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 _______________ A Thesis Presented to the Faculty of San Diego State University _______________ In Partial Fulfillment of the Requirements for the Degree Master of Science in Geological Sciences _______________ by Kaitlin Nicole Wessel Spring 2016 iii Copyright © 2016 by Kaitlin Nicole Wessel All Rights Reserved iv DEDICATION To my family, thank you so much for your love and support. To Aaron, thank you for your patience, love, and encouragement. v 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. vi TABLE OF CONTENTS PAGE 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 vii 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 viii LIST OF TABLES PAGE Table 1. Imperial Border Trench Radiocarbon Sample Ages ..................................................31 Table 2. Time Interval Between Events ...................................................................................55 ix LIST OF FIGURES PAGE 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