New Paleoearthquake Ages from the Western Garlock Fault: Implications for Regional Earthquake Occurrence in Southern California

Abstract

Abstract New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past ∼5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well‐defined events at Twin Lakes to post‐A.D. 1450 (event A), 720–395 B.C. (event G), and 3425–2200 B.C. (event I); two probable events are also constrained to 625–1525 A.D. (event C), and 155 B.C.–A.D. 615 (event E), and a possible additional event to 3425–2200 B.C. and prior to event I. Our findings offer new insights into mid‐ to late‐Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2–5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region. Online Material: Two input files written for the computer program OxCal 4.1.7 and two output files from the OxCal analyses. The first input file calibrates and re‐weights probability density functions (PDFs) for radiocarbon ages based on stratigraphic ordering, and the second input file calculates PDFs for earthquake ages based on radiocarbon ages and stratigraphy bracketing the events.