SCEC Award Number 22065 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Constraining a long history of paleolake and paleoearthquake activity at Coachella, CA
Name Organization
Seulgi Moon University of California, Los Angeles Sourav Saha University of California, Los Angeles
Other Participants Marina Argueta (graduate student), Thomas K. Rockwell, Katherine (Kate) Scharer, Nathan D. Brown
SCEC Priorities 5b, 5c, 5d SCEC Groups Geology, SAFS
Report Due Date 03/15/2023 Date Report Submitted 02/23/2024
Project Abstract
We collected two continuous boreholes (Figure 1), 33.5 and 40-m deep, from the Coachella paleoseismic site along the southernmost San Andreas fault, with the support from previous SCEC proposals (Awards 20144 and 21100). The first borehole was collected within the Coachella structural depression in 2020, and the second borehole was collected 135 m southwest of the first borehole in 2021, outside of the deformation zone. Using 25 corrected luminescence, we are able to extend the depositional history near the Lake Cahuilla shoreline to ~6 and 12 ka from the first and second boreholes, respectively. Several shells and grass materials were dated for 14C dating at California State University, Fullerton. However, the 14C dates are either too young (e.g., grasses) or too old (e.g., shells). We also performed grain size analysis for portions of both boreholes at SDSU’s Quaternary Geology lab. The age and grain size data from both boreholes reveal two different depositional histories recorded. The first borehole has ages from ~1 – 6 ka while the second borehole has ages ~5 – 12 ka. Additionally, the first borehole has thicker depositional units in comparison to the second borehole, as we expected due to the first borehole being extracted from the Coachella structural depression (Figure 2).
Intellectual Merit The project offers 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 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 and they are enormously affected by inheritance and reservoir effect.
Broader Impacts This project has provided ample opportunities for research and training at UCLA, SDSU, & CSUF. The project helps develop a series of other projects for the postdoctoral fellow at UCLA. A graduate and two undergraduate students are also being trained under this project. This project is part of a Ph.D. thesis at UCLA and two undergraduate research projects at SDSU and CSUF. Besides, the project contributes directly to addressing three primary goals of SCEC5: Science Objectives of "P5.b.", "P5.c.", and "P5.d."
Exemplary Figure Figure 1. Map showing the ancient Lake Cahuilla's 13-m shoreline (inset) and location of the MSD and CSD boreholes relative to the Central Trench of Philibosian et al. (2009, 2011).

Figure 2. Stratigraphic logs of the first borehole (left) and second borehole (right) showing alternative subaerial sandy and water-lain clayey/silty units, 6 published 14C ages (blue dots; Philibosian et al., 2011; Rockwell et al., 2022) plotted with respect to the first borehole’s depth. New 14C ages from the first borehole (left), and 21 new first borehole and 4 new second borehole corrected p-IR IRSL ages.