 1 & Core 4 in Wapato. Each core has a separate vertical depth scale. The pink plot on the right side of the core is the CT plot of WLC-04 placed side-by-side with the CT plot of WLC-01 for comparison.

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5.6 Leland Lake Results The primary resource for data and interpretation of the Leland Lake sediment cores, including bathymetry, geologic setting, geophysical properties, as well as intra-lake, inter-lake, and onshore-offshore stratigraphic correlations are found in the USGS NEHRP report (Goldfinger et al., 2017). The presence of a geochemically confirmed Mazama primary tephra airfall deposit at Leland Lake provides an opportunity for additional inquiry into the Mazama tephra's potential coevality and identified CSZ seismoturbidite deposits within Leland Lake core LLJ-07. In addition, an assessment of well log correlation between Wapato and Leland Lakes is conducted.

Wapato Lake (Chelan County, WA) and Leland Lake (Jefferson County, OR) are situated 205 km apart (Figure 2.2). While local geologic events inform part of each lakebed’s stratigraphy, regional events also impact lake stratigraphies depending on geographic location. Goldfinger et al. (2017) provide the geologic context of Leland Lake (among several other lakes in Washington), including a potential event bed record and regional correlation. Despite the distance between Wapato and Leland Lakes, a comparison in well log data provides an opportunity to determine potential common geophysical and stratigraphic characteristics between the lakes. This well log comparison includes assessing the CT and gamma density plots, magnetic susceptibility plots, the images of halved cores, and OxCal model ages. In this study, the determination of facies typology was used to estimate event bed thicknesses and how they correspond to plot peaks. 53

Figure 5.19 Leland Lake cores LLJ-01 and LLJ-07 with well log data (CT density, gamma density, and magnetic susceptibility plots) along with lithologic logs, radiocarbon ages, and tentative correlation lines marking possible CSZ event beds. "MA" represents the location of Mazama tephra beds. This correlation diagram represents the data that provides the stratigraphic anchor to correlate the offshore CSZ event record with Wapato and Bull Run Lakes in this project through additional well log data for Wapato and Bull Run. From Goldfinger et al., 2017 Figure 4-25. 54

Mazama Airfall in Leland Results

Goldfinger et al., (2017), constructed a regional correlation between the established marine CSZ event bed record and several inland lakes in Washington (Leland and Sawyer) and Oregon (Bull Run). Each lake contained a tephra bed that was geochemically identified as the climactic Mazama airfall deposit. The age model proposed by Goldfinger et al., (2017) for Leland Lake is revisited here and refined in this study by collecting smear slides and conducting point count ratios of tephra, diatoms, and lithics. The goal is to refine the OxCal age model to estimate the time difference between tephra-producing eruptions and CSZ turbidite deposits. Improving the depth control of potential event beds and tephra beds will provide detailed data to inform a refined OxCal age model. An improved age model provides correlative data to refine the intra- lake and regional well-log correlation.

When considering the timing relationships for deposition of the Mazama tephra bed in Leland and CSZ seismoturbidites, Leland Lake sediment core 7 between the depths of 760-800 cm were assessed by analyzing twenty smear slide samples collected at 2 cm intervals from the base of the tephra bed. An upright Leica DM4 P petrographic microscope using a 20x magnification eyepiece (Field-of-view is 1.1 mm with an additional 1.6x conoscopic magnification) to determine constituent ratios of tephra, diatom, and other lithic materials. In Figure 5.20, the percentage of tephra found within the event bed ranges 55-90% with very few diatoms (5-35%); above the tephra bed, the tephra percentage ranges from 10-40% with a large increase in diatoms (25-70%). Below the tephra bed base, the tephra percentage ranges between ~5%, with declining tephra ratios further away from the tephra bed base and an overall increase in diatoms (45-80%). The major peak in tephra ratio hovers in the 90% range from 785-791 cm, which corresponds to the core section containing the geochemically verified Mazama primary airfall tephra bed (Figure 5.21 & 5.22). In Goldfinger et al., 2017, the CSZ T14 event bed base is modeled in an OxCal age model as being located around 764 cm. In that previous age model, the Mazama eruption age was listed at 7700 years BP, while in the new age model, the Zdanowicz et al., 1999 age of 7630, is used. In addition, the new model is based on the fining upward sediment leading away from the top of the Mazama tephra bed—observable in the core images and CT density and gamma density data plots.

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Figure 5.20 Leland Lake, Washington sediment LLJ-07 Section I data. Point Count of Mazama tephra by depth is on the left flank of the core images with CT density, Gamma density, magnetic susceptibility, and select XRF elemental ratios. Appendix 10.4 will provide detailed images of each smear slide.

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Figure 5.21 Smear Slide (Sample 1) from the base of Mazama Tephra bed in Leland Lake Core 7. Large conchoidally fractured tephra glass shards mixed with a minority diatom population.

Figure 5.22 Smear Slide from the middle of Mazama Tephra bed (Sample 2) in Leland Lake Core 7.

Below the tephra bed base, the tephra percentage ranges between ~5%, with declining tephra ratios the further away from the tephra bed base and an overall increase in diatoms (45-80%). XRF data collected for LLJ-07 plots select elemental ratios to determine whether tephra detection is apparent based on the criteria detailed in Balascio et al. (2015).

5.6.1 Leland Age Model An updated Leland Lake OxCal P-Sequence age model is shown in Figure 5.23, encompassing most of the Holocene (~0-10 ka), with samples yielding model ages from 300-6115 cal. yrs BP. The model is comprised of 22 beds, including the identified tephra bed (Mazama 7630 ± 150). 57

Interevent and event bed thicknesses used to produce the model are from Goldfinger et al., 2017. Since event beds represent zero time, these thicknesses were subtracted to construct an event-free stratigraphy (Goldfinger et al., 2012; 2017; 2020). Radiocarbon and tephra samples collected inform the construction of the event free age model. As listed in the OxCal model output table (Appendix 10.2), the agreement indices value between 60 and 100 are acceptable (Ramsey, 2009)—the majority of agreement indices values within the Leland model are 90 or above, with the overall model index of 102.7%. OxCal model input is given in Appendix 10.1, with the output in Appendix 10.2 (Ramsey, 2009). 58

Figure 5.23 Leland Lake LLJ-07 OxCal P-Sequence Age Model. Twenty-two possible Post- Mazama event beds are recorded in LLJ-07, some of which could represent CSZ, crustal, slab earthquakes, or another turbidite-triggering event that produced a >5cm thick event bed. In Leland Lake, the Mazama tephra bed is used as a reference point for the age model.

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5.7 Bull Run Lake Results The data and interpretation of the Bull Run Lake sediment cores, including bathymetry, geologic setting, geophysical properties, as well as intra-lake, inter-lake, and onshore-offshore stratigraphic correlations, can be found in Goldfinger et al., (2020), as seen in Figure 5.24. The Bull Run Lake turbidite record correlates with the offshore record established in Goldfinger et al. (2012, 2017).

Figure 5.24 Bull Run Lake bathymetric map showing the core positions—from figure 4-6 of Goldfinger, et al., 2017. For this project, BRL-08 and -09 are used to provide smear slide and tephra-event bed depositional relationships. 5.7.1 Bull Run Lake Tephra Results Smear slides collected from BRL09 (Figure 5.25, Appendix D1) provide improved depth control of tephra, diatom, and lithics deposits in that core. The goal is to refine the OxCal age model to estimate the time difference between tephra-producing eruptions and CSZ turbidite deposits. As seen in Figure 5.25, BRL09, the tephra point count ratio is in the 80% range and can therefore confirm tephra prevalence is high within the section of the core that tentatively corresponds 60

stratigraphically to the Mount Hood’s Timberline eruption (1470 ± 40cal. yrs BP) and the CSZ T5 event bed with a Bull Run Lake model age of 1550 (1470-1620). Stratigraphic, lithologic, and geophysical data agree that the CSZ T5 event occurred close in time to the deposition of the tephra doublet observed from 8-13 cm in the Bull Run lake cores. The climactic Mazama tephra bed is located at the bottom of the Bull Run Lake sediment cores. Other tephra beds in Bull Run remain ephemeral but observable and need additional analysis, including geochemically identifying the tephra and further constraining tephra bed ages by collecting additional lake cores, radiocarbon data, and tephra samples.

Wapato Lake (Chelan County, WA) and Bull Run Lake (Washington County, OR) are situated 300 km apart. While local geologic events inform part of each lakebed’s stratigraphy, regional events also impact lake stratigraphies depending on geographic location. Goldfinger (2020) provides the geologic context of Bull Run, including a potential event bed record and regional correlation (Figure 5.26). Despite the distance between Wapato and Bull Run Lakes, a comparison in well log data provides an opportunity to determine potential common geophysical and stratigraphic characteristics between the lakes. This well log comparison includes assessing the CT and gamma density plots, magnetic susceptibility plots, images of halved cores, and OxCal model ages.

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Figure 5.25 Bull Run core with geophysical data plots and smear slide point count ratios between tephra, diatoms, and other lithics. Of specific interest is the prevalence of tephra in the same depths as the bright signals in the greyscale and color Osirix CT images that correspond to peaks in the CT density and Magnetic Susceptibility data plots. Appendix 10.4 will provide detailed images of each smear slide. For detailed images of each smear slide, refer to Appendix D1.

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Figure 5.26 A tentative well-log correlation of Bull Run Lake that is the basis from which this section builds upon by improving depth control of tephra, diatom, and lithics deposits to refine the timing of tephra-producing eruptions and CSZ-generated turbidite deposits. From Figure 5-13 of Goldfinger et al., 2020. 63

5.8 Inter-lake Well-log Correlation 5.8.1 Wapato-Bull Run Inter-lake Correlations In Figures 5.27 & 5.28, Wapato WLC-04 appears to possess stratigraphic material, post-dating the top of the BRL composite core data, and pre-dates the bottom of the BRL core. The BRL composite core provides insight into the mid-Holocene event bed record in addition to at least two distinct tephra populations. To properly correlate the two lakes, the BRL core log was stretched vertically for easier viewing and prevented the WLC core from becoming too compact to see distinct features. 64

Figure 5.27 Wapato & Bull Run inter-lake correlation diagram to illustrate a comparison between WLC-04 in Wapato Lake and a BRL-08 & -09 composite core. Each core has their own vertical depth scale. The pink plot on the left side of the BRL core is the CT plot of WLC- 04 placed side-by-side with the CT plot of BRL-08 & -09 composite for stratigraphic and well log comparison. OxCal model ages (blue) for each lake core as well as local event bed numbers (green) are used. See Results chapter Figure 6.34 for the flattened diagram 65

In Figure 5.28, the flattened version of the Wapato-Bull Run correlation is shown for easy comparison of well-log data, including distinct events and tephra beds. In both Figures 5.32 and 5.33, correlation lines have been added in red. The pink WLC-4 CT density plot overlaid with the black BRL composite CT plot into probable correlations. Correlation between lakes appears strong—in particular between WLC10-15 and BRL8-13. WLC3-9 with MSH Yn tephra 3660 (3505-3795) just below WLC9 and BRL1-7 also have corresponding well log data and model ages. The Mazama tephra's tentative location in WLC-04 at ~312 cm corresponds to the Mazama tephra bed in BRL at ~88 cm. Despite the 300 km distance between Wapato and Bull Run lakes, a regional correlation appears probable.

Tephra beds contained in Bull Run (Figures 5.27 and 5.28) include a doublet tephra deposit that may correspond stratigraphically to the Mount Hood Timberline eruption (1470 ± 40 cal. yrs BP) underlined in Figure 5.33 by the BT5 event bed and at the very bottom of Bull Run Lake Core 9GC containing a Mazama tephra fall bed corresponds tentatively to the CSZ T14 turbidite deposit. The BRL core 9 tephra ratio peaks around the doublet while it does not peak in core 8. Core 8 has a single bright signal above the doublet, not reflecting a peak in tephra. Smear slide sampling frequency throughout the core may be blamed for the lack of conclusive tephra ratio data between the two cores. 66

Figure 5.28 The stratigraphy, lithology, geophysical properties, and sample data of Wapato Lake WLC-04 compared with Bull Run (composite of Cores 8 & 9). CT Osirix, Magnetic Susceptibility RGB, Lithologic Column, and CT greyscale images are flanked by CT density and Magnetic Susceptibility plots. Radiocarbon and Tephra samples are marked. Additionally, the pink Wapato CT plot on the right is set next to the black Bull Run CT plot for well log correlation. Each core has a separate vertical depth scale. 67

5.8.2 Wapato-Leland Inter-lake Correlations

Figure 5.29 Leland Lake, LLJ-07 core data well log correlation with Wapato Lake, WLC-04 core data. Local event bed numbers are listed in green for each lake site. Model ages and well log data plots informed the choices in correlation of event beds. 68

Figure 5.30 The stratigraphy, lithology, geophysical properties, and sample data of Wapato Lake WLC-04 compared with Leland Lake Core 7. CT Osirix, Magnetic Susceptibility RGB, Lithologic Column, and CT greyscale images are flanked by CT density and Magnetic Susceptibility plots. Radiocarbon and Tephra samples are marked. Additionally, the pink Wapato CT plot on the right is set next to the black Bull Run CT plot for well log correlation. 69

5.9 Rogue Apron Tephra Results Goldfinger et al., (2012) found the Mazama tephra in all marine sediment cores collected from Rogue Apron. One of the turbidite deposit observable in Rogue Apron core TN0909-1JC is definitively well-log correlated to the CSZ T14 (7630 ± 140 cal. yrs BP) megathrust earthquake (Goldfinger et al., 2012). Volcanic tephra collected from the T14 turbidite deposit in the Rogue Apron marine sediment core has been identified at the Electron Microprobe Laboratory at the University of Alberta as the climactic Mazama tephra (7630 ± 150 cal. years BP). Smear slide samples were collected and analyzed from above, within, and below the Mazama tephra bed base to trace the extent to which down-core Mazama tephra is deposited (Figure 5.33 and Table 5.5).

Figure 5.31 Mount Mazama (Crater Lake), Rogue River, and Rogue Apron sites. Following the 7630 ± 150 cal. yrs BP cataclysmic (VEI 7) Mazama eruption. The Rogue River (marked with a red line) transported large quantities of tephra to the continental shelf, with ocean currents transporting the tephra a distance of ~58 km from the mouth of the Rogue River to the head of the Rogue Apron canyon. 70

Figure 5.32 Marine Sediment Core TN0909-1JC with Gamma density, CT density, and Magnetic Susceptibility data plots flanking the CT and RGB images. The corresponding data peaks indicate the presence of CSZ event beds and the Mazama tephra bed (towards the bottom of the core. Figure 5.35 will provide a zoomed-in view of the core section with the Mazama tephra and CSZ T14 event beds. Modified from Goldfinger et al., 2017, their Figure 4-25.

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Figure 5.33 A close-up view of the core section LLJ-07 section I contains the Mazama tephra and CSZ T14 event beds. CT density, gamma density, and magnetic susceptibility data plots are used to detect fining-upward event beds and tephra beds. The smear slides provided a microscopic view of the point count ratio of tephra versus lithic to ascertain the core depths that contain a majority Mazama tephra. This outcome provides some insight into comparing the timing of the CSZ T14 and Mazama tephra's deposition. Appendix 10.4 will provide detailed views of each smear slide. 72

Table 5.5 Marine Sediment Core TN0909-1JC Section 4 Smear Slide point count ratios of tephra and lithics. Tephrochronological analysis has confirmed the identification of the tephra of cataclysmic Mazama. Tracing the prevalence of this tephra through the established CSZ T14 event bed and below it may inform the timing interval between the Mazama tephra arrival at the head of the submarine canyon and deposition of the T14 CSZ event bed.

The Mazama tephra is undetectable 0.7 cm below the base of the T14 event bed. The Rogue Apron sedimentation rate informs the quantification of time from the deposition of the T14 event bed base and the pre-T14 prevalence of tephra. The first detection of tephra below the T14 event bed base is 545.6 cm—a 2.5 cm difference in core depth. However, the first discernable decline in tephra prevalence is at 543.8 cm—a difference of 0.7 cm. The sedimentation rate at the Rogue Apron is 15 cm every 1000 years (Goldfinger et al., 2012), or 0.1 cm represents ~7 years. It is possible that the presence of Mazama tephra below the base of T14 can be attributed to undetected load features and bioturbation. The depth of the volcanic ash compared to CSZ T14 suggests the earliest Mazama ash arrived in the submarine canyon via the continental shelf 0-50 years, or ± 25 years before the CSZ T14 deposit. However, geochemical comparisons between climactic Mazama and the earlier Llao Rock tephra signature could provide additional stratigraphic context.

5.9.1 Rogue Apron Tephra Geochemical Biplots All samples were collected from within the CSZ T14/Mazama event bed, samples M1, M2, and M3 (Figure 5.34). In M1 (534.6 cm core depth), located within the tail of the CSZ T14 turbidite deposit strongly correlates with Mazama (7630 ± 150 cal. yrs BP), despite some higher SiO2 points. M2 (542.1 cm) located in the tail of the turbidite strongly matches Mazama. However, 73

M3 appears to contain more than one tephra population, including Mazama. M3 is the deeper (543.4 cm) located directly below the base of the CSZ T14 event bed that contains Mazama, Llao Rock, and a third unidentified population.

Figure 5.34 Geochemical biplots of tephra sample M1, M2, and M3 from Rogue Apron Marine Core TN0909-1JC compare the published reference geochemical profiles of the ~7.7 ka climactic Mazama and the ~7.8 ka Llao Rock tephra. Panels A – B show that M1 and M2 are a significant match for Mazama while M3 appears to contain more than one tephra population, including Mazama. M3 is the deeper, and therefore older deposit that contains Mazama, Llao Rock, and another unidentified population.

6.0 Discussion We have identified several regionally extensive event beds in the study's lakes. Lake event beds can relate to local events such as fires, storms, and localized seismic shaking, and it is important to distinguish events caused by these processes from those caused by subduction zone earthquakes. In the following sections, we briefly review the causes and features of local event beds and discuss them within the context of this study. 74

6.1 Origin of Event Beds The geographic location, regional surface processes, and lake morphology dictate how event beds are emplaced. In this study, glacially scoured lakes receiving low fluvial input are best suited for paleoseismic studies. Additionally, several processes are identified that can lead to event bed emplacement within the stratigraphy of the lake. These mechanisms, as outlined in Goldfinger et al. (2020) and are discussed at length below, include 1.) seismic shaking that directly triggers destabilization of lake slope walls, producing turbidite deposits on the lake bottom; 2.) hillslope destabilization from earthquakes, fires, landslides, storms, floods, and other events; 3.) disturbance of lake floor sediments due to wave action; and 4.) disturbance of lake floor sediments by seismic or other processes (Goldfinger et al., 2020).

In addition to seismoturbidites and other types of event beds outlined above, volcanic ash and two types of tephra beds must also be included in this list: 1.) Primary airfall represents a short period in geologic time, depositing geochemically identifiable tephra into the lake and surrounding environs that can be dated through radiocarbon or other means; 2.) Reactivated tephra transported by a myriad of surface processes including fluvial, aeolian, seismic, and storms—these tephra deposits can provide some temporal framework for an event bed through geochemical identification and radiocarbon dating. For example, in core WLC-04 a non-primary tephra deposit is geochemically identified as MSH Wn (471 ± 0.5 cal. years BP) and Mazama (7630 ± 150 cal. years BP) with a radiometric sample age within the same deposit of 320 ± 50 cal. years BP—this suggests both Mazama and MSH Wn tephra were reactivated and emplaced within an event bed dated to 320 ± 50 cal. years BP. If the radiocarbon age was older than MSH Wn, it could be determined that the radiocarbon result is likely contaminated and incorrect. Thus, the identification of tephra within event beds is also an important tool to check the quality of radiocarbon outcomes and stratigraphic interpretations.

Goldfinger et al. (2017 & 2020) note that similar facies are identifiable and correlatable in lakes investigated (Leland and Sawyer Lakes in Washington and Bull Run Lake, Oregon) in the U.S. Pacific NW. This idea leads to the interpretation; there are both localized intralake, and regional processes generating event beds. It is important to look at the scope of possible events that produce event beds to understand the impact these processes have upon lake systems. 75

6.1.1 Fire Lake cores are used widely in climate studies and ecology, to document burn event records in the western US (Long & Whitlock, 2002; Long et al., 2007; Briles et al., 2008, Whitlock et al., 2008, and Walsh et al., 2010). Increased sediment input in lakes producing terrigenous silty-sandy deposits with low organic content relative to the background are attributed to post-fire erosion, (Millspaugh and Whitlock, 1995), meltwater events, (Noren et al., 2002), runoff following severe storms, (e.g., Noren et al. 2002), land clearance, and logging roads (Colombaroli and Gavin, 2010). Wapato Lake is situated in the Cascade Mountains rain shadow, where lower sedimentation rates are typical. Bull Run Lake lies on the north flank of Mount Hood and is subject to alpine runoff from precipitation and glacial melt, translating into a low sedimentation rate—both lakes in this study with low sedimentation rates also record numerous seismic and volcanic events.

Fire-triggered event beds are found to correlate to peaks in magnetic susceptibility and are used as a proxy for increased sediment grain size, along with shifts in vegetation trends such as a drop in vegetative diversity represented in pollen and seed content within core material (Goldfinger et al, 2017). Direct differentiation of fire and earthquake deposits is beyond the scope of this project; however, site observations such as physiography and inter-lake correlations may inform further investigations. Forest fires are seasonally present in the region and can impact the stratigraphic records in lakes adjacent to wildfires. These stratigraphies also may also contain event beds exhibiting classic attributes of turbidites such as fining upward-grain size, load features, and magnetic and density peaks. The deposition of charcoal during proximal fire events into lakes is expected. The similarity in turbidite event beds between the sampled lakes draws attention to the regionality and synchroneity of whatever processes triggered turbidity flows. In order for fire to produce the correlated event beds between the sites we have studies – including offshore locations – would require broadly synchronous fires that produce similar stratigraphies of event beds in lakes located in the Olympic Peninsula (Leland), southwest of Puget Sound (Sawyer), Central Washington (Wapato), and on the north flank of Mount Hood (Bull Run Lake). This seems unlikely.

It is unlikely that widespread, regional fires repeatedly produce event beds in lakes throughout the study area. Alternatively, there is a high potential that large-scale processes, such 76

as earthquakes trigger regional, correlatable event beds. The similarity in ages, timing intervals, and frequency found regionally in the lakes further supports the seismic origins of the event beds in question (Goldfinger et al., 2017 & 2020). Fire-triggered event beds are problematic due to the low probability of similarities in age, synchroneity, and magnitude of large fire events regionally. Similarly, basal erosion and load features exhibited by the majority of the event beds within lakes in this study are not representative of fire-related mechanisms. Therefore, while fires impact sediment supply into a given lake basin, the penultimate mechanism triggering final sediment delivery to the lake floor is consistent with a discrete event such as an earthquake rather than from landscape de-vegetation due to local or regional fires (Goldfinger 2017 & 2020).

6.1.2 Storms Runoff from severe storms can also trigger event beds (Noren et al., 2002). Increased sediment transport into lakes from storm runoff can increase due to post-fire de-vegetation and erosion (Millspaugh and Whitlock, 1995; Noren et al., 2002). Storm induced event beds can contain reactivated diatoms and tephra in abundance (Goldfinger et al., 2017). The extent of damage a diatom exhibit tends to correlate with the amount of transport diatoms undergo—both in distance and intensity of transport (Goldfinger et al., 2017). Therefore, event beds containing diatoms with little to no evidence of transportation, similar to the interevent beds, suggests such event beds are not a product of storm-induced flood input (Goldfinger et al., 2012; 2017; 2020). Storm- related event beds are unlikely to be regionally correlative, and so while radiocarbon dates and resultant age modeling alone do not provide a temporal resolution establishing synchroneity between process types and triggered turbidites, the combination of radiocarbon dates, tephrostratigraphy, well log data correlation, as well as the number, thicknesses, and intervals of the large (>5cm) event beds tentatively correlating with other inland lake and offshore paleoseismic studies (Goldfinger et al., 2012; 2017; 2020), support the interpretation these event beds are not products of storm-induced fluvial runoff. Despite the distances between Wapato, Bull Run, and Leland lakes and the offshore coring site at the Mid-Juan de Fuca submarine canyon, the well log data appears to correlate significantly. Region-wide storms in the study areas could produce similar storm-triggered lake bed depositional events and are interpreted by Goldfinger et., al (2012; 2017; 2020) to be part of the overall well log background noise present in lakes. 77

6.1.3 Crustal Earthquakes Site selection of lake sediment coring for this study avoided large drainage basins that would, in turn, have higher sedimentation rates and be susceptible to a myriad of surface processes. Additionally, lakes with adjacent steep slopes along lake margins and lake basins were not sampled due to the high susceptibility to non-seismic slope failure. A study at Crescent Lake located in the northern Olympic Peninsula (near Leland Lake) suggests several proximal crustal faults have triggered megaturbidites and seiching (Nelson et al.., 2007; Wegmann et al., 2014; Joyner, 2016; Goldfinger et al., 2017).

Shallow, crustal earthquakes can generate seismoturbidites in nearby lakes as well as subaerial landslides and rockfalls. The Seattle and Tacoma Faults are the closest crustal faults to

Leland and Sawyer. The Chelan Fault, suspected of a 7.2 Mw earthquake in 1872 CE, is the closest fault to Wapato Lake. Bull Run Lake is near the Portland Fault to the West, Blue Ridge Fault Zone and Gate Creek Fault to the North, and the Twin Lakes and Multorpor Mountain Faults to the South (Brocher et al., 2017). Since each lake in this study is relatively close to crustal faults, it is highly probable that each lake includes a record of crustal earthquakes. Local crustal earthquakes will produce a larger magnitude of shaking proximal to a rupture, so large crustal earthquakes emanating from the Chelan Fault could produce a relatively thick event bed within Wapato Lake as compared to a larger magnitude CSZ earthquake occurring over 300 km away. This local seismicity presents noise to the paleoseismic record.

6.1.4 Slab Earthquakes Slab earthquakes occur at a greater depth than crustal earthquakes and trend a certain distance from the convergence zone, dependent on the subducting slab's dip and obliquity (Cheng et al., 2017). These earthquakes also tend to occur at certain depths along the subducting slab at the

Moho (Cheng et al., 2017). The 6.8 Mw 2001 CE Nisqually earthquake is a recent example of a slab earthquake (Ichinose et al., 2004). It is, therefore conceivable that lake turbidites could be triggered by ground motion generated by slab earthquakes. Slab, CSZ, and crustal earthquakes can look similar in sediment cores, depending on the magnitude of the earthquake and the distance a sample is taken from the foci. The key to differentiating event beds from different types of earthquakes is to sample lakes from coast to inland and ascertain how the correlatable event beds change further inland. The thickness of the event bed indicates the amount of shaking 78

at a sample site. If the thickness of the event bed decreases further inland, then it is likely either a Slab or CSZ earthquake event bed. Uncorrelatable event beds at inland lakes likely indicate a local crustal earthquake. Slab and CSZ earthquakes can be difficult to differentiate since the foci tend to be relatively close, with the CSZ rupture zone situated to the west of the Olympic Peninsula and the Slab foci located within the Puget Sound area. Since the magnitude of CSZ earthquakes tend to be greater than Slab earthquakes, it is reasonable to tie the thickest regional event beds with CSZ earthquakes and smaller regional event beds with slab earthquakes.

6.1.5 Plate Boundary Earthquakes Building on the Adams (1985, 1990), Goldfinger et al. (2012; 2017; 2020) proposes a paleoseismic compilation in which several offshore submarine turbidite systems with separate sedimentary sources contain thirteen post-Mazama turbidites in the Cascadia and Juan de Fuca Channels offshore from Washington. All other marine coring sites south of the Astoria Fan off the northern coast of Oregon contain more than thirteen post-Mazama turbidities. All post- Mazama CSZ events are found in sediment cores collected from the Rogue Apron offshore southern Oregon. These offshore turbidite chronologies exhibit agreement along the coasts of Washington and Oregon and onshore paleoseismic sites (Atwater, 1987, 1990, 1992, Atwater el at., 1997; Priest et al., 2017). While recurrence intervals and ages vary slightly between sites, the well-log correlation provides strong evidence for a comprehensive regional paleoseismic record in the U.S. Pacific Northwest, particularly for the Cascadia Subduction Zone megathrust earthquakes along the JdF-NACP plate boundary.

The regional agreement is observed when comparing the Leland-Sawyer-Wapato-Bull Run lake records to the offshore-onshore paleoseismic record. The coastal and offshore records contain fourteen post-Mazama events. The cataclysmic Mazama eruption occurred 7630 ± 150 (Zdanowicz et al., 1999) and is found in the deepest lake cores at Leland, Sawyer, and Bull Run, while trace tephra is identified geochemically at Wapato. Inherent in the lake record is the diminishing seismic disturbance observed in the turbidite record further from plate boundary ruptures. For instance, Leland's corresponding turbidite will have a greater thickness than its equivalent turbidites in Sawyer, Wapato, and Bull Run lakes situated further inland and away from the rupture zones. Alternatively, as with slab earthquakes, larger turbidites could be produced in lakes closest to a given crustal earthquake's focal point. Leland and Sawyer's cores 79

contain eighteen post-Mazama events compared to the fourteen in the offshore record—the additional four event beds in Leland and Sawyer can be attributed to either large slab or crustal earthquakes close to those lakes. Corresponding event beds diminishing in thickness the further inland a lake suggest strong evidence that these event beds are associated with plate boundary earthquakes. Wapato Lake, in central Washington, Chelan County is the furthest inland lake considered in this study (~340 km from the Olympic Peninsula coast) contains eleven event beds above the top of a reactivated lapilli slope failure sequence. The event beds detailed in the results section as ≥ 5 are interpreted as likely CSZ-generated. Mazama tephra has been identified within the lapilli deposit, and there is a bright planar CT density signature within the lapilli deposit appears to agree well with T14 and Mazama event beds in Leland, Sawyer, and Bull Run.

6.2 Wapato Lake & Cascadia Marine OxCal Age Model Comparison The Wapato event bed stratigraphy is detailed in Figure 6.1 with WLC-04 representing the coring location. A color-coded stratigraphic model in Figure 6.1 provides a summary of the core sections are categorized as regional event or non-regional event beds. In the construction of this age model, background sediment as well as possible event beds with < 5 cm in thickness were categorized as non-regional event deposits. The remaining event beds were categorized as regional event beds and their thicknesses subtracted from the core stratigraphy to produce an event-free model. The reason to cull regional event beds of lesser thicknesses is to filter the noise of smaller beds that may have been triggered by fire, storm, slab, or crustal earthquakes that otherwise complicate well-log correlation over long distances. 80

Figure 6.1 Wapato event thickness diagram. Red blocks indicate the sections of the sediment core considered event beds. Blue blocks indicate the core sections that are background hemiplegic deposits, while the green blocks are possible event beds that are <5cm in thickness and therefore considered background deposition. This provides the event- free depths to build an OxCal age model.

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A preliminary OxCal P-Sequence age model was processed (Figure 6.2), encompassing most of the Holocene (~0-10 ka), with samples yielding model ages from 470-8880 cal. yrs BP. The calibrated and modeled ages are provided in the OxCal output table in Appendix 10.2. The model includes 22 beds, with two identified tephra beds (MSH tephra Wn 470 ± 0.5 calibrated by dendrochronology and Mazama 7630 ± 150). Interevent and event bed thicknesses were measured, and since event beds represent zero time, these thicknesses are subtracted to construct an event-free stratigraphy (Ramsey, 2001; Goldfinger et al., 2020). Radiocarbon and tephra samples collected inform the construction of an event-free age model. As listed in the OxCal model output table, the agreement indices value between 60 and 100 are acceptable (Ramsey, 2009)—the majority of these values within the Wapato age model are 90 or above, with only two event beds (WLC18 & WLC19) modeled ages yielding 68 and 70, respectively at the bottom of the core. With the uncertainty interval indicating WLC18 & WLC19 event beds event-free depths, these may be incorrect. WLC18 & WLC19 event beds are approximations located amid the chaotic lapilli deposit and are considered the best estimates given the nature of the data available within that deposit. The overall model index is 101%. OxCal model input is given in Appendix 10.1, with the output in Appendix 10.2 (Goldfinger et al., 2012; 2017; 2020).

When the Wapato event bed stratigraphy is compared to the CSZ age model time series in Figure 6.2, there is considerable overlap in model ages between Wapato and marine cores. The Cascadia marine age series is provided by Goldfinger et al. (2012; 2017) in which multiple marine coring sites are evaluated and propose a best fit age model for CSZ earthquakes during the Holocene. Of the fourteen Cascadia events thought to be full length ruptures in the past ~ 7600 years, all of these event beds overlap with Wapato event bed ages. 82

Figure 6.2. WLC-04 Wapato Lake OxCal P-Sequence Age Model. Compared to the CSZ time series, the Wapato event bed stratigraphy overlaps in model ages Wapato and marine cores. The Cascadia age series is provided by Goldfinger et al. (2012; 2017) in which multiple Wapato and marine sites are evaluated that proposes a best fit age model for CSZ earthquakes. 83

6.3 Bull Run Lake Discussion Local events and the geographic locations of individual sample lakes dictate some depositional variation. For instance, Bull Run is on the northwestern flank of Mount Hood—a Holocene- active volcano not known for ejecting large volumes of tephra airfall regionally, but does deposit tephra on its own slopes (reference). Bull Run Lake is situated southeast of MSH—therefore, depending wind direction during a given MSH eruption, tephrafall deposits may or may not reach Bull Run lake. As seen during the May 1980 CE MSH eruption, tephra was observed in nearby Portland (~140 km NW from Bull Run lake).

Figure 6.3 A photo of the Timberline tephra doublet, in profile, located on the southern flank of Mount Hood at Elk Meadows (45.3260° N., 121.6980° W.; UTM 10T 602038E, 5019767N; altitude 1695 m.). Photo from Figure 65b USGS Mount Hood Field- Trip Guide (Scott & Gardner, 2017).

Of particular interest in assessing the proximity of volcanic eruptions to CSZ events. Bull Run contains an event bed record from CSZ T1 through T14; however, a T5 event bed 1554 (1384- 1571 years BP) is missing at the stratigraphic location expected in Bull Run cores. A tephra 84

doublet at an approximate depth of 14-19 cm in BRL-09 is found instead of the T5 event bed. This tephra doublet is not geochemically identified, but it is most likely the Mount Hood Timberline eruption 1470 (1320-1620 years BP) an eruption that is known to have produced a doublet tephra deposit (Figure 6.3). The sigma-2 uncertainty of ages for CSZ T5 and the Mount Hood Timberline eruption overlap significantly and may reflect coeval events. The slightly older median age of the CSZ T5 event could mean the younger Timberline eruption completely eroded through the T5 event bed, or the two events occurred so near in time as to be indistinguishable in the sediment core record.

Bull Run core with geophysical data plots and smear slide point count ratios (Figure 6.4) between tephra, diatoms, and other lithics. Of specific interest is the prevalence of tephra in the same depths as the bright signals in the greyscale and color Osirix CT images that correspond to peaks in the CT density and Magnetic Susceptibility data plots. The tephra point count ratio is in the 80% range and can therefore confirm that tephra prevalence is high within the section of the core that stratigraphically corresponds, tentatively to the Mount Hood’s Timberline eruption (1470 ± 37 cal. yrs BP) and the CSZ T5 event bed with a Bull Run Lake model age of 1550 (1470-1620). Stratigraphic, lithologic, and geophysical data agree that the CSZ T5 event occurred close in time to the deposition of the tephra doublet observed from 8-13 cm in the Bull Run lake cores. Figure (6.5) was informed by the outcome of this smear slide analysis as well as age model comparison conducted between Wapato and Bull Run coring sites.

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ges of each smear slide. For detailed images of each smear slide, refer to Appendix each D1. of smear slide, refer ges smear slide. Foreach images of detailed Bull Run core withplotsdiatoms, Bull and Run core tephra, and datacount smear slide point between geophysical other ratios Of specific interest is the prevalence of tephra in the same depths as the bright signals in the greyscale and color Osirix Figure 6.4 lithics. inCT toand imagespeaks that Magneticdata theSusceptibility plots.10.4 will CT density correspond Appendix provide ima detailed

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6.3.1 Bull Run-Wapato Inter-lake Correlation

Figure 6.5 Wapato & Bull Run inter-lake correlation diagram illustrates a comparison between WLC-04 in Wapato Lake and a BRL-08 & -09 composite core. Each core has its vertical depth scale. The pink plot on the left side of the BRL core is the CT plot of WLC-04 placed side-by-side with the CT plot of BRL-08 & -09 composite for stratigraphic and well log comparison. OxCal model ages (blue) for each lake core and local event bed numbers (green) are used. See Results chapter Figure 6.34 for the flattened diagram. 87

6.3.2 Bull Run Lake OxCal Overall agreement between age models and event bed records in Bull Run Lake and the marine age series is evident, just as in the Wapato Lake-marine age series comparison (Figure 6.6). In comparing age models, a regional picture begins to become evident, with event beds of similar stratigraphic position and thickness providing reference points to provide further support to the hypothesis that inland lakes are good recorders of CSZ earthquakes and correlate well with the offshore paleoseismic record.

Figure 6.6 Bull Run Lake event- free age model compared to the CSZ Marine age model, From Figure 4-16 of Goldfinger, 2020. Local Bull Run event numbers are used and tentatively correlated to marine record CSZ event beds.

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6.4 Leland Lake Discussion 6.4.1 Leland Lake Age Model In Figure 6.7, the Leland Lake age model is compared with the marine CSZ event time plot. The Leland Lake core record appears to correlate with overlapping marine record PDFs significantly (Goldfinger et al., 2017). The new Leland age model (Figure 6.7) contains some refinement in event bed thicknesses in core section I, where both T14 and Mazama tephra event beds occur. The marine time series in Figure 6.7 goes back as far as CSZ T13, which correlates with Leland local event number LT20.

Of particular focus in the LLJ-07 core is the relative stratigraphic positions between Mazama airfall tephra and CSZ T14 event bed located within core section I. The Mazama tephra bed is located 786-790 cm with what appears to be load casting features leading vertically away from the base of the tephra bed to approximately 793 cm. The tephra fall's dissipating tail extends upward from the top of the tephra bed to around 783 cm. As seen in Figure 6.26, the point count ratio for tephra is located at 783-793. The Mazama tephra at LLJ-07 is geochemically identified as Mazama. A peak in the lithics point count in Figure 6.26 at 788 cm, extending up to 785 cm. This lithic peak could represent the base of an event bed, perhaps the base of CSZ T14.

The previous placement of the T14 event bed was modeled by Goldfinger et al. (2017) to 764 cm; however, due to new stratigraphic evidence presented above, the T14 event bed may be adjusted to ~785 cm. The event bed that Goldfinger formerly called T14 is now modeled to be ~7400 yrs BP—there is no CSZ event at the time, and likely represents another non-CSZ triggering event at 7400 yrs BP. The most likely type of triggering events include slab or crustal earthquakes, as well as other localized surficial events. To explore this hypothesis, the new OxCal P-Sequence age model produced considers that the T14 event bed and Mazama tephra bed are nearly indistinguishable. In the local event nomenclature, LT22 appears to overlap in the model with the lower boundary Mazama. LT21 may be the bright laminae found at 772 or 777 cm. These laminae could be seismoturbidites, tephra beds, or mixed tephra-laden event beds triggered by crustal or slab earthquakes. Further stratigraphic and tephra investigation is needed at Leland lake to improve the understanding of the sediment core record. As discussed in section 6.6, smear slide point counts in Leland Lake suggest the base of CSZ T14 eroded into the tail of 89

the Mazama tephra airfall and consistent with Rogue Apron. In Figure 6.26, the percentage of tephra found within the event bed ranges 55-90% with very few diatoms (5-35%); above the tephra bed, the tephra percentage ranges from 10-40% with a large increase in diatoms (25-70%). Below the tephra bed base, the tephra percentage ranges between ~5%, with declining tephra ratios further away from the tephra bed base and an overall increase in diatoms (45-80%). The major peak in tephra ratio hovers in the 90% range from 785-791 cm, which corresponds to the core section containing the geochemically verified Mazama primary airfall tephra bed (Figure 6.27 & 6.28). In Goldfinger et al. (2017), the CSZ T14 event bed base is modeled in an OxCal age model as being located around 764 cm. In that previous age model, the Mazama eruption age was listed at 7700 years BP, while in the new age model, the Zdanowicz et al., 1999 age of 7630, is used. In addition, the new model is based on the fining upward sediment leading away from the top of the Mazama tephra bed—observable in the core images and CT density and gamma density data plots. n suggesting deposition of the Mazama tephra bed and T14 event bed occurred at or near the same time. 90

Figure 6.7 Leland Lake, Washington sediment LLJ-07 OxCal age model compared with the Cascadia marine age model series. There are 22 event beds in Leland lake—18 of these event bed model ages overlap with the marine CSZ event model ages.

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6.5 Regional Well-log Correlations 6.5.1 Inland Inter-lake Correlations The Bull Run BRL-08 & -09 composite (Figure 6.8) includes event and tephra beds appear to correlate well with the marine stratigraphic and OxCal age models ranging from CSZ T3 through T14 and including tephra beds corresponding with Mount Hood’s Timberline eruption and the climactic Mazama eruption (Goldfinger et al., 2020). Additionally, this Bull Run composite core well log and age model appear to agree significantly with Wapato and Leland lake well log and model age data. Each coring site is recording localized and regional events. Localized events may be observable in one to two proximal sites, while all the lakes record regional events. The Mazama tephra fall is found in all lakes except for Wapato due to a large slope failure ~6200 yrs BP dominated by reactivated late-Pleistocene Glacier Peak lapilli interfered with Wapato lake’s regional record from the ~6200 yrs BP slope failure to the end of the ~13500 yrs BP Glacier Peak lapilli-producing eruption. It is likely Mazama tephra is intermixed within the reactivated lapilli deposits and is tentatively placed at WLC19 (Figure 6.6), directly above the CSZ T14- correlating WLC18 event bed.

6.5.1.1 Wapato-Bull Run Correlation The main correlative anchors between Wapato and Bull Run Lakes are the event beds and tephra beds that were closely timed to likely CSZ turbidite deposits. The well-log correlation started the 800 yrs BP (670-900) CSZ T3 event bed. The 1540 yrs BP (1280-1895) CSZ T5 event bed in Wapato correlates with the base of the bottom tephra doublet, tentatively identified as Mount Hood Timberline tephra 1470 yrs BP (1430-1510) found in Bull Run. The 3490 yrs BP (2970- 3985) CSZ T8 event bed in Wapato correlates with an event bed that is found in Bull Run dated to 3760 yrs BP (3590-3930). The T8 event bed in Wapato is also closely timed to the MSH Yn tephra dated to 3660 yrs BP (3505-3795). CSZ T9 4110 yrs BP (3715-4525) in Wapato appears to correlate with the Bull Run event bed dated to 4380 yrs BP (4220-4540). The CSZ T10, T10b, T10c, T10f may be represented in both Wapato and Bull Run cores with several event beds appearing closely spaced in the stratigraphy at similar intervals and relative depth locations in both coring sites. 92

Figure 6.8 Wapato & Bull Run inter-lake correlation diagram to illustrate a comparison between WLC-04 in Wapato Lake and a BRL-08 & -09 composite core. Each core has their own vertical depth scale. The pink plot on the left side of the BRL core is the CT plot of WLC- 04 placed side-by-side with the CT plot of BRL-08 & -09 composite for stratigraphic and well log comparison. OxCal model ages (blue) for each lake core as well as local event bed numbers (green) are used. See Results chapter Figure 6.23 for the flattened diagram Due to the lapilli deposit in the bottom of the Wapato cores, precise well-log correlation is more difficult. It is possible that the greyscale CT-image shows evidence of a climactic Mazama tephra band at ~312 cm and may correlate with the positively identified Mazama tephra bed in the Bull Run coring site.

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6.5.1.2 Wapato-Leland Correlation The main correlative anchors between Wapato and Leland Lakes are the event beds and tephra beds that were closely timed to likely CSZ turbidite deposits (Figure 6.9). The well-log correlation started the 470 yrs BP (420-530) CSZ T2 event bed in Leland to the base of the MSH Wn tephra bed dated to 480 yrs BP (285-665). The CSZ T3 event bed in Leland dated to 790 (790-890) correlates with the event bed that dates to 800 yrs BP (670-900). The 2510 yrs BP (2430-2580) CSZ T6 event bed in Leland correlates with the 2575 yrs BP (2080-3100) event bed in Wapato. The 4210 yrs BP (4090-4330) CSZ T9 event bed in Leland correlates with the 4150 yrs BP (3715-4525) event bed in Wapato. 94

Figure 6.9 Leland Lake, LLJ-07 core data well log correlation with Wapato Lake, WLC-04 core data. Local event bed numbers are listed in green for each lake site. Model ages and well log data plots informed the choices in the correlation of event beds. 95

6.5.2 Offshore-Onshore Correlation Geophysical wiggle traces and overall downcore patterns provide clear regional well log correlation. Event size and thickness are observable and tentatively correlatable between Bull Run, Wapato, and Leland Lakes, despite the distances between project sites (Goldfinger et al., 2012; 2017; 2020). The addition of radiocarbon age modeling and tephrochronology outcomes provides further support for the regional event bed correlations.

Additional evidence is provided by the well-defined marine CSZ event record published in Goldfinger et al. (2012; 2017; 2020). The Mid-Juan de Fuca marine core site downcore patterns and wiggle plot traces are similar to the downcore patterns observed in cores from inland lake sites (Figure 6.10). The same number of events pre-Mazama are observed in each site, except in the case of Wapato’s large lapilli deposit from T11 onward. The similarities in bed thicknesses, magnetic and density signals (as a proxy for grain size) observed between sites are significant. The overall number of events is similar, while model ages are compatible, with only minor differences between the onshore and offshore facies 1 and 2 sequences. The thicker event bed and tephra airfall facies are, the most useful stratigraphic markers that provide reference anchors to construct well-log correlations between lakes and offshore coring sites. The Mid-Juan de Fuca core CSZ T3 event bed dated to 796 yrs BP (687-913) correlates with the Leland Lake event bed that dates to 790 yrs BP (790-890), which, in turn correlates with the CSZ T3 event beds in Wapato and Bull Run. The Mid-Juan de Fuca core CSZ T6 event bed dated to 2536 yrs BP (2399-2683) correlates with the Leland Lake event bed that dates to 2510 yrs BP (2430- 2580), which, in turn correlates with the CSZ T6 event beds in Wapato and Bull Run. The Mid- Juan de Fuca core CSZ T11 event bed dated to 5959 yrs BP (5818-6094) correlates with the Leland Lake event bed that dates to 6080 yrs BP (5950-6190), which, in turn correlates with the CSZ T11 event beds in Wapato dated to 6180 yrs BP (5925-6295) and Bull Run 6090 yrs BP (5990-6190). The CSZ T14 event bed in the Mid-Juan de Fuca core is timed closely with the climactic Mazama tephra airfall deposit—the same close stratigraphic relationship is observed in Leland and Bull Run cores. 96

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6.6 Rogue Apron Discussion As presented in section 6.9, the earliest arrival of Mazama tephra at the coring site at the base of the Rogue Apron ranges from 0-50 years before the CSZ T14 deposit. The prevalence of tephra tapers off, along with the fining-upward grain size from the top of the Mazama tephra bed, with the ratio starting to drop off at approximately 528 cm where the ratio of lithics begins to climb. As seen in Figure 6.35, tephra sample M1 (534.6 cm core depth), located within the tail of the CSZ T14 turbidite deposit strongly geochemically correlates with Mazama (7630 ± 150 cal. yrs

BP), despite some higher SiO2 points. M2 (542.1 cm) located in the tail of the turbidite strongly matches Mazama. However, M3 appears to contain more than one tephra population, including Mazama. M3 is the deeper (543.4 cm) located directly below the base of the CSZ T14 event bed that contains Mazama, Llao Rock (7850 yrs. BP), and a third unidentified population. This outcome suggests that the Mazama tephra bed and CSZ T14 event bed are located in the upper tail of the older Llao Rock tephra deposit.

These depositional attributes lead to the interpretation that there is a relatively quick succession of distinct, identifiable beds that consist of a Llao Rock tephra bed (7850 yrs. BP) with minimal climactic Mazama tephra (7630 ± 150 cal. yrs BP) present in the uppermost tail of the Llao Rock deposit, followed by the CSZ T14 event bed (7625 ± 140 cal. yrs BP). The Rogue Apron sedimentation rate informs the quantification of time between the deposition of the CSZ T14 event bed base and the pre-T14 prevalence of climactic Mazama tephra. The sedimentation rate at the Rogue Apron is 15 cm every 1000 years (Goldfinger et al., 2012), or 0.1 cm represents ~7 years. The oldest core material where climactic Mazama tephra is observed is at 543.8 cm—a difference of 0.7 cm. The depth of the volcanic ash compared to CSZ T14 suggests the earliest Mazama ash arrived in the submarine canyon via the continental shelf 0-50 years before the CSZ T14 deposit. Potential triggering linkages between Mazama and CSZ T14 will be discussed in section 6.7. Determining an approximate minimum and maximum rate of tephra transportation is useful in looking at timing linkages between Mazama and CSZ T14.

6.6.1 Rogue River Tephra Transportation It is important to refine the Mazama eruption's empirical dating because the regionally ubiquitous Mazama tephra layer is widely used as a stratigraphic marker bed in numerous disciples, including archaeology and geology. The scientific community's best resolution for the 98

climactic Mazama eruption is based on Greenland Ice Sheet ice cores (Zdanowicz et al., 1999), dated to 7630 ± 150 cal. years BP. The general decrease in tephra prevalence and fining-upward grain size pattern within the Mazama tephra bed could indicate a waning airfall deposit or could represent decreasing tephra as the amassed tephra at the head of the submarine canyon was depleted. No extant data supports the supposition Mazama airfall reached the continental shelf adjacent to the Rogue Apron. The tephra's morphology within Rogue Apron suggests Mazama tephra was transported by fluvial and debris flow and not airfall. This observation leads to an analysis of possible river transport rates for the Rogue River, which empties onto the continental shelf and feeds sediment into the Rogue Apron submarine canyon.

Figure 6.11: Rogue River & Tephra Transport to Continental Shelf.

The rudimentary calculation chosen to compute the Rogue River's transport rate is = , where 𝑸𝑸 Rogue River values are first used with observed Rogue River historical (1861 and 1964𝑽𝑽 𝑨𝑨CE) flash flood conditions. A comparison of the observations made by Healy et al. (1987) concerning the arrival of the MSH 1980 volcanic debris at the mouth of the Columbia River. While there are numerous river flow rate formulae available, it is apparent no single formula surpasses others amongst river flow experts in the literature and appears to be an ongoing source of contention amongst those in that discipline (Major, 2004). For first-order calculations in this study, the 99

= formula is applied to the arrival time of Mazama eruptive material to the continental shelf 𝑸𝑸 𝑽𝑽via the𝑨𝑨 Rogue River.

The overall distance of the Rogue River is 346 km, with its headwaters located ~17 km north of Crater Lake (Mazama), Oregon, and a fluid discharge of between 187.5 m3/s and 8200 m3/s traveling in a southwestward direction. This estimate of the transport rate of Mazama tephra is based on the observed Rogue River channel from historical and modern records (USGS Peak Gage Height, Dec. 23 1964)—the assumption being while some minor channels may change in course, length, and gradient since Mazama’s eruption 7630 years BP ± 150, such changes would not meaningfully impact this first-order estimate. With this stated, however, it is likely that if a large flooding or lahar event occurred, that the effective area of the river channel would have expanded beyond currently held flood stage boundaries. Another assumption is the Rogue River discharge rate would be impacted by the rapid glacial melt from Mount Mazama's peak, thus increasing the volume of water flowing through the watershed and creating a conservative estimated discharge rate of 8200 m3/s (USGS Peak Gage Height, Dec. 23 1964). There would be rapid flushing of the river system of proximal tephra via a combination of lahar and flash flooding. Measuring a cross-section perpendicular to the Rogue River channel provides an estimated area of the river channel is 325 m2.

Applying the Flow Velocity Equation to the Mazama/Rogue River system, where V is Velocity, Q is the Flow Rate, A is the Area of the river channel, and T is Time, the following

/ equation holds: =  V=  25.231 m/s  90.8 km/hr = ~4 hrs 𝟐𝟐 . / 𝑸𝑸 𝟖𝟖𝟖𝟖𝟖𝟖𝟖𝟖 𝒎𝒎 𝐬𝐬 𝟑𝟑𝟑𝟑𝟑𝟑 𝐤𝐤𝐤𝐤 𝟐𝟐 𝑽𝑽 𝑨𝑨 𝟑𝟑𝟑𝟑𝟑𝟑 𝒎𝒎 𝑻𝑻 𝟗𝟗𝟗𝟗 𝟖𝟖 𝐤𝐤𝐤𝐤 𝐡𝐡𝐡𝐡 This represents the estimated potential maximum flow rate conditions and provides the earliest arrival of Mazama tephra to the coast at approximately 4 hours. A minimum flow rate is needed to provide the latest that Mazama tephra could have arrived at the coast. In comparing these findings with Healy’s study (1987), the accumulation of tephra from the 1980 MSH eruption to the continental shelf along the coast of Washington, with observations made five months after the eruption, indicating an observable accumulation of MSH tephra and markedly less observable tephra in succeeding months. This outcome was due to river-borne, tephra-laden material dispersing along the continental shelf by storms. The T14 turbidite generating CSZ megathrust earthquake is dated to 7630 years BP ± 140. 100

Comparison to the 1980 MSH Toutle River Lahar Observed Data

The major differences between Healy (1987) and this project are that the 1980 MSH eruption is a VEI 5, compared to the VEI 7 Mazama eruption, and distances between the river systems and flow rates are substantially different.

, =  = , V= 1.847 m/s = 6.65 km/hr . / = 52 days 𝟐𝟐𝟐𝟐 𝟕𝟕𝟕𝟕𝟕𝟕 𝒎𝒎 𝟑𝟑𝟑𝟑𝟑𝟑 𝐤𝐤𝐤𝐤 𝑽𝑽 𝟏𝟏𝟏𝟏 𝟎𝟎𝟎𝟎𝟎𝟎 𝒔𝒔 𝑻𝑻 𝟔𝟔 𝟔𝟔𝟔𝟔 𝐤𝐤𝐤𝐤 𝐡𝐡𝐡𝐡 The first arrival of volcanic material to the mouth of the Rogue River should match the 1980 Mount Saint Helens ash deposition rate at the mouth of the Columbia River. In this scenario, the estimated arrival time of Mazama tephra at the mouth of the Rogue River was ~52 days. The above-outlined transport calculations along the Rogue river suggest that tephra could have arrived at the river's mouth within 4 hours after the Mazama eruption. T14 is estimated as a magnitude Mw 9.01 (Goldfinger et al., 2012), considered one of the smaller CSZ Holocene events and may be regarded as potentially having less erosive capability but remains uncertain in the stratigraphic analyses.

The Healy (1987) observations provide an approximate maximum tephra river transport time of 5 months. The Rogue River has a much lower flow rate and channel length when compared to the Toutle-Columbia river system; however, magnitude differences between 1980 MSH and climactic Mazama as well as the likely flash-melting of Mount Mazama glaciers could negate the differences in the river systems to some degree. The morphology of the Rogue Apron tephra includes partially-rounded and broken shards, thus supporting the hypothesis that the tephra was likely transported by river and debris flow. Based upon the above calculations, the T14 deposit material comprises ~30% Mazama tephra in the marine sediment cores collected in Rogue Apron submarine canyons and suggests the Mazama eruption pre-dates the T14 CSZ event by a matter of days or weeks. This would be the minimum amount of time it would take for tephra to be transported down the Rogue River and deposited on the continental shelf at a volume dictating the majority of the subsequent T14 deposit is composed of Mazama tephra. It is estimated here that Mount Mazama’s earliest tephra arrival at the mouth of the Rogue River was ~ 4 hours. This interpretation assumes a large volume lahar with a flow velocity matching the 1980 Toutle River MSH lahar, and the volume of the Rogue River channel substantially increased due to the rapid glacial melt of glaciers capping Mount Mazama at the time of 101

eruption. In this interpretation, the lahar could have flowed all the way to the coast or the lahar could have traveled part of the way, injecting itself into flash flood waters that flowed the remainder of the channel length to the coast.

Assuming that the tephra was river transported, how much of the combined tephra bed and fining-upward, lithics-rich event bed is turbidite versus a tephra event bed? Does the lithics peak represent the beginning of the turbidite, or does the tephra bed base represent the CSZ event deposit composed mostly of Mazama tephra? These are questions that can be speculated based upon the results presented here.

A double peak of tephra is observed at the bottom of the tephra ratio plot, with one peak seen below the visible tephra bed base and the other just above the base. This doublet could represent two pulses of the same Mazama tephra, a pulse of an earlier pre-climactic Mazama eruption, bioturbation, or load casting features. The most likely answer is the peak below the base of the tephra bed is due to load casting features that can be observed as tendrils leading away from the base of the tephra bed, downward. These tendrils' morphology does not appear to be the same as the bioturbated core material seen from ~525-510 cm in the same core. An earlier tephra deposit would likely present as a relatively horizontal lamina. The question of whether the lithics peak or tephra bed base represent the T14 event bed suggests the two events are so closely timed within the stratigraphic data that the two events appear to be coeval with Mazama occurring shortly before the CSZ T14 event with Mazama erupting as little as hours to days before the CSZ T14 event. The exact transport time of tephra from the mouth of the Rogue to the head of the Rogue submarine canyon is unknown, but comparing this transport time to the Healy (1987) observations provide insight into a likely outer time range between the Mazama eruption and the CSZ T14 event to within hours or day to up to approximately five months. One additional possibility is that a hyperpycnal flow of mostly Mazama tephra extended out to sea, settling to the Rogue Apron without a turbidity current. It could be argued that the sediment core data could support a mixture of these processes is possible.

6.7 Tectono-Volcano Triggering This research project's key question is whether it is possible or likely there is a direct timing correlation between tectonic megathrust earthquakes and increased volcanic activity. Underlying causative mechanisms aside, there is a strong record of this potential timing relationship. As seen 102

in Table 6.1, global examples of volcanic activity present themselves as new megathrust earthquakes occur: examples include, the 1707 eruption of Mount Fuji 49 days after the Mw 8.6 Hoei megathrust earthquake (Chesley et al., 2012); the Cordon Caulle volcano 1 day after the

1960 Chilean Mw 9.5 earthquake (Lara et al., 2004), with Planchon-Peteroa and Tupungatito erupting in July, 1960 and 8 months after Calbucu and Villarrica volcanoes erupted (Walter &

Amelung, 2007); the November 4, 1952 Mw 9.0 Kamchatka earthquake with the Karpinski volcano erupting 2 days later, Tao-Rusyr Caldera erupted 7 days later, and Maly Semichik volcano erupted within the month—with Sarychev Peak and Tolbachik volcanoes erupting in 1954, and in 1955 Bezymianny volcano erupted (Walter, 2007); the 1964 Alaska following the

Mw 9.2 earthquake (Miller et al., 1998); and the Mw 8.7 Sumatra-Andaman megathrust earthquakes followed by the 2005 eruptions of Talang, Egon, and Barren Island volcanoes

(Walter & Amelung, 2007); the 8.8 Mw February 27, 2010 Maule Chile earthquake followed by the June 4, 2010 eruption of the Puyehue-Cordon Caulle volcano (Swanson et al., 2016); following the March 11, 2011 Mw 9.0 Tohoku, Japan (JAXA Space Technology Directorate)

Sakurajima volcano erupted on October 1-3, 2011; the 7.5 Mw Sulawesi, Indonesia earthquake on September 28, 2018 which was followed five days later by the eruption of Mount Soputan in North Sulawesi, Indonesia (Kim Hjelmgaard, 2018). All these examples had volcanoes erupt explosively following large magnitude seismic events. Additional timing linkages between megathrust earthquakes and volcanic eruptions are also identified in Kamchatka, Alaska, and Central America (Walter et al., 2009).

Previous work has been conducted to explore possible triggering relationships in subduction systems, but the Cascadia Subduction Zone in the North American Pacific Northwest is missing this perspective. This work explores the potential linkages between CSZ megathrust earthquakes and CVA eruptions. There are many potential hypotheses to choose from underlying triggering mechanisms, and none are mutually exclusive. Tectonic and volcanic systems are complex, but when considering these two systems as a whole, the combined system is multifaceted. It should not be limited by myopic research questions ignoring other possible mechanisms.

The temporal and spatial scales impacted and dictating behaviors of a combined tectono- volcanic system at any given subduction zone is reliant upon the rate of convergence, angle, and 103

length of the subduction zone and how much the subduction zone ruptures in a given megathrust event (Cheng, et al., 2017). The geographic position of a volcano from the rupture zone and the preexisting state of the volcano's magma chamber before and following a megathrust earthquake is equally as important. One hypothesis posed by Bebbington et al. (2011) is whether a megathrust earthquake can rapidly advance the eruptive clock of a volcano by strain transference from the tectonic convergence boundary to the continental crust. Compression and decompression are both possible depending on where the volcanic plumbing is located within (or away from) the tectono-fault system. Such a process could place critical strain on volcanic magma plumbing systems. Processes associated with magmatic volume and pressure, such as relaxation of crustal tensions, could release new magma into a near-surface magma chamber, inducing an eruption.

Table 6.1. Global Historically Observed Megathrust Earthquakes followed by Volcano Eruptions. 104

Further paleoseismic analysis is needed to determine possible triggering relationships at sites of other known large magnitude (VEI 6-7) volcanic eruptions near subduction zones such as the 3560 yrs BP VEI 7 Mount Thera (Santorini, Greece); the 1600 CE VEI 6 Huaynaputina (Peru) eruption; the 1815 CE Mount Tambora (Indonesia) eruption; the 1883 CE VEI 6 Krakatoa (Sumatra) eruption; the 1912 CE VEI 6 Novarupta (Alaska) eruption, 1991 CE VEI 6 Mount Pinatubo (Philippines) eruption.

Watt et al. (2009) have investigated the relationship between large earthquakes and volcanic eruptions by using historical records from the Southern Volcanic Zone of the Andes (SVZ) and the earthquake history of the adjacent Peru-Chile Trench subduction zone. Historic events increase temporal data points beyond relying upon instrumental seismology from the past hundred years. Watt et al.'s (2009) study looks at historical earthquake and eruption data, not just instrumentally measured events. This is a step beyond what Walter and Amelung (2007) achieve in reviewing 400 years of historical earthquake records. Nevertheless, records going back ~400 years are still not a long enough time to provide a data set big enough to make statistically significant observations of volcanic eruptive behaviors. Large clusters of eruptions occur 0-1.5 years are noted after large-earthquakes (Walter & Amelung, 2007). Walter and Amelung posit internal variables at any volcanic system, in addition to external variables, such as the time since previous large earthquakes are driving factors in whether a volcano is predisposed to triggering by tectonic megathrust earthquakes. Watt et al. (2009) suggest a variable magnitude of responses to great earthquakes, resulting in volcanoes 'primed' for eruption are highly variable. The SVZ megathrust earthquake record shows a remarkably well-defined cyclicity with main ruptures generally propagating southwards from epicenters and stepping southward along the subduction zone in a temporally clustered pattern. Adjacently locked tectonic segments are seismically loaded, while explosive eruptions at long quiescent volcanoes suggest individual cases of potentially earthquake-triggered eruptions (Watt et al., 2009).

Bebbington and Marzocchi (2011) explore the potential correlations between megathrust earthquakes and volcanic eruptions from eruption and earthquake catalogs in the Banda and Sunda arcs from 1900-2010 suggests the possibility of delayed triggering of eruptions days to months after a megathrust earthquake. Of the 35 volcanoes with at least three eruptions in their study region, seven (Marapi, Talang, Krakatau, Slamet, Ebulobo, Lewotobi, and Ruang) reflect 105

the statistical potential of volcanic triggering across both temporal and spatial scales. According to Bebbington and Marzocchi (2011), the volcanic system's internal state and internal magma plumbing appear to determine eruptive factors. Given that active volcanic systems undergo their unique eruptive cycles, eruptions triggered by earthquakes may appear indistinguishable from eruptions that are not triggered. The timing and synchroneity of megathrust earthquakes and volcanic eruptions hinge primarily on whether the magma source, plumbing system, and magma chamber are predisposed to erupt. A longer than expected delay in eruption triggering may suggest coupling via deeper tectonics and/or a nonlinear volcanic response to stress transference (Marzocchi and Piersanti, 2002; Bebbington and Marzocchi, 2011).

Another study of the Andean Southern Volcanic Zone (SVZ) by Bonali et al. (2012) posits that static stress changes are capable of triggering observed volcanic phenomena up to a distance of 350 km from the epicenter, and 11 out of the 18 new volcanic events occurred at volcanoes with shallow magma chambers (2-3 km) under conditions of unclamping or very weak clamping. New activity in volcanoes with deeper magma chambers (>7 km) occurred only by magma pathway unclamping. Considering the regional tectonics of the SVZ, Tupungatito erupted 183 days after the 1906 Valparaiso earthquake, while Villarrica erupted 262 days later. These results suggest that magma pathway unclamping plays a fundamental role in increased activity in volcanoes that already possess a magma source and primed chamber. Other research involving tectonic stress transference from megathrust earthquakes having triggered volcanic eruptions include Mexico (De la Cruz-Reyna et al., 2010); Japan (Ozawa et al., 2016); and Kamchatka, Russia (Walter, 2007).

Figures 6.12-6.18 are visual comparisons between the CSZ and CVA Holocene records, and certain patterns become noticeable. In Figure 6.12, the CSZ record displays four distinct clusters of megathrust earthquakes. Meanwhile, the CVA eruptions indicate three potential clusters of arc-wide volcanic activity. The oldest cluster of eruptions has been dated between ~12000-8800 BP, with the next cluster runs between ~8800-5800 BP. The third cluster runs between ~5800-3000 BP, a fourth cluster running between 3000-1200 BP, with the present cluster 1200-present. It is an interesting coincidence that both systems appear to have undergone four clusters over the past 10000-12000 years. The graphs (see figures 6.12-6.18) provide possible triggering relationship(s) between CSZ and CVA. 106

Figure 6.12 provides a first comparison of documented the Holocene event record of CSZ 8+ Mw earthquakes and VEI 0-7 volcanic eruptions in the CVA illustrating the frequency of tectonic and volcanic events in the region. The noisy signal in the volcanic record necessitates comparison by magnitude of volcanic eruptions as well as volcanic zonation into South, Central, and North. The following figures provide these comparisons. The x-axis represents years BP with present day located at the far right. The y-axis represents magnitude (unitless proxy for magnitude of CSZ earthquakes and VEI for volcanic eruptions). This figure is intended to provide a big picture view of the timing of Holocene, arc-wide timing. Figures 6.13-6.18 will offer magnified views of each section of the arc. Modified from Figure 4 in Goldfinger et al., 2013.

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The Southern CVA (Figure 6.13) includes Lassen Peak, Mount Shasta, Medicine Lake, and Mount Mazama. Notable volcanic eruptions that occurred close in time to a CSZ event include a VEI 4 Lassen Peak eruption with overlapping model ages of 284 (184-384) yrs BP with the CSZ T1 265 (139-371) yrs BP. A VEI 4 Mount Shasta eruptions 2500 (2250-2750) and 6000 (5750- 6250) have overlapping model ages with CSZ T6 2536 (2514-2558) and T11 5959 (5848-6070). The VEI 7 Mazama eruption 7630 (7477-7777) may have occurred within a year before the CSZ T14 event 7630 (7487-7763). In addition to this, there is a cluster of 5 (VEI 2-5) Medicine Lake eruptions and one VEI 4 Lassen Peak eruption that occurs between CSZ T3 and T4. No other clusters appear in the published record of the southernmost volcanoes during the Holocene. It is possible that caldera ring faults triggered by T4 set off a cluster of Medicine Lake eruptions or possibly a change in magma system pressure occurred due to tectonic-crustal strain transference.

While Mount Shasta was quiet during this period, it did erupt twice at VEI 4's in the 1000 years before T4 and again 1000 after T4. Mount Shasta eruption timing is closely aligned with four of eighteen CSZ events and Medicine Lake three CSZ events. 108

Figure 6.13: Holocene CSZ & Southern CVA Event Timing. Modified from Figure 4 in Goldfinger et al., 2013.

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The Central Cascades included in Figure 6.14 are Mount Mazama, Newberry Crater, and South Sister. Mazama is once again included since it could easily fit into either Southern or Central CVA. The defining characteristic of these volcanoes is that they are situated in a volcanism zone that may be the product of both the subduction of the CSZ and crustal extension. Newberry Crater has a cluster of three eruptions (VEI 4’s) between 1214-1480 yrs BP occurring between T5 1554 (1522-1586) and T4 1243 (1201-1285) and includes a single VEI 2 South Sister eruption 1510 (1310-1710).

Newberry Crater eruptions also coincide in time with CSZ T8 3443 (3375-3511), T10 4770 (4719-4821), T12 6466 (6364-6568), and T13 7182 (7138-7226). South Sister also has eruptions, both VEI 2—2740 (2670-2810) and 2560 (2360-2760) occurring close in time CSZ T6 2536 (2514-2558) and T7 3028 (2967-3089). Of the Central CVA, Newberry Crater may show the most timing linkage with CSZ 8+ Mw earthquakes, occurring near in time five of the eighteen CSZ events. 110

Figure 6.14: Holocene CSZ & Central CVA Event Timing. Modified from Figure 4 in Goldfinger et al., 2013.

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The Northern CVA volcanoes include Mounts Hood, Saint Helens, Adams, Rainier, Glacier Peak, and Mount Baker. Much of the subsequent discussion will focus on this volcanic zone since all lake coring sites are located within this northern region. Figure 6.15 displays the published Holocene Northern CVA and CSZ records for comparison. In subsequent figures, detailed looks at the region by VEI magnitude and temporal data will elucidate potential timing linkages between the CSZ and Northern CVA systems. In general, looking at Figure 6.15 frequency of eruptive activity at each volcano is observable. MSH awakens from a quiescent period in the first half of the Holocene, with eruptions resuming around 4.2 ka. Mount Rainier is quiet in the first 2.5 ka of the Holocene but resumed activity ~7.5 ka. Mount Hood has been quiet throughout most of the Holocene, only becoming intermittently active ~1.5 ka. There does appear to be a gap in volcanic activity from ~9.5-7.6 ka, in general, particularly following CSZ T16 8906 (8844-8968).

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Figure 6.15: Holocene CSZ & Northern CVA Event Timing. Modified from Figure 4 in Goldfinger et al., 2013.

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Figure 6.16 displays VEI 3-7 in the Northern CVA. It appears a cluster of eruptions from ~7-5 ka is observable involving six Mount Adams eruptions, four Mount Rainier eruptions, two Mount Baker, and one Glacier Peak eruption. The gap in the region's eruptive activity is visible between ~9.5-7.6 ka, with only one VEI 3 Mount Baker eruption occurring at 8675 (8500-8850). Volcanoes with eruption timing linkages to the CSZ record include Mount Baker erupting at or near in time to four of eighteen CSZ events; Glacier Peak with five of eighteen CSZ events; six of eighteen CSZ events; Mount Adams has a cluster of eruptions that are near in time to CSZ T11 and T12; at this VEI magnitude MSH may be too noisy to make linkage statements; and Mount Hood has two of eighteen CSZ events. 114

Figure 6.16: Holocene CSZ & Northern CVA Event Timing. Modified from Figure 4 in Goldfinger et al., 2013.

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In Figure 6.17, only VEI 4-7 show a Mount Adams eruption cluster around CSZ T11 and T12. Glacier Peak has potential timing linkages at 6045 (5905-6185), a VEI 4 eruption producing tephra D with CSZ T11 at 5959 (5848-6070); Glacier Peak underwent a VEI 4 eruption at 1662 (1512-1812) paired with T5 1554 (1522-1586). While MSH appears relatively noisy at VEI magnitude 3-7, likely timing linkages are observable at T2 481 (398-564)—MSH tephra Wn 471/2 yrs BP; T5 1554 (1522-1586)—MSH tephra Bu 1690 (1530-1830), and T8 3443 (3375- 3511) with MSH tephra Yn 3660 (3515-3805).

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Figure 6.17: Holocene CSZ & Northern CVA Event Timing. Modified from Figure 4 in Goldfinger et al., 2013.

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At the 5-7 VEI magnitude range displayed in Figure 6.18, a possible pattern in timing emerges with possible linkages with Mount Adams, MSH, and Mount Hood. All Glacier Peak’s VEI 5+ activity occurred in the late Pleistocene, beyond the temporal scope of the extant CSZ record. Of particular interest are the number of MSH eruptions and the single Mount Hood eruption loosely timed to CSZ events. MSH tephras Ye 3520 (3420-3620) and Yn 3660 (3515-3805) erupted near in time to T8 3443 (3375-3511). Additionally, MSH tephra Bu 1690 (1530-1830) and Mount Hood’s Timberline tephra 1470 (1320-1620) erupted around the same time as CSZ T5 1554 (1522-1586). A slope failure, or series of slope failures that was large enough to block the flow of the Columbia River—known as Bridge of the Gods Landslide 512 (500-524), occurred near in time to MSH We 468 (466-470), MSH Wn 471, and CSZ T2 481 (398-564). 118

Figure 6.18: Holocene CSZ & Northern CVA Event Timing VEI 5-7. Modified from Figure 4 in Goldfinger et al., 2013.

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6.8 Evidence for Triggering Linkages in Cascadia According to the literature outlined above and applied in this study indicate two types of triggering—a direct, immediate reaction of a volcano hours to days after a megathrust earthquake and mid-to-long-term timing—depending on what the state of a particular volcano is in, the megathrust earthquake could excite the gases within the magma chamber, causing turn-over and a subsequent eruption. The earthquake could trigger an edifice collapse—this may not be immediate; then an eruption. One of the ideas expressed in the literature is the theory of an earthquake placing a finger on the minute arm of the eruption clock for some volcanoes. How quickly an eruption occurs in response to a megathrust earthquake is based on how far away a volcano is from the earthquake rupture zone, each volcano's status within its eruption cycle, and the status of an individual volcano's magma chamber and plumbing system. With this idea, even neighboring volcanoes could react differently to the same earthquake since they are likely all on slightly different phases of their eruption cycle. Despite this, the same earthquake may have advanced each neighboring volcano's eruption clock.

This research includes an in-depth review of published dates of CVA Holocene volcanic eruptions while working dates of CSZ megathrust earthquakes are found in Goldfinger (2012 and 2017). Figures 6.12-6.18 are tentative 10000-year plots and illustrate the timing and Volcanic Explosivity Index (VEI) of CVA eruptions within the Southern, Central, and Northern Cascade Arc, as reported within published, peer-reviewed databases, compared with the timing of CSZ megathrust earthquakes (Goldfinger, 2012; 2017). These figures illustrate published eruptive histories of Holocene active CVA volcano eruptions compared to CSZ events to show potential overlapping events. This research narrows timeframes better to ascertain the coevality of volcanic and megathrust events. Please reference Appendix C for the Holocene CSZ and CVA data table to produce Figures 6.12-6.18. In comparing the timing of Mount Mazama eruptions and CSZ events, the 7630 ± 150 BP cataclysmic Mazama eruption (VEI 7) appears to be coeval with the T14 CSZ (7630 ± 140 BP) event notated by Goldfinger (2012; 2017). An earlier Llao Rock eruption (7850-7950 years BP) does not appear to have a temporally proximal CSZ event, but was found in our Rogue Apron tephra analysis.

At Lake Wapato, numerous tephra and cryptotephra samples were collected and analyzed, yielding definitive identifications of the MSH Wn (471 ± 0.5 cal. years BP); MSH Yn 120

(3660 ± 145 cal. years BP); Glacier Peak layer B (11250 ± 150 cal. years BP); and Glacier Peak layer G (13560 ± 150 cal. years BP). Many of the tephra samples yield a mixed geochemical profile consistent with redistributed tephra deposits and thus reflecting a movement of the lakebed and terrigenous material that, in some cases, correspond to established inland lake- marine core CSZ seismoturbidite record. The geophysical data, including sediment core CT and RBG data, identify fining upward grain size, rip-up clasts, and load-casting bodies that determine which sections of the cores likely record seismoturbidites.

The geophysical, stratigraphic, radiocarbon, and tephrochronological evidence suggests identification at Wapato Lake as CSZ T1 (150-280 cal. years BP) through T11 5960 (5820-6090 cal. years BP) as well as CSZ T2 481 (389-578) 471 ± 0.5 cal. years BP (MSH Wn). Additionally, 14C ages collected directly above the large slope failure deposit in Wapato Lake contains a mixed population tephra consisting of climactic Mazama (7630 ± 150 cal. years BP), lapilli from GP-B (11250 ± 150 cal. years BP). Glacier Peak layer G (13560 ± 150 cal. years BP) provides further age constraint for the local and regional stratigraphic records. The radiocarbon samples collected (6230 ± 31 R3 in WLC-01 and 6287 ± 18 in WLC-04 R9) directly above the upper boundary of this slope failure deposit suggest the slope failure occurred around 6300 cal. years BP which is near in time to CSZ T11 5960 (5820-6090 cal. years BP). The Wapato OxCal model confirms that the centimeters directly above the upper boundary of the lapilli deposit have a model age of 6180 (5930-6300). Finally, the active Chelan crustal fault near Wapato Lake makes it possible for Wapato lake paleoseismic data to record crustal ruptures. However, the recurrence and timing of Chelan fault ruptures are poorly understood. The historical 1872 CE

Entiat earthquake (7.2 Mw) could have triggered a slope failure on the Wapato lakebed; however, due to the recent age and core top disturbance, no definitive or circumstantial evidence appears to be recorded in the extant Wapato sediment cores.

7.0 Conclusion The primary question this research examines is whether it is possible to find timing linkages within the Cascadia Subduction Zone-Cascades Volcanic Arc system where volcanic eruptions having timing coincident with CSZ megathrust earthquakes. While many eruptions worldwide are not linked to observed earthquake activity, there is evidence of potential triggering relationships in geologic records and historic and modern observable events. Numerous 121

examples of volcanic activity occurring near in time to megathrust earthquakes are recorded globally, with the U.S. Pacific Northwest having similar potential for coeval tectonic and volcanic events. Tephra deposits within lake sediment cores offer additional insight into regional volcanoes' eruptive histories, while lacustrine turbidite sequences provide higher temporal resolution of subduction megathrust events (Goldfinger et al., 2012; 2017; 2017a). This study illustrates that lake and marine sediment cores provide evidence of timing linkages between megathrust earthquakes and volcanic events. Outcomes in this study include stratigraphic signal within sediment cores of turbidite and tephra deposits and closely timed radiocarbon OxCal model ages of CSZ and CVA events providing potential timing linkages between CSZ and CVA events. The hypothesis of this study is that some CSZ megathrust earthquakes are coincident in timing with eruptions in CVA volcanoes. To attempt to test this hypothesis, regional well-log correlations developed in this study provide evidence for potential triggering relationships between seismoturbidites attributed to the CSZ and volcanic tephra deposits analyzed in lake and marine sediment cores collected in the U.S. Pacific Northwest. Existing literature (outlined in sections 3.4 and 6.7) suggest two types of triggering—a direct, immediate reaction of a volcano hours to days after a megathrust earthquake and mid-to- long-term timing—depending on what the state of a particular volcano is in, the megathrust earthquake could induce bubble nucleation in oversaturated magma within the magma chamber, causing turn-over and a subsequent eruption. The earthquake could also trigger an edifice collapse, this may not be immediate, followed by an eruption due to depressurization of the magmatic system. One of the ideas expressed in the literature (Walter, 2007) is the theory of an earthquake placing a finger on the minute arm of the eruption clock for some volcanoes. How quickly an eruption occurs in response to a megathrust earthquake, if at all, is based on how far away a volcano is from the earthquake rupture zone, each volcano's status within its eruption cycle, and the status of an individual volcano's magmatic system. With this idea, even neighboring volcanoes could react differently to the same earthquake since they are likely all on slightly different phases of their eruption cycle. Despite this, the same earthquake may have advanced each neighboring volcano's eruption clock. The criteria in linking megathrust earthquakes in this study include temporal proximity of calibrated radiometric and tephra sample results and stratigraphic proximity of CSZ 122

seismoturbidite and identified primary tephra deposits. CSZ and volcanic events with similar timing results from calibrated radiometric data provide context and may support stratigraphic relationships between event deposits, although uncertainties with radiometric ages make this less conclusive than direct stratigraphic observations. The regional synchroneity of CSZ event beds and certain volcanic deposits offer significant timing linkage of deposition of seismoturbidites and tephra beds. The possible linkage between CSZ and volcanic deposits are interpreted where both radiometric and well-log results appear to agree. Where tephra beds are observed in the sediment core where a CSZ-generated event bed should occur, it is surmised that probable stratigraphic correlation is likely. It is also possible that a tephra could obscure the transition to hemipelagic sedimentation. Additionally, close stratigraphic deposition between tephra and CSZ event beds make it probable stratigraphic correlation is likely. Stratigraphic proximity is dependent on the sedimentation rate at an individual coring site, but at most sites discussed in this study, tephra deposits within 0-1.5 cm of a seismogenic event bed base appears reasonable (with sedimentation rates estimated as ranging from 0.1-0.25 cm/year); tephra found within the event bed and the event bed tail appears reasonable to interpret close depositional timing. This study refines the timing of U.S. Pacific Northwest seismic and volcanic events and evaluates whether the processes are temporally proximal to one another. Volcanic eruptive histories are increasingly detailed through geochemical analysis and regional well-log correlation of tephra found within sediment cores detailed in this study. Calibrated radiocarbon dating of organic material sampled stratigraphically adjacent to tephra and turbidite deposits offer geologists improved numerical ages with less than a century error. Radiocarbon AMS dates such as those provided in this study include uncertainty ranges notated as 1σ and 2σ. 1σ and 2σ are the resultant uncertainty from measuring radiocarbon ratios and represents a measurement of uncertainty with 2σ representing ~95% confidence that a particular sample is within a given age range and 1σ representing ~68% confidence the sample is within a given age range. Radiometric age 2σ uncertainty tends to be reported in the literature, though it provides a less precise age of a sample than the 1σ uncertainty (Bowman, 1990; Southon et al., 2005). In this study, OxCal is a radiocarbon calibration software that considers multiple external age constraints, including sedimentation rates and multiple 14C ages, by applying Bayesian statistics to consider overlapping probabilities (Goldfinger et al., 2012; 123

Ramsey, 1995, 2001). Where radiocarbon dates are missing due to lack of suitable material or a gap in sampling, sedimentation rate alone can inform OxCal of event dates (Goldfinger et al., 2012; Ramsey, 1995, 2001). Within OxCal, P-sequence age modeling is employed in which event-free depth values are input along with the radiocarbon ages. By subtracting observed event bed thicknesses from the full core stratigraphy, event-free depth values are achieved (Goldfinger et al., 2012, 2017, and 2020). Matching coeval deposits between cores provides further detail to the correlation of geophysical plot peaks and troughs and between greyscale and RGB images of the cores (Goldfinger et al., 2020). Although radiocarbon ages are important constraints, it is noted that this study's findings are not solely dependent on radiocarbon ages for linking stratigraphic beds and these ages provide temporal context to support the important outcomes from regional well-log correlations, geochemical identification of tephra, and analysis of stratigraphic proximity of CSZ event beds and identified tephra beds. This project's standard well-log practice produced regional stratigraphic correlations between Lakes Wapato, Bull Run, and Leland. The CSZ seismoturbidites previously identified by Goldfinger (2017) at Bull Run and Leland lakes, to the Mid-Juan de Fuca marine cores, are informative in identifying event beds in Lake Wapato in concert with this study's new stratigraphic, geophysical, tephrochronological, and radiocarbon analyses. In addition to these findings, several unidentifiable tephra deposits were collected and will help inform future tephrochronology investigations. While the historical and modern era offer data that may support triggering relationships between tectonic boundary megathrust earthquakes and volcanic eruptions (Table 6.1), application of paleoseismic and tephrochronological methods to this research question provides a 10000-year dataset to analyze potential timing linkages between CSZ and CVA deposits. In the four sampled coring sites assessed in this study, all sites appear to have well-log agreement between CSZ event beds and volcanic tephra beds (Table 7.1).

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Coring CSZ Site T# CSZ date (yrs. BP) Volcano Tephra Tephra Date (yrs. BP) Wapato T2 481 (389-578) MSH Wn 471 (470-472) Wapato T8 3443 (3290-3599) MSH Yn 3660 (3515-3805) Leland T14 7625 (7487-7763) Mazama Mazama 7625 (7475-7775) unidentified Mount Bull Run T3 796 (687-913) Hood? unidentified unidentified Mount Bull Run T5 1554 (1374-1724) Hood Timberline 1470 (1433-1507) Bull Run T8 3443 (3290-3599) MSH Yn 3660 (3515-3805) Bull Run T14 7625 (7487-7763) Mazama Mazama 7625 (7475-7775) Rogue Apron T14 7626 (7487-7763) Mazama Mazama 7625 (7475-7775) Rogue pre- Apron T14 7850 (7750-7950) Llao Rock Llao Rock 7850 (7750-7950) Table 7.1 Summary by coring site of well-log correlated CSZ event beds and volcanic tephra deposits. In Wapato Lake cores, well-log correlation is strongest for CSZ T2 481 (389-578) and MSH tephra Wn 471 (470-472). In this case, the tephra bed base appears where the CSZ T2 event bed is expected stratigraphically based on regional well-log correlation. The modeled, calibrated radiocarbon ages overlap, in terms of uncertainties. Also, in Wapato Lake, the CSZ T8 event bed 3443 (3290-3599) and MSH tephra Yn 3660 (3515-3805) appear within the same sediment core depth with the well-log data supporting a near-synchronous timing of events. In Leland Lake, this study's focus was refining the stratigraphic proximity of the CSZ T14 event bed 7625 (7487-7763) and the Mazama tephra bed 7625 (7475-7775). The results of sedimentological analyses in the Leland core show a 6 cm thick bed, 90% comprised of Mazama tephra with an abrupt fining-upward of lithics observed less than 1 cm above the base of the tephra bed. This fining-upward in grain size may reflect a fining of tephra shard size over time or a seismoturbidite. The regional well-log correlation suggests that this tephra bed is at the same stratigraphic position the CSZ T14 event bed would be expected while stratigraphic proximity of the tephra bed base and the possible base of the T14 event bed strongly suggest close timing of the Mazama eruption and CSZ T14 event. Additional stratigraphic agreement between Mazama and CSZ T14 appears evident in Bull Run Lake and Rogue Apron cores. The top of Bull Run core BRL-08 contains an unidentified tephra bed that may be the Old Maid tephra (~169 years BP) but appears to correlate in the regional well log data to CSZ T3 796 (687-913) alternatively. At Bull Run Lake, the tephra doublet's deposition from core depth 8-13 125

cm is tentatively identified as the Mount Hood Timberline eruption (1470 ± 37 cal. yrs BP). The tephra point count ratio is 80%, and can therefore confirm that tephra prevalence is high. According to the regional well-log data, the base of the tephra doublet corresponds to where CSZ T5 event bed 1550 (1470-1620) is expected. Stratigraphic, lithologic, and geophysical data agree that the CSZ T5 event occurred close in time to the deposition of the tephra doublet observed from 8-13 cm in the Bull Run lake cores. As with Wapato Lake, the CSZ T8 event bed 3443 (3290-3599) and MSH tephra Yn 3660 (3515-3805) appear within the same sediment core depth. The well-log agreement between the Wapato and Bull Run for CSZ T8 and MSH Yn supports a possible timing linkage despite an offset in modeled, calibrated radiocarbon median ages of ~220 years. The regional well-log correlation suggests that the Mazama tephra bed in Bull Run cores is at the same stratigraphic position the CSZ T14 event bed is expected, with stratigraphic proximity of the base of the tephra bed and the possible base of the T14 event bed strongly suggest close timing of the Mazama eruption and CSZ T14 event. Additional stratigraphic agreement between Mazama and CSZ T14 appears evident in Leland Lake and Rogue Apron cores. At Rogue Apron, a first-order estimate of the transport rate of the climactic Mazama 7630 ± 150 cal. years BP) tephra via the Rogue River to the Rogue Apron submarine canyon head, further providing an approximate transport rate of Mazama tephra ranging 4 hours to weeks from the time of the Mazama eruption to the arrival of the Mazama tephra at the head of the Rogue Apron submarine canyon. The CSZ T14 event bed deposit comprises 80-90% Mazama tephra, suggesting the Mazama eruption occurred before the T14 megathrust earthquake. Mazama tephra, positively geochemically identified as such in this study, is traced to the millimeter below the base of the CSZ T14 (7630 ± 140 cal. years BP) event bed. Given the established sedimentation rate of the TN0909-1JC Rogue Apron marine sediment core, the extent of the present Mazama tephra below the CSZ T14 event bed represents 50 years of deposition. It suggests the Mazama eruption occurred 0 and 50 years before the CSZ T14 megathrust earthquake. This research includes an in-depth review of published dates of CVA Holocene volcanic eruptions located throughout the literature. Working dates of CSZ megathrust earthquakes are referenced from Goldfinger et al.'s work (notably 2012, 2017, and 2020). Figures 6.11-6.18 of this manuscript are tentative 10000-year plots which illustrate the timing and Volcanic 126

Explosivity Index (VEI) of CVA eruptions within the Southern, Central, and Northern Cascade Arc, as reported within published, peer-reviewed databases compared with temporal markers for CSZ megathrust earthquakes (Goldfinger et al., 2012; 2017). These studies illustrate current and up to date literature concerning the eruptive histories of Holocene active CVA volcanoes compared to CSZ. While this study's focus is to explore the possible timing linkages observable in the well-log data between CSZ megathrust earthquakes and CVA eruptions in the Holocene, the overall record in the region is comprised of volcanic eruptions that do not appear to have been triggered by CSZ earthquakes. As seen in Table 7.1, out of the fours coring sites, we observed at least nine potential well-log pairings between the CSZ and CVA while in Tables 7.2- 7.4, there are possibly three timing linkages in the Southern CVA; ten possible timing linkages in the Central CVA; and seven possible timing linkages in the Northern CVA.

CSZ T# CSZ date (yrs. BP) Volcano Eruption Date (yrs. BP)

T1 265 (254-276) Lassen Peak 284 (184-384)

T11 5959 (5848-6070) Mount Shasta 6000 (5750-6250) T18 9795 (9701-9889) Mount Shasta 9600 (9500-9700) Table 7.2 Southern CVA eruption timing compared to CSZ megathrust earthquake timing Twenty possible linkages out of at least 122 CVA Holocene eruptions clearly does not equate to a majority share of eruptions linked stratigraphically with the eighteen Holocene megathrust earthquakes. Volcanoes are influenced by both subduction and crustal processes; thus, it is inarguable that tectonic boundary megathrust earthquakes do not trigger all volcanic eruptions. If the geologic record appears to suggest it possible that triggering occurs, it is arguable that the topic warrants further consideration.

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CSZ T# CSZ date (yrs. BP) Volcano Eruption Date (yrs. BP)

T4 1243 (1201-1285) Newberry 1240 (1140-1340) 1420 (1308-1532); T5 1554 (1522-1586) Newberry 1480 (1380-1580) T5 1554 (1522-1586) South Sister 1510 (1360-1660)

T6 2536 (2514-2558) South Sister 2560 (2360-2760)

T7 3028 (2967-3089) South Sister 3000 (2900-3100) T8 3443 (3375-3511) Newberry 3500 (3400-3600)

T10 4770 (4719-4821) Newberry 4860 (4740-4980) T12 6466 (6364-6568) Newberry 6400 (6270-6530)

T14 7630 (7586-7664) climactic Mazama 7627 (7477-7777)

T14 7631 (7586-7664) Llao Rock (pre-Mazama) 7850 (7750-7950) Table 7.3 Central CVA eruption timing compared to CSZ megathrust earthquake timing CSZ T# CSZ date (yrs. BP) Volcano Eruption Date (yrs. BP) MSH We 468 (467-469) T2 481 (398-564) MSH Wn 471 (470-472)

T5 1554 (1522-1586) Mount Hood 1470 (1320-1620) T5 1555 (1522-1586) MSH Bu 1680 (1530-1830) MSH Ye 3520 (3420-3620) T8 3443 (3375-3511) MSH Yn 3660 (3515-3805) Mount Adams Tephra T11 5959 (5848-6070) Layer 11-13 6000 (5500-6500) T11 5959 (5848-6070) Glacier Peak Tephra D 6045 (5905-6185) Mount Adams Tephra T18 9795 (9701-9889) Layer 1-4 9800 (8800-10800) Table 7.4 Northern CVA eruption timing compared to CSZ megathrust earthquake timing

Recommendations for future work This study's findings provide an exploratory first look at comparing the timing of CSZ megathrust earthquakes and CVA eruptions through paleoseismic and tephrochronological 128

methods. Continued work in lake coring in sites downwind from Holocene-active Cascade volcanoes can provide further seismic and tephra data. This future data will update Goldfinger's regional well-log correlations (2012; 2017; 2020) and in this current study. The identified possible timing linkages in Tables 7.2-7.4 can guide future research to confirm or refute the possible links. Well-log correlation of paleoseismic and tephrochronological deposits offer a much larger temporal dataset for exploring triggering relationships and relative timing of events, largely circumventing the errors associated with radiocarbon dating. Future work is recommended to include lake and marine sediment core analysis, well-log correlation, geochemical identification of primary tephra beds that will expand the sampling, in particular. Selection of other lakes to conduct coring should abide by the criteria that the lakes have limited fluvial input, situated downwind from volcanoes from which tephra was not uncovered in this study. To continue sampling lakes that will likely record CSZ megathrust earthquakes, sampled lakes should not further east than ~350 km from the Pacific coast. Logistics and access to lakes are key considerations with crucial lake characteristics such as depth, water access from the shore, and whether coring equipment can be transported to the water by vehicle or carried in by hand. Other issues to include in planning are procuring proper authorization to operate in a particular lake. Some lakes fitting the sampling criteria could be located on Indian Reservations, federal or state parks, and private property. More coring at Bull Run and Wapato Lakes would provide additional material to analyze. Future research could come from a multi-disciplinary project with partnerships forged between academic, governmental, and private industry. Prior, limited coring projects have been conducted at Lake Chelan, a lake neighboring Lake Wapato, through the Lake Chelan Research Institute and the University of Washington. Much of Lake Chelan's bathymetry has yet to be charted due to the depth (453 m) and Lake Chelan area. Lake coring, seismic survey, and sonar survey operations could be conducted during the same research cruises aboard large research vessels. It is possible that Lake Chelan also holds a paleoseismic and tephra bed record far more complete than the much shallower Wapato Lake (21 m). Other research questions about the Lake Chelan basin would also benefit such a research cruise, such as whether the lake is merely a deep glacial scour basin or possibly an active, flooded crustal fault. Other suggested lakes include Walupt Lake (Lewis County, WA at 46°25′01″N 121°27′47″W); Palmer Lake (Okanogan 129

County, WA at 48°54′00″N 119°37′03″W); Cultus Lake (Deschutes County, OR at 43.83766°N 121.86006°W); Davis Lake (Klamath & Deschutes Counties, OR at 43°36′57″N 121°50′39″W); Crescent Lake (Klamath County, OR at 43°28′32″N 121°59′32″W); Lake Abert (Lake County, OR at 42.647°N 120.224°W); and Eagle Lake (Lassen County, CA at 40°38′42″N 120°44′38″W). Finally, to improve the understanding of the extent to which the Rogue River was impacted by lahar and flash flooding following the climactic Mazama eruption, sediment coring should be sampled along the entire floodplain sediment profiles produced. This future research would help determine if a lahar traveled down the Rogue River, possibly answering questions about the lahar's estimated volume and how far west it reached.

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APPENDICES

144

Appendix A: OxCal Age P-Sequence Model Input Wapato Lake P-Sequence Model Input Plot() { Curve("IntCal13","IntCal13.14c"); P_Sequence(1) { { Boundary("WLC Base"); { z=127; }; Date("R11") { z=126.5; }; Date("Mazama") { z=109.1; }; Date("T14") { z=109.05; }; Date("T13") { z=102; }; Date("T12") { z=91; }; R_Date("R9",5475,15) { z=90; }; Date("T11") { z=88.75; }; Date("T10f") { z=88.25; }; Date("T10c") { z=84; }; Date("T10b") { z=82; }; Date("T10") { z=75.75; 145

}; R_Date("R15",4210,35) { z=75; }; Date("T9") { z=72; }; Date("A7 Tephra") { z=60.8; }; Date("WLC9") { z=58.75; }; Date("WLC8") { z=53; }; Date("WLC7") { z=47.25; }; Date("WLC6") { z=41; }; Date("WLC5") { z=34.25; }; Date("WLC4") { z=30.5; }; R_Date("R14",1240,15) { z=29.75; }; R_Date("R16",1165,15) { z=27.5; }; R_Date("R8",920,20) { z=23; }; Date("WLC3") { z=21.25; }; Date("WLC2") { z=13.4; 146

}; Date("A5 Tephra") { z=13.25; }; Date("WLC1") { z=9; }; Boundary("WLC_Core",2014) { z=0; }; }; }; }; Bull Run Lake P-Sequence Model Input Plot() { Curve("IntCal13","IntCal13.14c"); P_Sequence("BRL-09-08 composite3-5-20CG", 10) { Boundary ("Mazama Ash",-5700) { z=19.32; }; Date("BRT23") { z=18.58; }; Date("BRT22") { z=17.91; }; R_Date("BRL-09 65.8 20.45", 5665, 25) { z=17.65; }; Date("BRT21") { 147

z=16.58; }; Date("BRT20") { z=15.73; }; Date("BRT19") { z=15.31; }; R_Date("BRL-09 54.5", 4465, 25) { z=13.75; }; Date("BRT18") { z=13.66; }; Date("BRT17") { z=13.11; }; Date("BRT16") { z=12.20; }; Date("BRT15") { z=10.88; }; Date("BRT14") { z=9.41; 148

}; Date("BRT13") { z=8.92; }; R_Date("BRL-09 79.5", 2515, 25) { z=8.5; }; Date("BRT12") { z=8.39; }; Date("BRT11") { z=8.17; }; Date("BRT10") { z=7.06; }; Date("BRT09") { z=6.19; }; Date("BRT08") { z=5.96; }; Date("BRT07T") { z=4.81; }; 149

Date("BRT06T") { z=4.55; }; R_Date("BRL-09 6.5 4.4", 1460, 25) { z=4.1; }; Date("BRT05") { z=3.66; }; Date("BRT04") { z=3.14; }; Date("BRT03") { z=2.26; }; Date("BRT02") { z=1.7; }; Date("BRT01") { z=0.8; }; Boundary("surface", 2015) { z=0; }; }; 150

}; Leland Lake OxCal P-Sequence Model Input Plot() { Curve("IntCal13","IntCal13.14c"); P_Sequence("LLJ-07", 1) { Boundary ("Mazama Ash",-5677) { z=175.5; }; Date("LT22") { z=175.25; }; Date("LT21") { z=168; }; R_Date("LLJ-7H 48.5", 6020, 30) { z=164; }; Date("LT20") { z=160; }; Date("LT19") { z=153; }; Date("LT18") { z=146; }; R_Date("LLJ-7G 48.5", 5310, 45) { z=139; }; Date("LT17") { z=139; }; Date("LT16") { z=124; }; Date("LT15") { z=117; }; Date("LT13") { z=111; }; 151

Date("LT12") { z=99; }; Date("LT11") { z=93; }; R_Date("LLJ-7E 58.5", 3910, 90) { z=93; }; Date("LT10") { z=79; }; R_Date("LLJ-1C 72", 2940, 160) { z=67; }; Date("LT9") { z=67; }; R_Date("LLJ-7D 20.5", 2500, 20) { z=52; }; Date("LT8") { z=52; }; Date("LT7") { z=44; }; Date("LT6") { z=36; }; Date("LT5") { z=29; }; Date("LT4") { z=25; }; R_Date("LLJ-1B 27.5", 760, 80) { z=20.75; }; Date("LT3") { z=20.5; }; 152

Date("LT2") { z=13; }; Date("LT1") { z=10; }; R_Date("LLJ-7A 8.5", 250, 20) { z=9.5; }; Date("T0") { z=3; }; Boundary("surface", 2013) { z=0; }; }; };

153

Appendix B: OxCal Output Tables Wapato Lake OxCal Output Table

154

Bull Run Lake OxCal Output Table

Bull Run Lake OxCal P-Sequence Output Table as seen in Appendix 2 of Goldfinger, 2020

155

Leland Lake OxCal P-Sequence Output Table

156

Appendix C: Cascadia Subduction Zone Event Bed & Cascade Volcanic Arc Holocene Dates Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

Mount Baker A 70 2 0.002 Tephra/Historic Mount Baker R 80 2 0.002 Tephra/Historic

Mount Baker YP 87 2 0.002 Tephra/Historic

Mount Baker 97 2 0.002 Historic

Mount Baker 107 3 0.04 Historic Tucker and Scott, 2006, Tucker et al., Mount 2005; Tucker et al., Baker 130 2 0.002 Historic 2007 Mount Baker OP 5835 3 0.04 Tephra

Mount Baker 6500 3 0.045 C14

Mount Baker Ba 8675 3 0.0083 Tephra

Mount Baker 9800 3 0.04 C14

Mount Baker SC 10350 4 0.2 Tephra

Mount Baker RC 12000 4 0.32 Tephra Glacier Beget, 1982 Peak X 316 2 Tephra

Glacier Foit, 2004; Kuehn Peak 913 3 C14 2017

Glacier Foit, 2004; Kuehn Peak 1662 4 C14 2017

Glacier Foit, 2004; Kuehn Peak A 1990 4 Tephra 2017

Glacier Foit, 2004; Kuehn Peak 2901 C14 2017 157

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

Glacier Foit, 2004; Kuehn Peak 5803 C14 2017

Glacier Foit, 2004; Kuehn Peak D 6045 4 Tephra 2018

Glacier Foit, 2004; Kuehn Peak 6295 C14 2017

Glacier Foit, 2004; Kuehn Peak B 11600 5 Tephra 2017

Glacier Foit, 2004; Kuehn Peak M 11875 4 Tephra 2017

Glacier Foit, 2004; Kuehn Peak G 13560 5 Tephra 2017

Mount Mullineaux (1974) Rainier W 500 ? ? C14

Mount Mullineaux (1974) Rainier 1040 ? ? C14

Mount Mullineaux (1974) Rainier C1 & C2 1510 ? ? Tephra

Mount Mullineaux (1974) Rainier C 2200 4 0.3 Tephra

Mount Mullineaux (1974) Rainier 2350 ? ? Tephra

Mount Mullineaux (1974) Rainier SL5 2450 ? ? Tephra

Mount Mullineaux (1974) Rainier SL3 & SL4 2560 ? ? Tephra

Mount Mullineaux (1974) Rainier SL2 2600 ? ? Tephra

Mount Mullineaux (1974) Rainier SL1 2650 ? ? Tephra

Mount Mullineaux (1974) Rainier B 4500 3 0.005 Tephra

Mount Mullineaux (1974) Rainier H 4700 2 0.001 Tephra

Mount Mullineaux (1974) Rainier S, F 5600 3 ? Tephra 158

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

Mount Mullineaux (1974) Rainier N 5800 ? ? C14

Mount Mullineaux (1974) Rainier Da 6800 2 0.002 Tephra

Mount Mullineaux (1974) Rainier Db 6920 3 0.075 Tephra

Mount Mullineaux (1974) Rainier L 7335 3 0.05 Tephra

Mount Mullineaux (1974) Rainier A 7525 2 0.005 Tephra

Mount Mullineaux (1974) Rainier 9750 ? ? C14

Mount Mullineaux (1974) Rainier R 10000 3 0.025 Tephra

Pallister, et al. (2017; Kuehn (2017); Greeley & Hyde (1972); Crandell et al. (1973); Yamaguchi (1983 and 1985); Crandell Modern (1986); Scott (1988); MSH 2004 CE 2 0.0002 Observations Yamaguchi et al. (1990); Yamaguchi Modern & Hoblitt (1995); MSH 1990 CE 3 0.001 Observations Mullineaux (1986); Modern Michael Clynne & MSH 1986 CE 2 0.0003 Observations James Vallance (Unpublished) Modern MSH 1980 CE 5 3 Observations

Modern MSH 1857 CE 2 0.0002 Observations

Modern MSH 1854 CE 2 0.0004 Observations Modern MSH 1850 CE 2 0.0002 Observations

MSH T 1800 CE 5 4 Dendro/Tephra

MSH X 448 3 0.001 Dendro/Tephra 159

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

MSH We 468 5 2 Dendro/Tephra

MSH Wn 471 5 3 Dendro/Tephra

MSH D 1075 4 0.1 Dendro/Tephra

MSH Bu 1680 5 3 Dendro/Tephra

MSH Bi 2025 1 0.0003 Tephra

MSH Bd 2170 Tephra

MSH Bo 2200 Tephra

MSH Ps & Pu 2480 Tephra

MSH Pm 3130 Tephra

MSH Ya 3430 Tephra

MSH Ye 3520 5 1 Tephra

MSH Yn 3660 6 12 Dendro/Tephra

MSH Yd 4050 Tephra

MSH Yb 3960 3 0.2 Dendro/Tephra East Flank-- Mount Tephra Layer Adams 24 1000 2 0.001 Tephra/C14

Mount Tephra Layer Adams 23 1750 2 0.0025 Tephra/C14 Hildreth & Fierstein (1997) Mount Tephra Layer Adams 22 2250 2 0.0025 Tephra/C14

Mount Tephra Layer Adams 21 2320 2 0.001 Tephra/C14 160

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

Mount Tephra Layers Adams 19-20 2500 2 0.0025 Tephra/C14

Mount Tephra Layer Adams SSE Flank 3800 4 0.4 Tephra/C14

Mount Tephra Layers Adams 17-18 4600 2 0.001 Tephra/C14 Mount Tephra Layer Adams 16 4900 2 0.0025 Tephra/C14

Mount Tephra Layer Adams 15 5200 4 0.2 Tephra/C14

Mount Tephra Layer Adams 14 5500 4 0.5 Tephra/C14

Mount Tephra NNE Adams Flank 5750 4 0.35 Tephra/C14

Mount Tephra Layers Adams 11-13 6000 4 0.1 Tephra/C14

Mount Tephra Layer Adams 10 6550 4 0.5 Tephra/C14 Mount Tephra Layer Adams 5-9 7100 3 0.03 Tephra/C14 Mount Tephra Layer Adams 1-4 9800 5 4 Tephra/C14

Mount Historic Pierson et al. (2011) Hood 85 2 0.002 Observations

Mount Pierson et al. (2011) Hood Old Maid 169 4 0.25-0.5 Dendrochron

Mount Pierson et al. (2011) Hood Timberline 1470 4 0.25-0.5 C14 South Sister 1510 2 0.003 C14 Scott (1999)

South Sister 1970 2 0.005 Tephra Scott (1987) South Sister 2150 3 0.03 Tephra Scott (1987) 161

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference South Sister 2300 2 0.0065 Tephra Scott (1987) South Sister 2560 2 0.001 Tephra Scott (1987)

South Sister 2740 2 0.003 Tephra Scott (1987) South Sister 3000 3 0.08 C14 Scott (1999)

South Sister 9520 3 0.01 Tephra Scott (1987)

Newberry 1240 4 0.3 Tephra/C14 Foit & Kuehn (2000)

Newberry 1420 4 0.1 Tephra/C14 Foit & Kuehn (2000)

Newberry 1480 4 0.6 Tephra/C14 Foit & Kuehn (2000)

Newberry 2270 3 0.02 Tephra/C14 Foit & Kuehn (2000)

Kuehn (2002); MacLeod (1995; Newberry 3500 2 0.0025 Tephra/C14 2006)

Robinson & Trimble Newberry 4860 3 0.05 Tephra/C14 (1981)

Kuehn (2002); MacLeod (1995; Newberry 6400 2 0.003 Tephra/C14 2006)

Kuehn (2002); MacLeod (1995; Newberry 7020 3 0.01 Tephra/C14 2006)

Newberry 7320 1 0.0001 Tephra/C14 Foit & Kuehn (2000)

Mazama 5220 5 3 Tephra/C14 Bacon et al., 2017 162

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference

Tephra/C14/Gree Zdanowicz et al., Mazama 7630 7 58 nland ice core 1999

Mazama Llao Rock 7850 6 24 Tephra/C14 Bacon et al., 2017 Medicine Lake Glass Nathenson, et al Lake Mtn 884 3 0.1 Tephra/C14 (2007)

Medicine Nathenson, et al Lake Callahan Flow 1062 2 0.01 Tephra/C14 (2007) Medicine Little Glass Nathenson, et al Lake Mtn 1065 3 0.4 Tephra/C14 (2007)

Medicine Nathenson, et al Lake Paint Pot 1162 4 0.4 Tephra/C14 (2007) Medicine Nathenson, et al Lake Hoffman Flow 1207 2 0.04 Tephra/C14 (2007)

Medicine Burnt Lava Nathenson, et al Lake Flow 2768 4 0.5 Tephra/C14 (2007) Medicine Lake Glass Nathenson, et al Lake Flow 4317 1 0.0002 Tephra/C14 (2007)

Medicine Nathenson, et al Lake Giant Crater 10580 3 0.01 Tephra/C14 (2007)

Mount Donnelly-Nolan Shasta 600 4 4.4 Tephra/C14 (1990)

Mount Donnelly-Nolan Shasta 1800 4 4.4 Tephra/C14 (1990)

Mount Donnelly-Nolan Shasta 2500 4 4.4 Tephra/C14 (1990) Mount Donnelly-Nolan Shasta 6000 4 4.4 Tephra/C14 (1990)

Mount Christiansen, et al., Shasta 8500 4 4.4 Tephra/C14 2017 Mount Donnelly-Nolan Shasta 9600 4 4.4 Tephra/C14 (1990)

Mount Donnelly-Nolan Shasta 10310 5 4.4 Tephra/C14 (1990) 163

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference Mount Shasta 10940 5 15 Tephra/C14 Gardner, et al. 2013

Lanphere, et al. Lassen Historic (1999 & 2004); Peak 35 4 0.25 Observations Clynne, et al. (2002)

Lanphere, et al. Lassen Historic (1999 & 2004); Peak 284 4 0.25 Observations Clynne, et al. (2002) Lanphere, et al. Lassen (1999 & 2004); Peak 1103 4 0.25 Geochron; 14C Clynne, et al. (2002)

Goldfinger, et al. 265 CSZ T1 N/A N/A C14 (2012)

Goldfinger, et al. 481 CSZ T2 N/A N/A C14 (2012)

Goldfinger, et al. 796 CSZ T3 N/A N/A C14 (2012)

Goldfinger, et al. 1243 CSZ T4 N/A N/A C14 (2012)

Goldfinger, et al. 1554 CSZ T5 N/A N/A C14 (2012)

Goldfinger, et al. 2536 CSZ T6 N/A N/A C14 (2012)

Goldfinger, et al. 3028 CSZ T7 N/A N/A C14 (2012)

Goldfinger, et al. 3443 CSZ T8 N/A N/A C14 (2012)

Goldfinger, et al. 4108 CSZ T9 N/A N/A C14 (2012) Goldfinger, et al. 4770 CSZ T10 N/A N/A C14 (2012)

Goldfinger, et al. 5959 CSZ T11 N/A N/A C14 (2012) Goldfinger, et al. 6466 CSZ T12 N/A N/A C14 (2012)

Goldfinger, et al. 7182 CSZ T13 N/A N/A C14 (2012) 164

Median Eruption Volcano Tephra Age (years Est. Volume Name Designation BP) VEI (cubic km) Data Type Reference Mazama Goldfinger, et al. 7630 CSZ T14 7630BP N/A N/A Tephra/C14 (2012) Goldfinger, et al. 8173 CSZ T15 N/A N/A C14 (2012)

Goldfinger, et al. 8906 CSZ T16 N/A N/A C14 (2012) Goldfinger, et al. 9101 CSZ T17 N/A N/A C14 (2012)

Goldfinger, et al. 9795 CSZ T18 N/A N/A C14 (2012) 165

Appendix D Smear Slides D1. Bull Run Lake Smear Slides BRL-08 Smear Slides

166

D2. BRL-09 Smear Slides

167

D3. Leland Lake Smear Slides

168

D4. Rogue Apron Smear Slides

169

Rogue Apron Smear Slides (continued)