FAULT SYSTEM BEHAVIOR OF THE EASTERN CALIFORNIA SHEAR ZONE: UNSTEADY LOADING RATES AND CLUSTERED EARTHQUAKE ACTIVITY

Clustering of major earthquakes over periods of years to centuries is observed historically and in paleoseismic records. The relationship of such behavior to fault loading processes is uncertain, and multiple mechanisms of earthquake triggering are likely to drive clustering over different time scales. Active faulting in the Mojave Desert region of California presents a well-constrained example of fault-system-wide trade-off in fault activity that may be associated with earthquake clustering. Dextral fault slip-rates from the Eastern California shear zone (ECSZ) determined from displaced alluvial fans and lava flows sum conservatively to 6.2±1.9 mm/yr, with at most 30% additional slip rate absorbed via distributed deformation. This long-term rate is only half the present-day loading rate of 12±2 mm/yr across the 60 km-wide shear-zone determined from geodesy. This discrepancy began prior to the 1992 M_W7.3 Landers earthquake, and its magnitude precludes residual post-seismic deformation following other large historic earthquakes in southern California. An opposite discrepancy exists on the conjugate, sinistral Garlock fault. Its long-term fault slip rate is similar to the sum rate across the ECSZ, yet geodetically its loading rate is indistinguishable from zero. These observations support that significantly elevated regional strain accumulation rate may be associated with clustered earthquake activity in the ECSZ, and that interseismic loading may oscillate between different fault sets within a hierarchal fault system. Redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear-zones.