Elevated shear zone loading rate during an earthquake cluster in eastern California

We compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern California shear zone (ECSZ) during a cluster of major earthquake activity. New slip rates are presented for six dextral faults that compose the ECSZ in the central Mojave Desert. These rates were determined from displaced alluvial fans dated with cosmogenic Be-10 and from a displaced lava flow dated with 40Ar/39Ar. We find that the sum geologic Mojave ECSZ slip rate, <= 6.2 +/- 1.9 mm/yr, is only half the present-day geodetically measured velocity of 12 +/- 2 mm/yr. These rates account for cumulative fault slip and geodetic observations that span the 60-km-wide shear zone; therefore this difference cannot be attributed to postseismic relaxation. 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.