IMPORTANCE OF RECONCILING PALEOSEISMIC, AVERAGE RUPTURE DISPLACEMENT AND GEOLOGIC SLIP RATE DATA FOR THE GARLOCK FAULT, CALIFORNIA

Reconciliation of paleoseismic events, average slip per event and geologic slip rates should, in theory, be straightforward. Offset geomorphic features require a discrete number of ground-rupturing earthquake events (that leave paleoseismic evidence) to accumulate displacement. Even with the inherent uncertainties related to dating of both paleoseismic events and displaced geologic features, there should still be rough agreement, within uncertainty, among slip rate, number of events and their size. Yet when the Garlock fault (GF) is analyzed, there is a discrepancy.

A 90 and 68 m displacement of a ca. 13.2 ka highstand shoreline and a younger incised stream in Pilot Knob Valley (PKV) (McGill and Sieh, 1993), respectively, suggests an average slip rate of 5-7 mm/yr along the central GF. The presence of a 3.5-4.2 ka debris flow displaced 43-50 m in PKV indicates large temporal fluctuations in strain release and a Late Holocene (LH) slip rate of 10.2-14.3 mm/yr. Existing paleoseismic data from El Paso peaks (EPP) (e.g., Dawson et al., 2003) shows a highly irregular earthquake clustering pattern during the last 7 ky with 2 events between 5,000-0 B.C., and 4 events post-0 B.C. McGill and Sieh (1991) measured offset geomorphic surfaces along the GF and argued that average displacements per event are ~2-3 m.

Here we show that none of these data are in particularly good agreement with each other, and that (1) average earthquake displacement is likely closer to 5 m and/or (2) that numerous paleoseismic events are missing from the record. Assuming a 5-7 mm/yr slip rate (e.g., McGill and Sieh, 1993) over the 7 ky interval spanning the paleoseismic record at EPP, 14-20 events with ~2-3 m average displacements are necessary, with only 5-10 events required if displacement is 5 m. In order for 43-50 m of slip to have accrued since ~4 ka, 17-20 events with displacements of ~2-3m are required. If slip-per-event is ~5 m, only 8-10 events are required. Morphological evidence from displaced channels evident on LiDAR-derived topography in PKV suggests that ~5 m displacements are tenable. Our new constraints on LH slip, combined with a re-evaluation of existing slip rate and paleoseismic data, imply that earthquakes along the central GF likely exhibit larger displacements and/or that existing paleoseismic records are incomplete.