500 Year Rupture History of the Imperial Fault at The

Home , Cerro Prieto Fault, Elsinore Fault Zone, Rose Canyon Fault, Salton Sink

500 YEAR RUPTURE HISTORY OF THE IMPERIAL FAULT AT THE

INTERNATIONAL BORDER THROUGH ANALYSIS OF

FAULTED LAKE CAHUILLA SEDIMENTS,

CARBON-14 DATA, AND CLIMATE DATA

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A Thesis

Presented to the

Faculty of

San Diego State University

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In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Geological Sciences

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Kaitlin Nicole Wessel

Spring 2016

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DEDICATION

To my family, thank you so much for your love and support. To Aaron, thank you for your patience, love, and encouragement.

ABSTRACT OF THE THESIS

500 Year Rupture History of the Imperial Fault at the International Border Through Analysis of Faulted Lake Cahuilla Sediments, Carbon-14 Data, and Climate Data by Kaitlin Nicole Wessel Master of Science in Geological Sciences San Diego State University, 2016

We excavated a trench across a sag pond created by a 30 m-wide releasing step along the Imperial fault 1.4 km north of the U.S.-Mexico international border to test earthquake recurrence models. The stratigraphy at the site exhibits distinct pulses of lacustrine and deltaic deposition with localized zones and layers of well-sorted sand deposits interpreted as the result of liquefaction. Evidence for five events is observed in the upper 3.5 m of stratigraphy, which corresponds to deposition from three full lake episodes over the past 400- 550 years. Age control is by 14C dating of detrital charcoal from the trenches and correlation to the well-constrained regional chronologic model of Lake Cahuilla. Evidence for events is based on production of accommodation space and associated growth strata, upward fault terminations and fissures, massive liquefaction in the form of sand dike intrusions and sand blow deposits, and significant vertical offset in the step-over area. The two most recent events appear to be significantly larger than the earlier two events based on event-by-event palinspastic reconstruction and correlation to previous trenching studies. Six meters of strike slip passed through the sag in the 1940 Imperial Valley earthquake. The penultimate event produced nearly identical vertical displacement as in the 1940 earthquake, implying that it was also large and likely slipped about 6 m. In contrast, events 3 and 4 produced little vertical displacement from which we infer that displacement in these earthquakes was small at our site. We interpret these as moderate 1979-type earthquakes and that the southern end of these ruptures was likely close to our site. Event 5 is interpreted to be large based on its expression in nearby trenches; our trenches were not deep enough to capture the vertical separation for this event. Together, if each event interpreted as large experienced a similar amount of displacement as in 1940, this implies something on the order of 18 m of displacement in the past 400-550 years, yielding a slip rate of 32-45 mm/yr. This rate is indistinguishable from all geodetically-inferred rates for the Imperial fault and implies that most of the plate motion is accommodated by the Imperial fault at the international border. The CD-ROM, an appendix to the thesis, is available for viewing at the Media Center of the San Diego State University (SDSU) Library.

TABLE OF CONTENTS

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ABSTRACT ...... v LIST OF TABLES ...... viii LIST OF FIGURES ...... ix ACKNOWLEDGEMENTS ...... xi CHAPTER 1 INTRODUCTION ...... 1 Purpose ...... 2 Tectonic Setting of the Imperial Valley ...... 5 Previous Trenching Investigations on the Imperial Fault ...... 7 Prior Work on the Chronology of Ancient Lake Cahuilla ...... 12 2 METHODOLOGY ...... 15 Fault Trenching Methods ...... 15 Radiocarbon Sample Collection and Processing ...... 19 3 LOCAL STRATIGRAPHY ...... 21 Stratigraphic Units Described in Previous Investigations ...... 21 Units Exposed at the International Border Site ...... 23 Unit 10 - Artificial Fill ...... 23 Units 50 And 100 - Plow Pan ...... 24 Units 120-150 And Unit 297 - Liquefaction Sands ...... 25 Units 200, 300, And 400 - Lakebed Clays ...... 25 Units 215-265, Units 270-295, and Units 350-365 - Deltaic Sediments ...... 26 4 AGES OF LAKE CAHUILLA SEDIMENTS ...... 28 Lake Inundation and Desiccation Based on Paleoclimate ...... 28 Historical Constraints...... 29 Radiocarbon Dating of Deposits in the Border Trench ...... 30

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Discussion of Age Data ...... 33 5 IDENTIFICATION OF EARTHQUAKE EVENTS ...... 39 Evidence for Faulting Events at the Border Site ...... 39 Event 1 ...... 39 Event 2 ...... 40 Event 3 ...... 40 Event 4 ...... 44 Event 5 ...... 45 Correlation to Previous Trench Studies ...... 45 Discussion ...... 51 Long-Term Rupture Behavior ...... 51 Constraints on Earthquake Recurrence ...... 53 6 CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK ...... 56 REFERENCES ...... 58

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LIST OF TABLES

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Table 1. Imperial Border Trench Radiocarbon Sample Ages ...... 31 Table 2. Time Interval Between Events ...... 55

LIST OF FIGURES

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Figure 1. Northwest oblique Google Earth Image showing generalized fault traces, Lake Cahuilla shoreline, and Colorado River Delta...... 2 Figure 2. Map of surface ruptures along the plate boundary fault system with black stars representing earthquake epicenters...... 3 Figure 3. Slip Distribution for the 1940 earthquake from Rockwell and Klinger (2013)...... 4 Figure 4. Cumulative displacement for the past 500 years along the Imperial fault as seen from evidence at the Dogwood Road site...... 5 Figure 5. 1940 Aerial image 1.4 km north of the international border showing the 30 m wide sag pond produced from the 1940 rupture...... 6 Figure 6. Google Earth image showing the location of paleoseismic studies along the Imperial fault...... 9 Figure 7. Stratigraphic section from Sharp’s (1980) paleoseismic study...... 10 Figure 8. Event ages determined from observed events at the Dogwood Road site on the northern Imperial fault...... 12 Figure 9. Map of Lake Cahuilla shorelines...... 14 Figure 10. Google Earth image of trench location...... 16 Figure 11. Photos taken during excavation of the trench...... 17 Figure 12. (Upper right image) 1940 aerial image of sag pond...... 18 Figure 13. Comparison of lake clay deposition between sites...... 23 Figure 14. Zoomed in image showing the burn layer (unit 25) positioned above unit 50...... 25 Figure 15. Foreset beds that have been laterally displaced and are now juxtaposed against flat lying deltaic sediments...... 27 Figure 16. Suggested period of inundation for Lake 1 at Border Trench site overlaid on precipitation plot...... 36 Figure 17. Suggested period of inundation for Lake 2 at the Border Trench site overlaid on precipitation plot...... 37 Figure 18. Different scenarios for inundation and desiccation of Lake 3...... 38

Figure 19. General down-warping that is exhibited by the plow pan deposits in response to Event 1...... 41 Figure 20. (a) Aerial image from 1940 with an overlay of the trench site and fault strand locations from Google Earth...... 42 Figure 21. Fault strands terminating at the top of Lake 1...... 43 Figure 22. Massive lateral deposition of Liquefaction sands above Lake 1...... 44 Figure 23. Collapse of Lake 1 deposits into deltaic deposits below...... 44 Figure 24. Evidence for Event 3 as seen in the trench along the western fault strand from Jerrett (2015)...... 46 Figure 25. Fissure fill and vertical displacement as a result from Event 4...... 47 Figure 26. Liquefaction sand dikes and lateral deposition of liquefaction sands above Lake 2 as evidence for Event 4. layout...... 48 Figure 27. Large fissures terminating at the top of Lake 3 and capped by deltaic sediments from above providing evidence for Event 5...... 49 Figure 28. Southeast facing trench log from the Dogwood Road site...... 54

ACKNOWLEDGEMENTS

I would like to express my deepest appreciation to those organizations and individuals that provided financial, material, and moral support throughout the duration of this project. First and foremost, I would like to thank the Southern California Earthquake Center (SCEC) and U.S. Geological Survey (USGS) for funding of this project, which allowed us to study the project site in its entirety. Certainly this project would not have been possible were not for the continuous guidance and support from my advisor Thomas Rockwell. I started working with Tom for my undergraduate thesis work in early 2010 where his passion for neotectonics and geomorphology as well as his experience with not only local tectonics, but faulting all around the globe inspired me to return to school and pursue my Master’s degree. His in depth knowledge on the subject at hand provided me with an educational experience that cannot be paralleled. There are many people that I would like to thank who were involved in the laborious effort that is fault trenching. Yann Klinger, Neta Wechsler, Petra Štěpančíková, and Jakob Stemberk travelled internationally to assist us in trench excavation, preparation, and interpretation. Their prior knowledge on paleoseismic trenching helped immensely, especially in interpreting evidence for past rupture events. I am also very grateful to backhoe operators Jim Little and Andy Hoyt, as well as Erik Gordon and Erik Haaker for assistance with movement of hydraulic shores and GPS data on the trench. Lastly, I would like to thank the Imperial Irrigation District (IID) for allowing us to trench on their land, with special thanks to Socrates Gonzalez who assisted us in the permitting process and provided us with aerial images and historic information on the Imperial Valley. My trenching partner, Andy Jerrett, and I spent four weeks at the Keck Carbon Cycle AMS Laboratory (KCCAMS) at the University of California Irvine for 14C dating where co- founder John Southon and lab prep manager Chanda Betrand spent countless hours teaching us how to process our radiocarbon samples. The entire team at KCCAMS was incredibly

xii helpful and very accommodating. From this team I would also like to thank Daniella Laset and Charity Sotero, who took time out of their busy work schedules to help us process some of our samples. I am very grateful to the San Diego State University Geology Department for all of their hard work in creating an environment where students not only learn about conceptual ideas in the classroom, but are able to apply those principles out in the field. I would further like to thank my committee members- Thomas Rockwell, Jillian Maloney, and Trent Biggs- for review of my thesis. I am also very appreciative of the friendship and support from graduate students in the department. I had the opportunity to work closely with Liz White who supplied me with Lake Cahuilla figures and data as well as Yuval Levy whose witticisms led to a fun and light-hearted work atmosphere. Further, I am immeasurably grateful to my partner in crime, Andy Jerrett, who worked with me every step of the way.

CHAPTER 1

INTRODUCTION

The Imperial fault is the main plate boundary structure that transfers slip across the U.S. – Mexico international border, accommodating as much as 70% to 80% of the relative motion between the Pacific and North American plates (Bennett, Rodi, & Reilinger, 1996; Genrich, Bock, & Mason, 1997; Sharp, 1982). This northwest-striking dextral slip fault is located in the Imperial Valley, California and displaces lakebed and deltaic sediments of known age from the inundation and desiccation of ancient Lake Cahuilla (Figure 1). The fault is approximately 70 km long, terminating at major right steps located at Mesquite Basin in the north and the Cerro Prieto geothermal field in the south, both of which are characterized by high heat flow and dense microseismicity (Lomnitz, Mooser, Allen, Brune, & Thatcher, 1970). What distinguishes the Imperial fault from other known faults in the region is that it produced two historical surface ruptures (Figure 2). The fault was first recognized after the 1940 Mw 6.9 earthquake where it ruptured along its the entire length (Buwalda & Richter, 1941). The rupture nucleated at the juncture between the Imperial and Brawley faults and propagated southeast, with the maximum amount of slip occurring near the All American Canal just north of the international border (Doser, 1990; Rockwell & Klinger, 2013). The most recent event occurred in 1979 with a moment magnitude of 6.5. In contrast to the 1940 event, this rupture nucleated to the south in Baja California and propagated unilaterally to the northwest, where it then ruptured beneath the 1940 high slip region and surfaced again north of Heber Dunes (Archuleta, 1984; Rockwell & Klinger, 2013; Sharp, 1982). Intriguingly, the northern third of this rupture closely matched the slip distribution of the 1940 event (Figure 3), leading Sieh (1996) to propose that the Imperial fault is segmented into “slip patches” where the northern end exhibits its own characteristic slip. Recent trenching studies at sites along the northern portion of the fault contradict this theory. As many as 17 surface ruptures

2 have occurred over the past 1200 to 1400 years at the Dogwood Road site (Rockwell, Meltzner, &Tsang, 2011). The displacement for the past six rupture events was resolved showing approximately 20 cm of displacement for the 1940 and 1979 ruptures (plus about 10 cm of afterslip for each) and 1.5 m displacements for three of the earlier four events (Buwalda & Richter, 1941; Rockwell et al., 2011; Sharp, 1982; Figure 4). This, in turn, favors a bi- modal distribution of the displacement along the northern Imperial fault, possibly suggesting that because displacement increased to the south near the international border, stress loading could have caused the northern low-slip section to re-rupture (Rockwell et al., 2011).

Figure 1. Northwest oblique Google Earth Image showing generalized fault traces, Lake Cahuilla shoreline, and Colorado River Delta. Locations include northern Baja California, Mexico, southwest Arizona, and southern California. Abbreviations for fault traces are as follows: ABF: Agua Blanca fault; SMF: San Miguel fault; DF: Descanso fault; RCF: Rose Canyon fault; CPF: Cerro Prieto fault; LSF: Laguna Salada fault; IF: Imperial fault; BSZ: Brawley seismic zone; SAF: San Andreas fault; SJF: San Jacinto fault; EF: Elsinore fault.

PURPOSE Constraining the timing of past ruptures on the Imperial fault can assist in settling the debate as to whether this simple plate boundary fault behaves in a more characteristic manner with only the northern portion of the fault failing with partial stress drops, or if earthquakes

3 exhibit a more variable distribution of displacement. Through this study, we aim to test this model by resolving the timing and relative displacement of past ruptures in the area with maximum 1940 slip and determine if we only see the larger Dogwood events, or whether some of those are absent, indicating the occurrence of a northern Imperial event with large slip.

Figure 2. Map of surface ruptures along the plate boundary fault system with black stars representing earthquake epicenters. Map also shows surface rupture for the Mw 6.9 1940 (black dots) and Mw 6.5 1979 (white dots) earthquakes and points out the paleoseismic studies conducted on the Imperial fault. Image from Thomas and Rockwell (1996).

Figure 3. Slip Distribution for the 1940 earthquake from Rockwell and Klinger (2013). Triangles are placed over interpreted rupture segments. The sub-events of Trufinac and Brune (1970) and Doser (1990), along with their estimate of percentage of the total moment magnitude for each sub-event, are shown at the top at their respective location. The percentages within triangles denote the proportion of each cross-sectional area for each triangle, based on surface slip.

Remapping of the 1940 surface rupture by Rockwell and Klinger (2013) showed the formation of a sag pond along a 30-meter right step on the fault 1.4 km north of the international border (Figure 5). This proved to be an ideal site as sag ponds provide for exceptional depositional environments for the preservation of paleoearthquake evidence (McCalpin, Rockwell, & Weldon, 1996). This is based on the relatively low-energy, episodic accumulation of thin strata in the tectonic depression. Coincidentally, both strands of the step-over intersect an access road owned by the Imperial Irrigation District (IID) of which permissions for trenching were easily obtained. Our objectives for this project were to: (1) excavate a slot trench on the eastern strand of the step-over to collect samples for radiocarbon dating and observe and record all evidence for paleoearthquakes; (2) correlate stratigraphy and rupture events to previous trenching studies completed along the Imperial

5 fault; and (3) incorporate a comprehensive record of Lake Cahuilla data to further constrain the dates of interpreted events.

Figure 4. Cumulative displacement for the past 500 years along the Imperial fault as seen from evidence at the Dogwood Road site. From Rockwell et al. (2011).

TECTONIC SETTING OF THE IMPERIAL VALLEY The Imperial Valley is the southernmost extent of the Salton Trough, which can be characterized as a transitional tectonic environment, where merging of the East Pacific Rise with the North American-Pacific Plate boundary has resulted in a region of structurally complex faulting. Ultimately, within the last 5-6 Ma, large strike slip faults have created the oblique pull-apart basin which currently sits at an elevation of roughly 90 m below sea level (Rockwell et al., 2011). Well-known faults associated with the southern San Andreas fault system transect the basin at an average orientation of N37oW (Genrich et al., 1997; Sharp, 1982). The primary fault zones seen in the Imperial Valley consist of the San Andreas fault

zone that borders the eastern shoreline of the Salton Sea, the San Jacinto fault zone transecting the western flank of the Salton Trough, the Elsinore fault zone that skirts the southwestern edge of the trough, and the Brawley fault zone, Superstition Hills, Superstition Mountain, and Imperial faults situated near the axis of the trough (Sharp, 1982). Johnson and Hill (1982) indicate that the Salton Trough area is one of the most active sections of the San Andreas fault system based on historical right-lateral movements and high levels of seismicity on several of these faults.

Figure 5. 1940 Aerial image 1.4 km north of the international border showing the 30 m wide sag pond produced from the 1940 rupture. Six meters of dextral slip passed through this area that is evident from offset crop rows and furrows dragged down into the sag feature. Image from Rockwell and Klinger (2013).

As previously mentioned, the Imperial fault sustained two historical surface ruptures and is the most historically active fault in southern California, resulting in the attention of the

7 paleoseismic community as a means of observing the general characteristics of a simple plate boundary fault. The best documented event was the October 15, 1979 earthquake, as this event not only caused 30 km of surface rupture on the Imperial fault, but also triggered small displacements on the Superstition Mountain fault, Brawley fault zone, and the southern San Andreas fault, revealing the complexities of this faulting network (Johnson, Rojahn, & Sharp, 1982). Further studies on the relationship between these faults show that the Imperial fault and Brawley seismic zone, a linear zone of high seismicity connecting the Imperial and San Andreas faults, are intimately connected. Both the northern Imperial fault and Brawley seismic zone are characterized as having episodes of swarm events. Two different models suggested by Hill (1977) and Johnson (1979) explain that these swarms may be caused by either shear failures due to fluid pressure in dikes or episodic creep events along the Imperial fault. It is expressed that these models be viewed as complementary, as both explain aspects of Imperial Valley seismicity (Johnson & Hill, 1982). Furthermore, creep-meters that were installed in 1975 produced extraordinary data on the coseismic slip as well as afterslip of the 1979 event (Johnson et al., 1982). A study by Cohn, Allen, Gilman, and Goulty (1982) found that, interestingly, there was no discernable creep that occurred in the days and hours prior to the earthquake that would suggest an imminent event. The coseismic data revealed that displacement inferred a brittle rather than plastic response of the soil at depth, as well as demonstrated that the minimum rate of surface fault displacement was 1.8 cm/s. Combining all associated data with the afterslip data yielded 60 cm as the maximum amount of slip on the fault for the 1979 event.

PREVIOUS TRENCHING INVESTIGATIONS ON THE IMPERIAL FAULT Four published paleoseismic investigations have previously been conducted on the Imperial fault, two of which are within 1.8 km of this study (Figure 6). Robert V. Sharp carried out the first paleoseismic study in 1980 between the All American Canal and the international border. The site location was determined using the photogrammetric technique where Sharp overlaid the 1940 aerial images onto current aerial photographs to locate the position of the fault with respect to present roads and canals (Sharp, 1980). From the photos it was found that the maximum amount of displacement was just north of the international

border. Sharp (1980) excavated a preliminary trench that was soon extended to a 3D trench network based on the exposure of a buried channel oriented perpendicular to the fault. Sharp’s concluding remarks suggest that the channel, as well as a laminated sand unit above it, had only been offset by the 1940 event. Pre-1940 movement was, however, found in the fault zone stratigraphically below the channel. Here, the flat-lying laminated sand remains undisturbed up until the fault where it is then truncated by the 1940 rupture trace. All units below the laminated sand are tilted in towards the fault implying that there was at least one episode of fault movement prior to the deposition of the laminated sand (Sharp, 1980) (Figure 7). A charcoal rich silt layer among the tilted sediments yielded a 14C date of ca. AD 1300, which indicates that there was movement along the fault between AD1300 and the time of deposition of the laminated sand. A suggestion in Sharp’s 1980 report to trench across the border in Mexico prompted a paleoseismic study by A. P. Thomas and Rockwell (1996). A subsurface investigation only 100 meters south from Sharp’s site revealed a buried channel that was offset 5 meters in the 1940 earthquake. Evidence for a previous rupture was seen above the 300-year-old lake sediments of the last Lake Cahuilla highstand, but no further structural evidence was recognized for any other events in the past 550 years, suggesting that the slip rate on the Imperial fault for the past 300-550 years is only about 15-20 mm/yr. (A. P. Thomas & Rockwell, 1996). This is substantially less than rates inferred from geodetic models, indicating that either slip is accommodated by other faults in the area or that the Imperial fault has been relatively slow for this time period.

Figure 6. Google Earth image showing the location of paleoseismic studies along the Imperial fault.

To the north, nearing the southern terminus of the Brawley fault zone, the Imperial fault transitions from a simple fault structure to a more complex fault zone with multiple strands. The primary dextral displacement seen along the southern and central portion of the fault transitions to oblique slip in the north, with both dextral and normal components. Little was known about the long-term history at the northern end of the fault until a paleoseismic study was performed by Meltzner (2006). In that study, a trench site was selected just south of Harris Road, adjacent to Mesquite Basin, where the down dropped side of the fault

Figure 7. Stratigraphic section from Sharp’s (1980) paleoseismic study. Adapted from A. P. Thomas and Rockwell (1996). Note evidence for paleoearthquake seen as an angular unconformity between units C and D.

exposed the most recent lakebed deposit of Lake Cahuilla. There was evidence for four surface rupturing events after the most recent highstand of Lake Cahuilla, based on the presence of fissures and flower structures in the hangingwall block. Two of the observed events represent the 1979 and 1940 events based on the incorporation of post-1920 glass in fissures associated with the penultimate event. An event that caused massive liquefaction above a lakebed is inferred to be the same event (ca. 1680 in Meltzner, Rockwell, & Owen, 2006) that was seen in paleoseismic studies along the central portion of the fault (A. P. Thomas & Rockwell, 1996). A possible intermediate deformation event is thought to be the result of either the 1906 or 1915 earthquake, or alternatively, a 1979-type event that followed the ca. 1680 event by several decades (Meltzner, 2006). As there was no other evidence in the trench for repeating 1979-type events, a new rupture model was proposed in which the northern portion of the Imperial fault normally ruptures less frequently than every 40 years, with higher amounts of slip than experienced in 1940 and 1979 (Meltzner, 2006). Rockwell et al. (2011) continued to develop the long-term rupture history of the Imperial fault by conducting a paleoseismic study along Dogwood Road just 700 meters south of the Harris Road site. They were able to extend the previously recognized 500-year rupture chronology back to 1200-1400 years based on evidence of 17 surface ruptures found in the 3D trenches (Rockwell et al., 2011). They determined an average interval of recurrence of about 80 years but with a large coefficient of variation at 0.84, which would indicate non- periodic behavior (Rockwell et al., 2011; Figure 8). In addition, they found a strong connection between the northern Imperial fault and the southern San Andreas fault where rupture on one may trigger rupture on the other. Because the 3D trenching studies at Dogwood demonstrated a more non-characteristic slip, it can be suggested that the best-fit model would be the one proposed by Meltzner et al. (2006) which points to the Imperial fault having some ruptures with complete stress drops while others are more complex with only partial stress drops.

Figure 8. Event ages determined from observed events at the Dogwood Road site on the northern Imperial fault. Image from Rockwell et al. (2011).

PRIOR WORK ON THE CHRONOLOGY OF ANCIENT LAKE CAHUILLA Much of the geologic history of the Salton Trough is recorded in sediments derived from the Colorado River. Throughout the late Pleistocene and Holocene, the basin periodically filled to create the ancient Lake Cahuilla (Figure 9). This freshwater lake formed from the inflow of water and sediments from the Colorado River, which lies just beyond the Orocopia Mountains to the east. During the Plio-Quaternary, sediments that eroded from the Colorado Plateau were deposited in the Colorado River delta to form a natural sediment dam across the trough (Downs & Woodward, 1961). Over time, the same actions that formed the dam also altered the course of the Colorado River to flow to the north and into the below-sea level Salton Trough (Buckles, Kashiwase, & Krantz, 2002). The basin would fill to elevations between 9 and 13 meters above sea level, which is evident from exposed shoreline berms (Gurrola & Rockwell, 1996; Orgil, 2001; Rockwell & Sieh, 1994; Sieh, 1986; Sieh &

Williams, 1990; Stanley, 1963, 1966; R. G. Thomas, 1963; A. P. Thomas & Rockwell, 1996; Van de Kamp, 1973; Waters, 1983). After filling to an elevation of 13 meters, continued water inflow would breach the delta and change the course of the river back to the Gulf of California in the south (Meltzner, 2006). The chronology of Lake Cahuilla has been established through the dating of charcoal, wood, shells, and peat at paleoseismic and archeological sites. Radiocarbon dating of charcoal and shells interstratified between lakebeds led Waters (1983) to recognize four lacustrine intervals between AD 700 and AD 1700. The dates obtained from the shells have been inferred to be unreliable due to the old carbon reservoir effect and thus, resulting estimates of lake ages have been disagreed upon by subsequent researchers. Following this, a study of the Superstition Mountain fault by Gurrola and Rockwell (1996) recognized six lake highstands of which five were dated. The samples of shoreline peat and detrital charcoal produced ages of AD 817 to AD 964 for Lake 5 and AD 1687 for the most recent desiccation of Lake 1. Philibosian, Fumal, & Weldon (2011) was also able to distinguish six late Holocene lakebeds. Dates determined that the earliest highstand occurred around AD 800 and that the most recent desiccation happened by AD 1700. In 2012, a study by Haaker and Rockwell showed that the most recent lake was actually after AD 1700 by dating ancient tree and bush stumps in coppice dunes drowned by lake sediments: they resolved that the last highstand of Lake Cahuilla post-dates AD 1710 but must have started to desiccate by the mid-1720’s to allow time for evaporation of the lake by the time Anza came through in AD 1774.

Figure 9. Map of Lake Cahuilla shorelines. Image from Cleland, York, and Johnson (2001) and modified by White (2016, in progress).

CHAPTER 2

METHODOLOGY

This chapter describes the field methods used for site selection, trenching excavation, data collection and interpretation. Also discussed are the methods used for radiocarbon sample collection and processing. Further supporting data and results can be found in the subsequent chapters.

FAULT TRENCHING METHODS A trench was excavated sub-perpendicular to the eastern strand of the Imperial fault at a step-over in the region of the large 1940 displacement near the U.S. – Mexico international border. The chosen trench site was situated on an access road within an agricultural field between Gunterman Road and the Alamo River at the WGS84 coordinates 32N41’10.67’’ and 115W22’08.33’’ (Figure 10). The access road is 5.5 m wide and sits just 35 cm above the current agricultural field. A backhoe was used to excavate the 25 m long slot trench that was extended to the depth of groundwater at 3.5 m (Figure 11). The length of the trench was restricted in the west due to a drainage pipe that empties into the Toland Irrigation Canal just north of the access road. To gain a more complete view of the entire step-over region, a second trench was excavated from the western side of the drainage pipe across the western fault strand. Information regarding this trench can be found in Andy Jerrett’s (2015) Master’s thesis entitled “Paleoseismology of the Imperial Fault at the US- Mexico Border and Correlation of Regional Lake Stratigraphy Through Analysis of Oxygen/Carbon Isotope Data.” The Imperial Irrigation District (IID) owns this land, so a permit was required for operations to begin. Because this was adjacent to an active agricultural field, the trench site was barricaded and flagged to prevent injury to those working and operating vehicles in the field. The permit also required that aluminum hydraulic shores be used to prevent the trench

16 walls from caving in. Per OSHA standards, the shores needed to be at a spacing of 1.5 m in saturated soil zones and double stacked when past a depth of 2.7 m. Fortunately, the soil was not saturated.

Figure 10. Google Earth image of trench location. Trench 1 represents trench excavated for this study while Trench 2 denotes the trench opened for Jerrett (2015).

The potential trench site was recognized while remapping the 1940 surface rupture on aerial imagery (Rockwell & Klinger, 2013). The remapping effort employed low altitude, stereo-paired aerial photographs that were taken soon after the earthquake along a 15 km stretch of the Imperial fault north of the border. The photographs revealed that a sag pond was produced at a ~30 meter right step on the fault (Figure 12). Six meters of displacement passed through the sag, based on offset of crop rows. It was also found that crop furrows were dragged down into the sag, further indicating that this feature was created during the

1940 event (Rockwell & Klinger, 2013). Additionally, a suite of aerial photos from 1937 to 1979 were acquired from the USDA and IID and were compared to the 1940 images. Of interest was the 1937 photo that showed no deformation or displacement in the existing agricultural field. This indicates that the sag pond and offset crop rows were created after 1937 and were almost certainly entirely formed during the 1940 event.

Figure 11. Photos taken during excavation of the trench. The photo on the left is showing the backhoe excavating the eastern side of the trench. The photo on the right is of inside the trench and shows spacing of hydraulic shoring.

Figure 12. (Upper right image) 1940 aerial image of sag pond. Figure 12. (Lower left image) Comparison to 1937 aerial image where there is no apparent sag feature. Image from Jerrett (2015)

This location was also considered ideal in that it has the potential for excellent stratigraphy. Lakebed sediments from ancient Lake Cahuilla tend to be laterally extensive, show episodic deposition, and commonly contain organic material (charcoal, peat-like horizons) necessary for radiocarbon dating. Lastly, there have been two previous trenching studies done below the lake shoreline and in close proximity to this trenching project. This should allow for easy correlation of the stratigraphy between sites. Backhoe excavation for the “Border site” was completed on February 10th, 2015. Following this, scrappers were used to clean and flatten the trench face to expose the alternating sequences of deltaic and lakebed deposits, which were immediately apparent. The massive clays derived from lake sediment deposition proved difficult to scrape, especially after having dried for a couple of days. This caused the trench face to be somewhat irregular. Once the trench face was entirely cleaned, artist brushes and sculpting tools were used to

remove dust and etch out important features such as laminations, krotovina, seismites, and small fractures. A 50 cm horizontal by 1 m vertical grid was fashioned on both the north and south trench faces in order to digitally mosaic the trench. This grid was created by emplacing nails at the designated intervals and then lining string both horizontally and vertically. The nails were labeled to keep track of the exact location of the corners of the panels, and string was separated by color; neon pink and green to represent horizontal for the north and south walls, respectively, and white string for all verticals. At this point, samples of charcoal and other detrital material were collected in small vials and labeled. Sample locations were also marked with orange flagging on the trench wall and logged into the note book. Photos of the trench were then taken with a high resolution Nikon DSLR camera and later uploaded into Adobe Photoshop. The trenching team of Thomas Rockwell, Yann Klinger, Neta Wechsler, Petra Štěpančíková, Andy Jerrett and myself corrected and rectified the images based on the dimensions from the grid and mosaicked them together to form a single large image for each wall. The large trench wall mosaics were printed on a plotter and used in the field for interpretation. When the team was finished with interpreting the exposed trench face, shores were moved so the process could be repeated for the parts of the trench that were hidden behind the shores. Once both trench walls were completed, the photo mosaics were exported into Adobe Illustrator where final logging and interpretations were added.

RADIOCARBON SAMPLE COLLECTION AND PROCESSING A total of 52 samples were collected from both the north and south walls of the trench for radiocarbon dating. Care was taken while extracting the samples so as to not contaminate them. This was done by taking a vial and pressing it straight into the trench wall to capture the target sample without touching it. Most of the samples were composed of single chunks of charcoal that were recovered from both the clayey lake and silty deltaic units. Other samples included detrital material such as twigs and old carbonized roots, as well as some fragments of mollusk shells found only within the upper deltaic deposits. The samples were taken to the Quaternary Research lab at San Diego State University for initial cleaning and processing. Samples that met the determined criteria for radiocarbon dating, those with an organic content of 5 mg or greater, were carefully divided into two

small vials of which one would be sent in for further processing while the other remained in the lab as an archive sample. Chosen samples were taken to the Keck Carbon Cycle AMS laboratory at the University of California, Irvine. Here, samples were first cleaned with acid and base washes to remove contaminants and secondary organics. Following this, samples were heated and combusted to isolate carbon dioxide which was then reduced to graphite in a reaction with hydrogen in the presence of an iron powder catalyst. The graphitization lines at the Keck Carbon Cycle AMS preparation lab each have 12H2/FE reduction reactors where samples have demonstrated 0.3% precision and 55,000 year backgrounds (W. M. Keck Carbon Cycle Accelerator Mass Spectrometer, 2014). The final graphite sample is then pressed into a wheel and run through a compact AMS particle accelerator. It is important to note that the radiocarbon dates from these samples may not represent the time of deposition of the host strata. Charcoal inherently returns older dates, as it is representative of the growth and death of the tree rings that comprise the charcoal sample. A significant amount of time may occur between the death of the dated tree rings, the burning of the tree wood, as well as the time before the sample is transported and buried in the delta or lake sediments. Considering that sediment in the Salton Trough is carried in from the Colorado River, charcoal could have been transported and deposited a long distance from its source. Further, the Colorado basin has very old living trees that may be as old as 6,000 years (bristlecone pines), which emphasizes the potential amount of age inheritance that may be present because the heartwood of these living trees will date back to their time of growth (and death, as they are annual rings). This lends to dates on charcoal yielding a maximum age of deposition. On the other hand, dating of burned or carbonized root material that is mistakenly thought to be charcoal may lead to young dates in old stratigraphy.

CHAPTER 3

LOCAL STRATIGRAPHY

The following chapter discusses the stratigraphic units that have been described during previous studies in the Salton Trough region. This mainly entails descriptions of the alternating sequences of deltaic and lake deposits from the periodic filling of Lake Cahuilla. Further, this chapter will describe and discuss those units found in the trench at the Border site. A stratigraphic column with complete lithological descriptions can be found on Plate 1 (see appendix at SDSU media center).

STRATIGRAPHIC UNITS DESCRIBED IN PREVIOUS INVESTIGATIONS As stated previously in Chapter 1, Prior Work on the Chronology of Ancient Lake Cahuilla, the Salton Trough periodically filled to create a freshwater lake. The episodic nature of lake inundation and desiccation is easily distinguished as alternating sequences of either massive or interbedded clays or fluvial and deltaic deposits. These are clearly visible in open exposures of washes and arroyos throughout the Salton Trough, as well as in trenches during paleoseismic investigations. From recent studies, six lake highstands have been documented to have occurred during the past 1200-1400 years (Rockwell et al., 2011). Although the lakes are laterally extensive, lake deposit thickness and composition are highly variable throughout the basin based on their proximity to certain fluvial sources. For example, those sites closest to Carrizo Wash will exhibit thinner clay lenses interbedded with coarser-grained clastic sediments as the wash drains the Fish Creek and Vallecito Mountains and the Cuyamaca Range. In contrast, lake deposits that are in close proximity to the Colorado River will have high clay content promoting the accumulation of massive clay beds. Some areas that are distant from major drainages have no Holocene sediments at all.

The lake deposits from the two most recent highstands of Lake Cahuilla tend to be relatively thin compared to other highstands. This would suggest a rather quick period of infilling and desiccation. A number of studies record thicknesses ranging from ~25-50 cm for each lake (Ragona, 2003; Rockwell et al., 2011; Sharp, 1980; A. P. Thomas and Rockwell, 1996). Comparatively, the thickness of lakes 3, 4, and 5 are observed to be around 50 cm to more than 1 m. Clay beds within the lakebed deposits vary in size depending on location with respect to their source. Trenching studies near the international border have exposed massive, blocky clays as this locale is close to the apex of the Colorado River Delta (Figure 13). These clay deposits typically display alternating colors from dark brown to reddish-brown or from blueish-grey to brown. In contrast, lake deposits that were observed at the Dogwood and Harris Road trench sites farther north are interbedded with clay, fine grained sand, and laminated silt. Generally the clay beds tend to be between 5-10 cm in thickness. These sites are close to the New River delta, where clayey sediments from the Colorado River are still transported, but not at the same volume as at the international border sites. Channel sands have been found at numerous sites and have been useful in determining lateral displacement along faults. These channel deposits can represent either channelized flow during periods of desiccation or incision during an initial infilling of the lake, and they are composed of coarse-grained sands with some thin clay and silt laminations. Inherently, those sites in close proximity to modern washes or river channels will have a higher occurrence of these channel deposits. A paleoseismic trenching study in Carrizo Wash documented over ten fluvial units that were incised into both lakebed and deltaic deposits (Ragona, 2003). The deposition of deltaic units in this region is again variable based on proximity to certain rivers. Delta deposits from the Colorado River are strictly due to progradation of the river delta, because when the river switches direction back to the Gulf of California, no more sediment deposition occurs. In contrast, delta deposits from other rivers flowing into the basin will form from both progradation and retrogradation of the respective river deltas following the ebb and flow of lake level and the occurrence of regional storms. Deltaic deposits have been seen to have alternating beds of orange-tan silt and grey-blue tabular clayey silt, and typically range in thickness from 20-50 cm.

Figure 13. Comparison of lake clay deposition between sites. The A. P. Thomas and Rockwell (1996) site is in close proximity to where the apex of Colorado River Delta would have been during infilling of Lake Cahuilla. This would promote the deposition of thick clays during lake highstands. Rockwell et al. (2011) or the Dogwood road site is located near the New River where clay deposition will be thinner.

UNITS EXPOSED AT THE INTERNATIONAL BORDER SITE The trench at the international border site exposed ten main units, five of which are interpreted to represent Lake Cahuilla deposits from the three most recent cycles of lake inundation and desiccation. Liquefaction sand dikes and lateral deposition of liquefaction- related sand deposits were also present at this site. Below are the detailed descriptions of these units, which are also described in the stratigraphic column on Plate 1 with corresponding unit numbering.

Unit 10 - Artificial Fill Unit 10 is classified as artificial fill and ranges in thickness from 1.5 m within the sag depression to about 75 cm just outside of the depression. It can generally be described as a

beige- colored, poorly sorted sand with pebble sized clasts, and many of the clasts are rip-ups of local bedded strata. We interpret that this fill unit was emplaced to fill the depression created from down-warping during the 1940 event and was extended above the current elevation of the agricultural field for construction of the current access road. Between the 15 and 17.5 m marks on the trench logs on both the north and south trench walls (Plates 2 and 3; see appendix at SDSU media center), there is a large excavation cutting down to the third lake which has been backfilled with this artificial fill. It is interpreted to be an old trench excavation because the walls appear to be almost vertical and there is an accumulation of clay chunks and debris lining the bottom. The purpose of this trench is unknown but may have been used to drain the sag of water after the 1940 earthquake to facilitate the construction or restoration of the road.

Units 50 And 100 - Plow Pan Plow pan deposits are anthropogenic in nature and are the consequence of agricultural plowing. Units 50 and 100 are interpreted to be plow pan deposits and are described as moderately sorted, fine grained silty sand units that are slightly indurated. Both units are only preserved within the sag feature and follow the warped nature of the dropped down area, indicating that they were active field surfaces prior to the time of the 1940 rupture event. There is a very localized organic burn layer, labeled unit 25, capping unit 50 (Figure 14). Current agricultural practices show that farmers often burn piles of plant and crop material along the edges of the fields at the end of the growing season. We interpret this burn horizon as such a layer, which further supports the interpretation that unit 50 is in fact a plow pan deposit. Unit 75 - Flood Deposits From 1905-1907 Unit 75 is a beige, finely laminated silty sand stratum that is positioned between the two plow pan units. It is only found within the sag feature where grading during the emplacement of unit 50 likely removed much of the deposit outside of the sag. This deposit is interpreted to result from inundation of the agricultural fields during the 1905-1907 flood that produced the Salton Sea, because this site is less than 250 m from the Alamo River and flooding adjacent to the Alamo River was extensive (Cory, 1915).

Figure 14. Zoomed in image showing the burn layer (unit 25) positioned above unit 50. This suggests that unit 50 is a plow pan deposit.

Units 120-150 And Unit 297 - Liquefaction Sands There are two sections that are interpreted as sand deposition resulting from liquefaction, hereafter referred to as liquefaction sands. The first such stratum, units 120-150, is situated just below the plow pan, unit 100, and above Lake 1, unit 200. It is a tan, moderately well-sorted, fine grained, friable sand that is finely laminated. There are also a number of krotovina (filled animal burrows) that perforate this unit, as well as cementation of some of the laminations. This deposit is interpreted to be derived from the upward movement of the deltaic sediments from below Lake 1 during strong ground shaking. This is inferred from the observations of collapse of a number of segments of Lake 1 into the deltaic sediments below. The second stratum interpreted to result from liquefaction of delta deposits is unit 297, which is situated above Lake 2. It is lithologically similar to the liquefaction sand of units 120-150 and appears to have emanated from sand pipes that penetrate through Lake 2 from the lowest exposed deltaic unit.

Units 200, 300, And 400 - Lakebed Clays Three of the most recent Lake Cahuilla lakebeds are exposed in this trench. Units 200-210 are interpreted to represent Lake 1, which is a 15 cm-thick clay stratum overlying a

5 cm-thick grey silty clay base. The top of the clay is dark grey resulting from the incorporation of organic material and also contains some detrital charcoal fragments. A reddish-brown clay lies below the organic-rich layer which in turn gives way downward to the grey silty clay. This deposit is only preserved within the local sag depression and pinches out towards the east. It was likely present prior to construction of the road, but deep grading of the native sediments disturbed all units to a depth of about 60 cm below the elevation of the field. Unit 300, interpreted as Lake 2, is a massive, reddish and dark grey clay that is about 1 m in thickness. What differentiates this lake bed from the other two is that it exhibits root casts and iron oxide staining that progressively increase in density up section and terminate at the top of the clay, indicating that the clay sat at the surface for a period of time. As stated above, this lake bed has a number of sand dikes, interpreted as liquefaction features, extending through to the top of the unit. These dikes are filled with tan, fine-grained, well- sorted sand. Towards the bottom 50 cm of the clay, there are laminar beds of similar fine grained sand. Finally, unit 400 is interpreted to represent Lake 3 and is exposed in the lowest portion of the trench, extending from the outer edge of the sag pond to the farthest eastern portion of the trench. This clay stratum exhibits alternating beds of reddish brown (units 400 and 404) and grey colored clays (units 402 and 405). Sand blows are also seen to pierce through this lake deposit as sand-filled dikes.

Units 215-265, Units 270-295, and Units 350-365 - Deltaic Sediments There are two sections of deposits in the trench that are interpreted as resulting from deltaic deposition during progradation of the delta, as this site is in close proximity to the Alamo River, which is one of the primary pathways used by the Colorado River during inundation of the Cahuilla Basin. The units interpreted as delta sediments are laminated fine sand and silt strata that are laterally extensive (not channelized) but contain only minor clay (i.e., they are quite different from the fat clay strata interpreted as lake deposits). The first delta section is situated between units 200 and 300 and is divided into upper and lower sections. The upper deltaic section comprises units 215 to 265 and consists of alternating

27 beds of orange silt with grey clayey silt with bed thicknesses varying from 2 to 10 cm. Unit 225 is a thick, grey clayey silt stratum that, in most areas, directly underlies unit 200 and shows several deformed layers interpreted as seismites towards the western side of the trench within the sag depression. There is an interesting geologic feature within this segment that can be observed between the 9 and 11 m marks on the southern trench wall (Figure 15). This feature is interpreted to represent strike-slip movement along a cross-fault located at the 9 m mark, where angled foreset beds are now juxtaposed against flat lying deltaic sediments.

Figure 15. Foreset beds that have been laterally displaced and are now juxtaposed against flat lying deltaic sediments.

The lower deltaic section is composed of units 270 to 295. This section exhibits the same alternating orange and grey colored sequence of fine sandy silt and silt with a general compositional trend towards a more clayey consistency. A second section of deltaic deposition is interpreted to correspond to units 350 to 365 and is positioned between units 300 and 400. Bedding in this section is thicker than in the upper deltaic section (units 215-295), where beds exhibit an average thickness of 10 cm. This section is only observed outside of the sag feature, as it is presumably too deep in the sag and is likely preserved below the base of the trench. Units 350 to 365 pinch out to the west beneath unit 300.

CHAPTER 4

AGES OF LAKE CAHUILLA SEDIMENTS

The purpose of this chapter is to discuss the age constraints of Lake Cahuilla sediments based on: (1) new research relating paleoclimate data to periods of lake inundation and desiccation; (2) historical accounts from California expeditions, and (3) radiocarbon dates obtained from the trench site that are correlated to dates from the regional chronologic model of Lake Cahuilla.

LAKE INUNDATION AND DESICCATION BASED ON PALEOCLIMATE We can constrain the dates of inundation and desiccation of Lake Cahuilla by analyzing the Late Holocene climate record of the Colorado Plateau and Basin. Recent work by Rockwell (2016) shows that there is a strong correlation between the dates of lake inundation, as established through radiocarbon dating and archeological findings, and periods of high precipitation in the Colorado Basin (Plate 4; see appendix at SDSU media center). Not only is there a relationship between longer periods of precipitation with the longer lake highstands (represented by the thick lake deposits seen in paleoseismic trenches), but there is also a correlation between the smaller partial lake infillings observed by explorers and shorter lived periods of high precipitation. Furthermore, Rockwell (2016) found that there is a correlation between rates of discharge of the Colorado River and precipitation, suggesting that high precipitation leads to high discharge rates, which lead to the possibility of major flooding events along the Colorado River. It was suggested by Waters (1983) that avulsion of the Colorado River into the Salton Trough could have been triggered by either frequent large floods or by tectonic movement. The recent correlation by Rockwell et al. between high precipitation and lake inundation points to the probability that flooding more readily caused diversion of the Colorado River.

Furthermore, many of the large San Andreas and Imperial fault earthquakes appear to have occurred during the highstand of the lake, which implies they may have been triggered by the filling events (Brothers, Kilb, Luttrell, Driscoll, & Kent, 2011; Philibosian et al., 2011; Rockwell, 2016). Based on current climate conditions, Wilkes (1978) calculated that it would take 10 years for the lake to completely fill (provided that the entire flow of the Colorado River was captured) and 60 years for evaporation to cause desiccation of the lake once the Colorado River switched back to its normal route to the Gulf of California. Additionally, Waters (1983) calculated how stable the lake would be based on how precipitation, runoff, and evaporation would affect the total volume of the basin. He used a calculation derived from Snyder and Langbein (1962) and Leopold (1951) where he determined a total gain of water at 2.04x104 hm3/yr and a loss of 1.02x104 hm3/yr which points to a change in volume of +1.02x104 hm3/yr. This suggests that only half of the flow from the Colorado River would be needed to maintain a lake with any additional discharge flowing over the delta and towards the Gulf of California. This indicates that the Salton Trough is fully capable of sustaining stable lake highstands provided there is continued flow into the trough from the Colorado River. However, when the lake is full, the gradient to the Gulf is steeper, which would favor an avulsion back to the original course of the Colorado River if any water was flowing in that direction.

HISTORICAL CONSTRAINTS Historical accounts from early expeditions to the southern California region can also assist in constraining highstand ages of ancient Lake Cahuilla. Separate reports from Alarcon and Melchior Diaz in AD 1540, Juan Oñate in AD 1603, and Eusebio Francisco Kino in AD 1700, all travelling along the Colorado River, made no mention of a freshwater lake (Lippincott, 2007). The expeditions of Anza in AD 1775 to 1776 also suggested the absence of a lake (Bolton et al.,1930; Lippincott, 2007). Based on the hydrologic regime calculations by Wilkes (1978), is it very possible that the basin could have had ten years to fill starting immediately after Kino’s expedition, with sixty years to desiccate producing a newly dried lake bed by the time of Anza’s arrival. It is also very possible that there was still a lake in existence for all of these accounts; however elevations did not permit the explorers to view the lake from their location.

Lippincott (2007) calculated the visibility that Kino would have had when he reportedly climbed 1,000 m up Mt. Pinacate to assess his surroundings. Given perfect visibility, he would only have been able to see as far as 133 km. This is because of the “just beyond the horizon” effect, where the curvature of the earth and the refraction of light allows for only 33 m of visibility at ground surface. With the nearest Lake Cahuilla highstand shoreline at a distance of 190 km from the Pinacate Mountains, Kino would not have been able to see a lake even if it was at its highest level. Considering that all other expeditions, except for Anza, remained along the Colorado River, they too would not have been able to visibly observe an existing lake. Another commonality between the accounts is that they all observed that the Colorado River was flowing south into the Gulf of California based on strong currents which made it difficult for their ships to progress north from the gulf. This suggests that water flow from the Colorado River was cut off to the Salton Trough at those times, which means that at least partial desiccation of the lake had already begun prior to arrival of the explorers.

RADIOCARBON DATING OF DEPOSITS IN THE BORDER TRENCH Of the 52 samples that were taken from the trench, only 20 charcoal samples met the 5 mg organic weight criteria for lab processing. This weight criterion is important in that initial cleaning of the sample with acid and base washes can reduce the weight of the sample significantly, resulting in little to no sample left for further processing. Additionally, smaller sample sizes can result in higher uncertainties. Table 1 presents those samples processed at the KECK Carbon Cycle AMS facility with bolded ages signifying the best fit dates. Two samples were lost due to very small sample sizes. Of the final 18 samples, the majority had uncertainties ranging from ± 15 to ± 20 years, four samples resulted in ±70 to ± 90 year uncertainties, and one very small sample yielded an uncertainty of ± 230 years. As stated in Chapter 2, Methods, the dates of these charcoal samples represent the maximum age of deposition due primarily to: (1) old living trees in the Colorado basin, and (2) deposition of sediments long distances from their source. Because of this, many of the radiocarbon samples yielded very old dates that are not representative of the age of their host strata. From this we decided to highlight only those samples with the youngest dates, especially those that agree with previous work done on the ages of Lake Cahuilla highstands.

Table 1. Imperial Border Trench Radiocarbon Sample Ages Uncalibrated 14C Calibrated Calendric Stratigraphic Sample # Age, 2σ Max-Min Date Probability Unit Years B.P. Range AD 1669-1706 0.167 AD 1719-1780 0.341 IBT-3 Fill 145 +/- 20 AD 1798-1819 0.115 AD 1832-1883 0.188 AD 1914-1945 0.188 AD 1494- 1601 0.785 IBT-12 135 315 +/- 20 AD 1616-1644 0.214 IBT-24 200 1890 +/- 230 398 BC-AD 610 1.00 AD 908-912 0.011 IBT-25 200 1060 +/- 15 AD 969-1019 0.989 AD 1522-1575 0.406 IBT-47 200 275 +/- 20 AD 1626-1664 0.594 AD 1311-1359 0.722 IBT-35 230 585 +/- 15 AD1387-1408 0.278 AD 662-903 0.925 IBT-28 275 1230 +/- 70 AD 918-965 0.075 IBT-8 275 835 +/- 20 AD 1166-1253 1.00 IBT-10 280 -305 +/-20 N/A N/A IBT-30 280 3275 +/- 20 1611-1506 BC 1.00 2196-2171 BC 0.016 IBT-29 280 3580 +/- 90 2146-1691 BC 0.984 AD 896-926 0.045 IBT-20 300 990 +/- 70 AD 942-1208 0.955 IBT-49 350 560 +/- 70 AD 1286-1446 1.00 AD 1015-1058 0.45 IBT-50 350 975 +/- 25 AD 1069-1071 0.001 AD1075-1154 0.594 AD 1490-1602 0.808 IBT-15 355 330 +/- 15 AD 1613-1637 0.192 AD 1277-1308 0.657 IBT-17 365 675 +/- 20 Ad 1361-1386 0.343 (table continues)

Table 1. (continued) Uncalibrated 14C Calibrated Calendric Stratigraphic Sample # Age, 2σ Max-Min Date Probability Unit Years B.P. Range AD 1647-1668 0.612 IBT-18 400 (upper ) 230 +/- 15 AD 1782-1797 0.377 AD 1949-1950 0.011 IBT-2 400 (Mid) 2700 +/- 20 897-811 BC 1.00 1) All samples were single chunks of charcoal or dark organic material that looked like charcoal. 2) Stratigraphic Units. See Stratigraphy Chapter for more details. 3) All results were corrected for isotopic fractionation according to the conventions of Stuiver and Polach 1977. 4) Uncorrected 14C ages were dendrochronologically calibrated using Calib 7.1 based on Stuiver and Reimer (1993) 5) Relative area under 2 probability distribution

We found that the most representative age for unit 200, or Lake 1, was 275 ± 20 years B.P. resulting in a 2σ calendar age of AD 1626-1664. The charcoal sample came from the upper portion of Lake 1 implying that it was deposited close to the age of desiccation of the lake. However, as trees don’t grow in lakes, nor do they burn under water, it most likely that this sample was derived from a bush or tree that grew during the previous dry period before lake inundation, or even an earlier period, or came from beyond the lake shoreline. Previous dating of charcoal and tree stumps from other paleoseismic studies placed dates of lake desiccation within the early 1700’s (Haaker & Rockwell, 2012; Philibosian et al., 2011). The best fit date for the deltaic sediments between Lake 1 and Lake 2 came from a sample collected in unit 230. This sample returned a 14C date of 585 ± 15, which calculates to a calendar age of AD 1387-1408. Unfortunately, there was only one charcoal sample that was collected for Lake 2 which resulted in a very old 14C age of 990 ± 70 B.P. or AD 942-1208. Dates from previous studies as well as from archeological data suggest that this lake desiccated by AD 1680, pointing to a large amount of age inheritance for the samples collected for this study. We did not distinguish a representative date for Lake 3 as the two samples that were collected from the lake yielded both an extremely old age (2700 ± 20 B.P; BC 897-811) and a relatively young age (230 ± 15 B.P; AD 1647-1668/ AD 1949-1950). The sample that produced the young date is positioned directly underneath the area that was previously trenched and filled during some previous excavation that was possibly associated with the post-1940 reconstruction of the site and road. This location in Lake 3 is also very

fractured with some fissure fill. We interpret the young age to possibly be a result of either fissure fill from above, or from disruption during the excavation. One of the samples from the trench (sample 10) resulted in a “future date” yielding a 14C age of -305 ± 20 B.P. We have several theories as to why the sample produced this date. The first is that there was possible contamination of the sample either during collection or during lab processing. This is an unlikely scenario considering that none of the samples were directly handled in the field and were only touched by clean vials and tweezers. All of the samples that were taken to the lab were processed in batches. If one of the samples were contaminated at some point during the process, it is likely that others would have been as well, which was not the case. The second possibility is that the sample could have been dislodged from a higher stratigraphic unit during backhoe excavation and smeared along the trench wall. This was not evident when the sample was collected. Finally, it may be that the sample is charcoalized root material and we didn’t recognize the sample as such. In any case, the date is obviously too young as it is not possible that any of the native strata are post- bomb (ca 1950).

DISCUSSION OF AGE DATA Combining all associated Lake Cahuilla age data can reveal not only the dates of regional inundation and desiccation of the lake, but the dates at which we should expect to see those lake cycles in deposits at our site. This is based on using the Wilkes (1978) inundation and desiccation rates to calculate how long it would take to either fill or shrink the lake level to our site elevation. Work by Rockwell (2016) established a comprehensive, long- term chronology of Lake Cahuilla which provides us with an easy correlation to known lake dates from all previous studies (Plate 5; see appendix at SDSU media center). From this, Lake 1 shows a 95.4% probability of being between AD 1710 and AD 1734. Work by Haaker and Rockwell (2012) shows that the lake had to have formed after AD 1710 based on growth of tree stumps, and paleoclimate evidence points to a period of high precipitation starting in AD 1719. From this we infer that lake inundation started after AD 1710, but continued no later than AD 1725, because Anza’s arrival in AD 1775 suggested that there was no lake. By constraining the date of inundation to AD 1725, we are allowing for enough time for filling of the lake to its highstand elevation, but this date is only 50 years before

Anza’s expedition, which would mean that the lake would not have fully desiccated (if using Wilkes’ rate of evaporation of 1.8 m/yr). A possible explanation for a fully dried lake bed within 50 years would be a higher temperature causing faster rates of evaporation. However, continued work on the relationship between temperature and evaporation rates in the Salton Trough would be needed to prove this theory. Our trench site is at an elevation of 8 m above sea level, which is a 5 m difference from the Lake Cahuilla highstand elevation. This suggests that it would take almost the full ten years of inundation for the lake level to reach our site, placing the initial deposition of lake deposits anywhere from AD 1720 to AD 1725 (Figure 16). Because we are assuming that the lake had started to dry out by the time of Anza’s arrival in AD 1775, the lake would have had to start to desiccate immediately after it filled to its highstand. Using Wilkes’ 1.8 m/yr rate for lake desiccation, we get a period of 3 years that is needed to drop the lake level from the highstand elevation to 8 m elevation. This would imply that our site would have been completely dry sometime between AD 1723-AD 1728. We interpret that the Lake 2 inundation began around AD 1640 and infilled until AD 1650. This is based on climate data which shows a period of high precipitation starting in AD 1640 as well as radiocarbon data from Rockwell et al. placing the 2σ probability between AD 1647 and AD 1669. We place more emphasis on the climate data for the initial date of filling because it has a better fit with our hypothesis on Lake 2 desiccation, which is constrained by the radiocarbon date of a tree stump which grew after drying of the lake. The stump collected at an elevation of -16 m by Haaker and Rockwell (2012) yielded radiocarbon dates of AD 1654 – AD1680, of which we estimate AD 1668 to be the apex of the probability curve. This suggests that the lake level was below this elevation by around AD 1668 to allow for the growth of the plant. We calculated that it would take 16 years for the lake to desiccate to this elevation, suggesting that the latest that the lake could have started to desiccate was around AD 1652. From this, we suggest that our site would have been inundated by the lake around AD 1650 with desiccation starting between AD 1653 and AD 1655 (Figure 17). Age constraints for the third lake highstand are more varied than the two previously mentioned lakes, with 2σ radiocarbon dates ranging from AD 1484 to AD 1579, in part because of a double peak in the calibration curve. Consequently, we have developed three

35 different scenarios for lake inundation as well as two scenarios for lake desiccation (Figure 18). The first scenario for inundation proposes infilling of the lake anywhere from AD 1484 to AD 1494, which is based on both the radiocarbon dates and an increased period of precipitation. We suggest that the second scenario of inundation could have been from AD 1508 to AD 1518 due to another period of high precipitation immediately following the latter. In AD 1540, accounts from Alarcon note that the flow of the Colorado River was exiting through the Gulf of California. If this is true, desiccation of the lake would have commenced by the time of Alarcon’s arrival. The third scenario for lake inundation aligns with the next period of high precipitation with possible infilling starting between AD 1548 to AD 1570. The corresponding time of desiccation for this scenario would have to at least been by AD 1600, as a paleoseismic study at Salt Creek showed that there was a definite gap between deposition of Lake 3 and Lake 2 (Seitz & Williams, 2007). Salt Creek is at an elevation of -61 m, which equates to 40 years of lake desiccation needed for the area to have dried up. Because the data indicates that Lake 2 started around AD 1640, then the Salt Creek site would need to have a dry lakebed by that time.

Figure 16. Suggested period of inundation for Lake 1 at Border Trench site overlaid on precipitation plot. Based on radiocarbon dates from the regional chronologic model of Lake Cahuilla, paleoclimate, and historical accounts. Inferred that the site would start to inundate with water around AD 1729 with it drying up around AD 1732. Precipitation plot from Rockwell (2016).

Figure 17. Suggested period of inundation for Lake 2 at the Border Trench site overlaid on precipitation plot. Based on radiocarbon dates from the regional chronologic model of Lake Cahuilla, paleoclimate, and historical accounts. Inferred that the site would start to inundate with water around AD 1650 with it drying up from around AD 1653 to 1655. Precipitation plot from Rockwell (2016) layout.

Figure 18. Different scenarios for inundation and desiccation of Lake 3. Based on radiocarbon dates from the regional chronologic model of Lake Cahuilla, paleoclimate, and historical accounts . The first inundation scenario could have taken place between AD 1484 to AD 1494. Scenario 2 could have been between AD 1508 to AD 1518. For either of these scenarios, the lake would have to at least start to desiccate by the time of Alarcon’s arrival in AD 1540. The third scenario of inundation could be from AD 1548 to AD 1570. If this were the case, the lake would have to start to desiccate by AD 1600 to allow for a depositional gap of lake deposits that was seen at the Salt Creek site. Precipitation plot from Rockwell

(2016). 38

CHAPTER 5

IDENTIFICATION OF EARTHQUAKE EVENTS

This chapter discusses the evidence for surface ruptures exposed at the border trench site and attempts to correlate those events to ones recognized during previous trenching studies. During trench logging and observation, we aimed to identify any of the six general types of evidence for typical recognition of paleoearthquakes on strike-slip faults as defined by McCalpin et al. (1996). These include: (1) upward termination of fault displacement, (2) abrupt changes in vertical separation of strata as faults are traced upsection or downsection, (3) abrupt changes in thickness of strata or of facies across a fault, (4) fissures and sand blows in the stratigraphic sequence, (5) angular unconformities produced by folding and tilting, and (6) colluvial wedges shed from fault scarps. We also use our observations of the development of accommodation space, as the site is a small sag depression that sinks during large earthquakes, as it did in 1940.

EVIDENCE FOR FAULTING EVENTS AT THE BORDER SITE In total, we were able to identify evidence for five events in the trench based on upward termination of fault displacement and fissures, the production of accommodation space and associated growth strata, and massive liquefaction deposits. Below are the detailed descriptions of the observable evidence for these events, which are presented in order of increasing age. Trench logs for the north and south trench faces are found on Plates 4 and 5 (see appendix at SDSU media center), respectively.

Event 1 Event 1 is interpreted as the most recent faulting event seen in the trench. Because excavation and fill emplacement needed for the agricultural field and road have removed or mixed at least the top 90-95 cm of stratigraphy, we were not able to observe any evidence for

the 1979 rupture, although Sharp (1982) does not record observations that this event ruptured this far south at the surface. Evidence for the 1940 rupture, however, was marked by the down warping of plow pans (units 50 and 100) within the sag depression (Figure 19). Based on the comparison of 1940 and 1959 aerial images with Google Earth, it was deduced that this site was, in fact, within the bounds of the agricultural field instead of on the access road as it is today (Jerrett, 2015; Figure 20). This reveals that plow pan deposits would have been at the surface in 1940. Because both plow pan deposits follow the general slope of the stratigraphy that’s dragged down into the depression, we can infer that they were emplaced prior to 1940 and were disrupted during the 1940 event. These deposits are now buried by artificial fill that post-dates road reconstruction after the 1940 earthquake.

Event 2 The primary evidence for event 2 is based on the observation that several fault strands terminate at the top of unit 200 (Figure 21). A number of these fault strands continue into the liquefaction sand above and are interpreted as a product of afterslip due to a significant decrease in displacement, or perhaps as minor slip during 1940. Secondary evidence for the precise positioning of Event 2 is based on a massive liquefaction sand deposit above unit 200 (Figure 22). It is inferred that the liquefaction deposit was derived from deltaic sediments below unit 200 and resulted from strong ground shaking. Additionally, sand vents penetrate through unit 200, as well as the deltaic units 215- 295 below, and caused collapse of unit 200 into the underlying sediments (Figure 23). We interpret that this event occurred when the lake was full or during the waning stages of the lake because the large lateral deposition of liquefaction sands tend to occur when near surface sediments are saturated. Additionally, the accumulation of organic material at the top of unit 200 leads us to believe that the shoreline was close to our site elevation, as detrital organic material tends to collect along shorelines.

Event 3 Evidence for Event 3 is only seen in the western trench by Jerrett (2015). Such evidence consists of fault strands that terminate at the top of unit 245, with observed growth

Figure 19. General down-warping that is exhibited by the plow pan deposits in response to Event 1. Zoomed in image of the southern trench wall. 41

Figure 20. (a) Aerial image from 1940 with an overlay of the trench site and fault strand locations from Google Earth. Note how the trenches are within the bounds of the agricultural field. (b) Aerial image from 1959 showing how the roads have been moved since 1940. Notice how there is now an access road where the trench locations are. Image taken from Jerrett (2015).

Figure 21. Fault strands terminating at the top of Lake 1. Some strands continue into liquefaction sand deposit above the lake, but minimal to no displacement suggests that these are a result of afterslip. strata on the western side of the fault (Figure 24). Vertical displacement along the two main strands appear to be minimal compared to the 1940 and penultimate events. Unfortunately, the mobilization of deltaic sediments during liquefaction from Event 2 has caused deformation and warping of deltaic beds within the sag feature, resulting in a decreased ability to discern whether there is any further evidence of faulting for Event 3 in the eastern trench.

Figure 22. Massive lateral deposition of Liquefaction sands above Lake 1. Taken from zoomed in image on south trench wall.

Figure 23. Collapse of Lake 1 deposits into deltaic deposits below. A result of mobilization of deltaic sediments during liquefaction. Zoomed in image from northern trench log.

Event 4 The primary evidence for Event 4 consists of large fissures, vertical displacement on fault strands, and sand dikes that extend through unit 300 but are capped by overlying unit 295 (Figure 25). At the eastern end of the trench, these sand dikes lead upward to a well- sorted sand interpreted as a liquefaction deposit that is present above unit 300; this unit has been, for the most part, scoured out by the placement of artificial fill (Figure 26). Within the local depression, the fissures and sand dikes are capped by the deltaic units 220-295 signifying that the top of unit 300 was at the surface during the time of the rupture event. Around 25 cm of vertical displacement was seen on unit 300 in the fault zone (between the

12-15 m marks on both the north and south trench logs). It is important to note, however, that a portion of this displacement could be from movement during the more recent events.

Event 5 Event 5 is the oldest event for which was observed in the trench, and it is recognized by pervasive sand dikes and fissures within unit 400 (Figure 27). The deltaic sediments of units 350 to 365 cap the fissures and sand dikes and there was no observable evidence for the lateral deposition of liquefaction sand deposits above unit 400 preserved at this site. Units 350 to 365 thin onto unit 400 towards the fault zone, suggesting that during deposition of those units there was a structural high in this area. We interpret that the western fault block was uplifted during a locally transpressional event prior to the deposition of units 350 to 365.

CORRELATION TO PREVIOUS TRENCH STUDIES We wanted to compare event findings from this study to those seen in other paleoseismic trenches on the Imperial fault to ensure that we would gain the most complete view of the long term rupture history of the fault. We noticed a significant correlation to events seen in paleoseismic studies that were along the central portion of the fault, such as the events documented by A. P. Thomas and Rockwell (1996). We were able to correlate our stratigraphy to theirs based on similarity in the deposits from the last three lake highstands. Lakes 2 and 3 were easily correlated between the two trenches based on the deposition of thick clays; however Lake 1 proved difficult to correlate because it was of a siltier composition in Thomas and Rockwell’s trench, at least based on their unit descriptions, and there were limited photographs from the Thomas and Rockwell trench with which to compare directly with the Border site stratigraphy. We infer that the siltier nature of the sediments associated with the last lake is because the Thomas and Rockwell site is closer to the Lake Cahuilla highstand shoreline, which would have decreased the potential for deposition of massive clays (or perhaps the units were not correctly described). Once the stratigraphy was correlated, it became apparent that the same events were observed at the two trench sites (Plate 6; see appendix at SDSU media center). First, it was noted that the 1940 event affects plow pan deposits in both of the trenches. The 1940 event horizon in the Thomas and Rockwell trench does not exhibit the broad down warping that

was observed at our site (as it is not within a step-over sag area), but significant down drop of the western fault block and large fissures filled with plow pan deposit suggest that: (1) faulting was on the same order of magnitude, and (2) this was a recent faulting event. In both trenches, vertical displacement and liquefaction deposits above Lakes 1 and 2 mark Events 2 and 4, respectively, with evidence for Event 5 observed as pervasive sand dikes throughout Lake 3.

Figure 24. Evidence for Event 3 as seen in the trench along the western fault strand from Jerrett (2015). Fault strands terminate under unit 245 and 6 cm of vertical displacement is seen along the strands.

Figure 25. Fissure fill and vertical displacement as a result from Event 4. Vertical displacement would actually be much smaller than what is represented in the image as continued displacement and deformation along the fault zone from events post- dating Event 4 would further displace sediments. Zoomed in image taken from northern trench log.

Figure 26. Liquefaction sand dikes and lateral deposition of liquefaction sands above Lake 2 as evidence for Event 4. layout.

Figure 27. Large fissures terminating at the top of Lake 3 and capped by deltaic sediments from above providing evidence for Event 5. 49

In the Jerrett (2015) trench, we determined that Event 3 occurred during a lake filling phase due to fault splays terminating within a deltaic unit. In the Thomas and Rockwell trench, evidence for Event 3 is not seen within a deltaic deposit, but is instead identified by a highly liquefied silt unit between Lakes 1 and 2. We interpret that because the site is in close proximity to the shoreline, the delta would not have reached this area until the lake was almost full. There also appears to be a strong correlation between the events observed at the Dogwood Road site (Rockwell et al., 2011) and the majority of our events, with the exception of Event 3, of which there was no evidence at Dogwood Road (Figure 28). Event 1, or the 1940 rupture event, correlates to their Event 2, which was denoted by fissures within a local flood deposit. The Dogwood site has been completely undisturbed by farming, so flood deposits from the 1905-1907 flood are still well preserved at this locale. Event 3 at Dogwood, which correlates to our penultimate Event 2, is seen within the upper portion of Lake 1 (their Lake A) and shows fissures and upward terminating fault splays. Because our site is at a higher elevation, and we interpret that our Event 2 occurred during the waning stages of the lake, it makes sense that the rupture event would have occurred within the lake deposit as the Dogwood site would still have been inundated with water. In both studies, Event 4 occurs at the top of Lake 2 (Lake B at Dogwood). At the Dogwood site, this event was associated with a filled fissure and one upward fault termination, and was designated as a smaller event. We also consider our Event 4 to be a smaller event based on the minor amount of vertical displacement seen along fault strands. Conversely, Event 5 at the Dogwood site appears to be a large event based on filled fissures, liquefaction pipes, and 1.4 m of lateral slip on beheaded buried channels. We were able to correlate this event to our Event 5 based on fault splays and fissures terminating at the top of Lake 3 (Lake C at Dogwood), however, we were not able to resolve vertical separation of this event at our site due to trench depth (not deep enough). The Dogwood site also has an Event 6 that could correlate to our Event 5. This could point to one of three ideas on event correlation: (1) our Event 5 represents both Dogwood Events 5 and 6; however a lag in sedimentation inhibits us from observing Event 5 at our site, (2) our Event 5 correlates to Dogwood Event 6 with Event 5 missing at our site, or (3) our Event 5 correlates to the Dogwood Event 5 and we are

not able to observe Event 6 in our trench, again due to the depth of the trench being too shallow.

DISCUSSION

Long-Term Rupture Behavior Correlation of the large versus small interpreted events between paleoseismic sites suggests that the larger magnitude events are ubiquitous along the fault, with smaller events On this page, only being observed at some sites. Although lateral displacement is hard to determine in trenches without channels present and the associated 3D trenching to resolve the displacement, we can infer that the vertical displacement along fault strands crossing the sag feature at our site represent the general magnitude of the event because large lateral offsets should be represented by significant dropping in the step-over area. When viewing the trench “as is”, it is difficult to resolve vertical displacement per event in the fault zone due to accumulated deformation and displacement from events over time. Through creating event- by-event palinspastic reconstruction of the trench strata, we can essentially “flatten” to each event horizon to view the amount of vertical displacement that was present prior to that event. Palinspastic reconstruction for the Border trench log was done by Jerrett (2015) and shows that when flattening above the 1940 and penultimate events, the amount of vertical separation in each event appears to be similar (Plate 7; see appendix at SDSU media center). Vertical displacement for the 1940 event was measured to be 93 cm while the penultimate event showed 87 cm of displacement. Because we know that the 1940 event was large and caused 6 m of lateral displacement through the sag feature (Rockwell & Klinger, 2013), we can then infer that the penultimate event also caused a similarly large lateral offset, which we take to be around 6 m. We were also able to come to the same conclusion based on retrodeformation of the northwest trench log from A. P. Thomas and Rockwell (1996; Plate 8; see appendix at SDSU media center). Reconstruction of the vertical slip in the sag that was produced by Event 3 shows that only a minimal amount (6 cm) of vertical displacement occurred at our trench site (Jerrett, 2015). Furthermore, the only evidence for this event at the A. P. Thomas and Rockwell (1996) site is that it caused liquefaction of a silt bed, and evidence for this event was not even

52 observed at the Dogwood Road site (Rockwell et al., 2011). These observations all suggest that this was either a relatively small event or one that ruptured primarily to the south and terminated close to our Border site. Evidence for Event 4 also appears to indicate that this was a small event at the Border trench site based on retro-deformation of the Thomas and Rockwell trench log, which resulted in 13 cm of vertical displacement. However, that site just south of the border was not in a significant step, so vertical separation is not a good indicator of the amount of strike-slip that may have occurred. The lack of significant vertical motion in the sag at our Border site, however, is consistent with relatively small displacement in the border area, and the A. P. Thomas and Rockwell (1996) observations support that this was a rupture of the Imperial fault and not liquefaction attributable to rupture of a nearby fault. Retro-deformation of Event 5 on the Thomas and Rockwell trench log as well as vertical displacement seen at the Dogwood site implies that this event was large and on the same order of magnitude as Events 1 and 2. The Thomas and Rockwell log also provided us with some interesting information on previous movement along the fault for Event 5. The reconstruction shows that the central portion of the fault zone was uplifted, indicating a local transpressional stress regime instead of transtensional as it is today. This was also mentioned earlier in this chapter where we interpreted that there was a structural high due to onlapping of sediments onto Lake 3. This may imply that some ruptures bypass the step-over and do not activate the sag. Alternatively, it could be that the slightly transpressive structural high was adjacent to but not in the sag and lateral slip in the past four earthquakes has translated the transpressively deformed section into the sag area. In any case, based on the expression of Event 5 at our Border trench site, along with its strong expression at the A. P. Thomas and Rockwell (1996) site, Event 5 appears to be a large event similar to what occurred in 1940. Consequently, we attribute up to 6 m of lateral slip to Event 5. By assessing the relative magnitude of events, we can estimate the slip rate for the past 400 to 550 years. It is evident that Events 1, 2, and 5 produced larger magnitude earthquakes that we infer to have experienced around 6 m of lateral displacement, based on what was measured for the 1940 event. We suggest that lateral displacement from the smaller Events 3 and 4 are negligible based on how minimal the vertical displacement was. This equates to around 18 m of total displacement in the past 400-550 years, which corresponds to

a short-term slip rate of 32-45 mm/yr. Geodetic models of the Imperial fault by Bennett et al. (1996), Lisowski, Savage, and Prescott (1991), and Becker, Hardebeck, & Anderson (2004) have proposed rates of 35 mm/ yr, 37 mm/yr, and 39 mm/yr, respectively, which agrees well with our estimated slip rate.

CONSTRAINTS ON EARTHQUAKE RECURRENCE By incorporating the ages that we established for the time of deposition of Lake Cahuilla sediments at our site based on the regional chronologic model for the lake, we can further constrain the timing of rupture events and assess the recurrence interval of ruptures on the Imperial fault (Table 2). We determined that there was a long time interval of 212-217 years between the 1940 rupture and Event 2 as we inferred that Event 2 occurred sometime during the waning stages of Lake 1 around AD 1723-1728. In contrast, there appears to only have been 3-4 years between Event 2 and 3. This is because Event 3 occurred sometime during the infilling phase of Lake 1 at our site which places it in a date range of AD 1710 to early 1725. We conclude that there was 73 to 76 years between the Event 3 and 4 ruptures based on Event 4 taking place sometime during the desiccation of Lake 2. We interpret that there was a dry lake bed at our site at this time because of root casts seen at the top of the lake deposits. This places the date of the rupture around ca AD 1654. Evidence for Event 5 at our site pointed to the rupture event occurring during the desiccation of Lake 3. We have a wide range of ages for the desiccation of this lake, which would place the event between AD 1494 to AD 1600. This suggests that the timing between these rupture events was somewhere between 53 and 161 years. Combining these results yields a recurrence interval of 83-104 years for our site, however ages between events appear to be highly varied and could imply a non-episodic behavior for the Imperial fault. Alternative recurrence intervals could be established based on two events seen at the Dogwood site (Event 6 at AD 1550 and Event 5 at AD 1635), both of which could correlate to our Event 5. We propose three scenarios for ages of these events and their subsequent recurrence intervals. For our first scenario, we interpret that both events occurred around the same time, where a lapse in sedimentation at our site may have made evidence for Event 5 impossible to see. If this is the case, then we could have six events that occurred since AD

Figure 28. Southeast facing trench log from the Dogwood Road site. Our Events 1 and 2 correspond to Events 2 and 3 respectively. Our Event 3 was not observed at this site. Event 4 appears to be a smaller event with Event 5 being on the same order of magnitude as Events 1 and 2, which was also evident at our trench site. Figure from Rockwell et al. (2011). 54

1550, placing the recurrence interval at around 80 years. Alternatively, our second scenario suggests that our Event 5 correlates to the Dogwood Event 6, signifying that we are missing their Event 5 at our site, which further implies that the Dogwood Event 5 only occurred along the northern fault segment. This indicates that there were five events since AD 1550 at our site, yielding a recurrence interval of around 90 years. Finally, for our third scenario, we infer that our Event 5 correlates to the Dogwood Event 5, with Event 6 not being observed at our site due to our trench not being deep enough. This would imply that there were five events since AD 1635, resulting in a recurrence interval of 75 years. In any case, these recurrence intervals all remain within the recurrence interval range given for our site with ages between events still exhibiting a large variability.

Table 2. Time Interval Between Events Stratigraphic Timing of Time Interval Event Designated Age of Event Event Between Events 1979 Historical 1979

39 years

1940 Historical 1940

212-217 years

2 Waning stage of Lake 1 AD 1723-1728

3-4 years

3 Filling stage of Lake 1 AD 1720 - AD 1725

73-76 years

4 Desiccation of Lake 2 AD 1653 - AD 1655

53-161 years

5 Desiccation of Lake 3 AD 1494 - AD 1600

CHAPTER 6

CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK

The results from this study indicate that the Imperial fault has behaved in a non- characteristic fashion, at least for the past several large earthquakes. This inference is based on the correlation of surface rupturing events between our trench site and sites along the central and northern segments of the fault, as well as our inference of the size of these events. All of the paleoseismic sites present evidence for the larger magnitude events, while smaller magnitude events are not recognized to be coeval from site to site. This suggests that the Imperial fault has experienced a bi-modal distribution of the displacement along the fault and follows the hypotheses by Meltzner (2006) and Rockwell et al. (2011). From this, we infer that the fault will likely experience complete stress drops causing end to end ruptures, as denoted by the large events observed in the trenches. Rockwell and Klinger (2013) suggest that a shallow asperity near the border region causes an increase in displacement during these large ruptures, such as what occurred during the 1940 event. We propose that this would cause an increase in stress, therefore leading to smaller rupture events along the low-slip areas. As these smaller events would not rupture along the entire length of the fault, evidence for them would not be seen to correlate between the north and central fault segments. Palinspastic reconstruction of our trench log, along with correlation to a reconstructed trench log from the Thomas and Rockwell site, allowed us to determine the relative magnitude of events at our site for the past 400-550 years. Based on the similarity between our estimated slip rate of 32-45 mm/yr, as inferred from these observed events, and the documented geodetic rate of the Imperial fault, we interpret that most of the plate motion is accommodated by the Imperial fault at the international border.

By utilizing the long-term chronology of Lake Cahuilla as well as the precipitation plots based on paleoclimate by Rockwell (2016) we were able to constrain the dates of rupture events. We determined that rupture recurrence is highly sporadic, leading us to propose that the Imperial fault exhibits a non-episodic rupture behavior at our site. Further work on the Imperial fault should include a paleoseismic study of the southern segment. Little is known about this end of the fault except for an observation made during a survey by Rockwell and Klinger (2013) indicating that 2.7 m of right lateral strike- slip had passed through the small town of Tamaulipas, Mexico during the 1940 rupture. Their resolution of this displacement was based on offset telephone poles that had been erected prior to the event. Paleoseismic trenching along this segment of the fault would help to provide additional information for age constraint of ruptures as well as assist in the further development of the long-term rupture history on the Imperial fault.

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