AT LEAST TWELVE-EVENT HOLOCENE TO LATE PLEISTOCENE PALEO-EARTHQUAKE RECORD ON THE CENTRAL GARLOCK FAULT AT KOEHN LAKE, SOUTHERN CALIFORNIA

To improve the paleo-earthquake record of the Garlock fault, a major sinistral strike-slip fault that extends approximately E-W across the Mojave Desert in southeastern California, we revisited a paleo-seismic site at Koehn Lake on the western portion of the central section of the fault. This site was recognized in previous trenching studies by Burke (1979) and Madden & Dawson (2006) as having excellent playa-margin/distal alluvial fan stratigraphy and numerous well-defined structural event markers. We excavated a 37-m-long, 2.5-m-deep trench across the Garlock fault at this site ~20 m east of the previous trenches. This trench revealed evidence for at least twelve surface rupturing events during Holocene and latest Pleistocene time, extending back into the lacustrine sediments of terminal pluvial Koehn Lake at this site. In order to constrain individual event ages, we will employ the newly developed post-IR infrared stimulated luminescence (p-IR IRSL) protocol (Rhodes, 2015), together with radiocarbon dating. We collected 71 p-IR IRSL and 194 radiocarbon samples from throughout the trench, providing the opportunity for numerous 1:1 comparisons of ages based on these two geochronometers. These results should yield the longest record of paleoearthquakes on the Garlock fault, providing insight into the fault's long-term behavior. Specifically, these well-constrained ages of individual paleo-surface ruptures at Koehn Lake will extend the paleo-earthquake record at nearby El Paso Peaks (EPP) site (Dawson et al., 2003) on this section of the Garlock fault back into latest Pleistocene time. This will allow us to determine whether the pronounced temporal clustering of earthquakes observed at the EPP site has been a repeated feature of displacement along the fault. Moreover, our detailed paleo-earthquake record will facilitate comparisons with records at other nearby sites along the central and western segments of the Garlock fault, providing constraints on the lateral extents of previous ruptures. In turn, these results, together with comparisons of paleo-earthquake records from the eastern California shear zone faults both North and South of the Garlock fault, will shed light on potential interactions between the Garlock fault and nearby faults such as those of the eastern California shear zone and the San Andreas fault.