CHARACTERIZING THE LATE HOLOCENE SLIP RATE OF THE CENTRAL GARLOCK FAULT: IMPLICATIONS FOR VARYING STRAIN ACCUMULATION AND RELEASE THROUGH TIME

Detailed measurements of small geomorphic offsets and optically stimulated luminescence (OSL) dating results reveal an elevated late Holocene slip rate of the Garlock fault that is twice as fast as the long-term rate. Specifically, new advances in single grain feldspar IRSL (Infra-Red Stimulated Luminescence) dating have allowed us to accurately date 29 new luminescence samples from three alluvial fans (7 pits). Small stream gullies that have been shallowly incised into these alluvial fan surfaces have been laterally offset 23±2 meters along a 1.7 km stretch of the central Garlock fault at Christmas Canyon. Our calculated late Holocene slip rate of ~12 mm/yr is significantly faster than previous measurements of the longer-term latest Pleistocene-early Holocene rate of ~5-7 mm/yr (e.g., McGill et al., 2009; Ganev et al., 2012), suggesting that the ~10 ka rate represents an average between millennia-long periods of fast slip interspersed with periods of slow/zero fault slip. The new slip rate data provide a rare opportunity in which we can compare Holocene slip rates at multiple time intervals with a well-constrained late Holocene paleoseismic record, which reveals a temporal cluster of four surface ruptures between 25 AD and 1550 AD, preceded by an ~3,000-year-long seismic lull and followed by the ongoing period of seismic quiescence (Dawson et al., 2003). Moreover, geodetic measurements record little to no current elastic strain accumulation across the fault, suggesting the fault is in a strain accumulation lull that appears to correspond to the absence of large events since ~1550 AD. Inasmuch as our 2 ka rate includes the ongoing ~500-year-long lull since the most recent earthquake, the slip rate during the four-event cluster between ~2 ka and ~1550 AD was likely to have been even faster during the cluster than the ~12 mm/yr rate we measure (≥~15 mm/yr if all slip occurred during the seismic cluster). These data are all consistent with the idea that the Garlock fault experiences two different modes, with periods of rapid strain accumulation coinciding with periods of elevated earthquake activity, and intervening periods of slow/no strain accumulation corresponding to intervals of seismic quiescence.