Poster #022, San Andreas Fault System (SAFS)

Reconstructing long-term subsidence and paleoseismic history of the ancient Lake Cahuilla along the southern San Andreas fault in Coachella, California

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Poster Presentation

2020 SCEC Annual Meeting, Poster #022, SCEC Contribution #10290
The most recent ground-rupturing earthquake (MRE) along the southernmost ~100 km of the San Andreas fault (SSAF) occurred ca. 300 years ago, just after the last filling of ancient Lake Cahuilla in the Salton Trough. At the northern end of the Coachella Valley, the average interval between earthquakes on the SSAF over the last 1000 years is ~116–221 years. This stretch of the SSAF, therefore, likely has accumulated a large amount of elastic strain since the MRE and thus has some of the highest earthquake probabilities in formal forecast models. Other workers have proposed that lake levels of ancestral Lake Cahuilla modulate the regional stress field and tie earthquake occurrence to particul...ar stages of lake fill. Due to a high sedimentation rate (~5 mm/year) in the Salton Trough near Coachella, the existing trench records only date back to ~1 ka. These stratigraphic and temporal limitations, coupled with conflicting interpretation of lake stratigraphies, make it challenging to evaluate whether the lack of surface rupture since the MRE on the SSAF is due to the extended dry period in the lake. Extending the paleoseismic record back to the Mid-Holocene or older provides chronologic data to test the relationship between lake filling history and earthquake cycles. We therefore extracted a 33.5-m-deep core from the Coachella site south of Avenue 44, Indio, using a CME-85 continuous coring rig. The core was drilled at a narrow (~30 m wide) structural depression near the shoreline of ancient Lake Cahuilla and therefore is poised to capture both the changes in lake filling and rupture/subsidence triggered by the movement along the SSAF. Detailed logs of this continuous core record the deep subsurface stratigraphic units and are used to guide sampling locations for geochronological dating. This core had ~85% recovery and consists of alternate lacustrine and subaerial units. We plan to measure 12 luminescence ages from the subaerial units to date lake regression events. This project aims to provide a crucial insight into the long-term: (i) lake filling/desiccation chronology, (ii) a subsidence history of the ancient Lake Cahuilla, and (iii) sedimentologic context for paleoearthquake and slip rate studies in the Coachella Valley.
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