SCEC Award Number 20144 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Constraining a long history of paleolake and paleoseismicity at Coachella, CA using deep borehole samples
Name Organization
Seulgi Moon University of California, Los Angeles Sourav Saha University of California, Los Angeles Thomas Rockwell San Diego State University
Other Participants Katherine Scharer (USGS)
Nathan Brown (UC Berkeley)
SCEC Priorities 5b, 5c, 5d SCEC Groups SAFS, Geology, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/14/2021
Project Abstract
The project's main objective is to test the hypothesis that the lack of surface rupture since the most recent earthquake (MRE) on the southernmost ~100 km of the San Andreas fault (SSAF) is due to Lake Cahuilla's extended dry period. The average earthquake recurrence interval on this stretch of the SSAF in the previous 1000 years is ~180 years. However, the MRE was recorded around 1726±7 AD, just after the last filling of the ancient Lake Cahuilla. Why this stretch of the SSAF remains quiescent for ~300 years is still poorly understood. To address this relationship between lake cycles and earthquake frequency, we collected a 33.5-m-deep continuous borehole (~86% recovery) near the northeastern end of Lake Cahuilla's shoreline at Coachella, CA. The borehole's detailed stratigraphic log suggests approximately 20 lake highstands, interrupted by thick sandy subaerial units. The uncorrected thirteen luminescence ages show that constraining the timing of the longest lake filling history (up to ~7.0 ka) of Lake Cahuilla is possible. These uncorrected ages also tentatively suggest that the average interval of lake highstands is 0.2–0.3 ka, and the average sedimentation rate is ~5±0.3 mm/year in the past ~7 ka. We expect to further refine this high-resolution age model by incorporating 30 new and published 14C ages and historical information.
Intellectual Merit The project offers the potential to reconstruct the longest lake filling and desiccation cycles of the ancient Lake Cahuilla in the Salton Trough. These results are crucial to test whether SSAF is susceptible to lake loading and associated porewater pressure or is just a mere coincidence. In addition, the project has intellectual merit to offer long-term sedimentologic context for paleoearthquake and slip rate studies in the Coachella Valley. In combination with other geophysical (e.g., GPR) and geotechnical data (e.g., CPT), borehole data also has the potential to estimate the cumulative vertical displacement at the Coachella site. The new luminescence chronology and analytical improvements are valuable for providing a means to date deep borehole sediments, especially in contexts where no organic material for 14C exists or are enormously affected by inheritance and 14C reservoir effect.
Broader Impacts This project has provided ample opportunities for research and training at UCLA. The project helps develop a series of other projects for the postdoctoral fellow at UCLA. A graduate and an undergraduate student are also being trained in the UCLA luminescence lab. An extension of the project also has the potential to be part of a Ph.D. thesis in the future. Besides, the project contributes directly to address three primary goals of SCEC5: Science Objectives of "P5.b.", "P5.c.", and "P5.d."
Exemplary Figure Figure 2. Coachella (shoreline) core stratigraphic log (CSD-01) showing the alternative subaerial (sandy/silty) and lacustrine (clayey/silty) units. As many as 20 lacustrine units are identified of the ~33.5-m core. Thirteen luminescence samples (brown circles) are dated, and 30 14C samples (yellow circles) are collected to constrain the lake Cahuilla's long-term lake filling and desiccation cycles. Published 14C and uncorrected luminesce ages are provided for reference. The upper ~6-m of the log was adapted from Philibosian et al. (2009, 2011).