Confirmation of a rapid Late Holocene slip rate for the central Garlock fault, southern California

Sally F. McGill, James E. Burns, Kyle A. Peña, Ed J. Rhodes, & James F. Dolan

Published September 8, 2024, SCEC Contribution #13888, 2024 SCEC Annual Meeting Poster #101

Prior work at the Christmas Canyon West (CCW) site on the central Garlock fault, in Searles Valley, measured a ~26 m left-lateral offset of ~1.9 ka alluvial deposits to establish a Late Holocene, left-lateral slip rate of ~14 mm/yr that is almost twice the rate averaged over the Holocene epoch (Dolan et al., 2016; Fougere et al., 2024). This fast rate was attributed to a cluster of four earthquakes since 2 ka at the El Paso Peaks (EPP) paleoseismic site, about 28 km west of CCW (Dawson et al., 2003). To test this interpretation, we (1) dated several offset, Late Holocene alluvial features within 2 km of the EPP paleoseismic trench to determine the amount of slip produced by the four most recent earthquakes there, and (2) excavated a paleoseismic trench at CCW, in an active wash next to the 1.9 ka alluvial deposits, to determine the number of earthquakes responsible for the 26 m offset. All dating of sediment samples from both sites used the post-IR-IRSL dating method.

Near the EPP trench, we dated a small alluvial fan left-laterally offset ~23 m (range 19-25 m) and its age indicated it had experienced the four most recent earthquakes recorded in the EPP trench and not any of the older events. This suggests an average slip-per-event of 5-6 m. The resulting Late Holocene slip rate was 10.3 – 13.6 mm/yr, almost as fast as the Late Holocene rate at CCW.

In the trench excavated (in 2018) at the CCW slip-rate site, at least 9 events have occurred in the past 7-8 ka, including two events in the past 0.4-0.7 ka. Constraints on the ages of individual events are being refined. There was a depositional hiatus at the trench site between 0.7-3 ka, during which the third event occurred, and it is possible that an additional event(s) occurred during this hiatus, which cannot be distinguished from event 3. In fact, published paleoseismic records to the west (El Paso Peaks) and east (Echo Playa; Kemp et al., 2016), each record two slip events during the depositional hiatus at the CCW trench. Thus, the third event at CCW likely represents two surface ruptures with no sediment deposited between them. If so, a total of at least 10 events have occurred at CCW in the past 7-8 ka, and four events may have occurred in the past 1.9 ka. This supports prior interpretations that the ~26 m offset of alluvial fans at CCW was accomplished in four events (Dolan et al., 2016), and it suggests an average of 6-7 m of left-lateral slip per event at CCW.

Key Words
Garlock fault, slip rate, earthquake recurrence

Citation
McGill, S. F., Burns, J. E., Peña, K. A., Rhodes, E. J., & Dolan, J. F. (2024, 09). Confirmation of a rapid Late Holocene slip rate for the central Garlock fault, southern California. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology