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Behavior of Repeating Earthquake Sequences in Central California and the Implications for Subsurface Fault Creep

Lawrence Livermore National Laboratory, 700 East Avenue, Livermore, California 94550

Robert M. Nadeau and Roland Bürgmann

Berkeley Seismological Laboratory, 215 McCone Hall, University of California, Berkeley, Berkeley California 94720-4760

Online Material: Cross sections, timing, and source information for repeating earthquakes.

Repeating earthquakes (REs) are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. We investigate the occurrence of these REs along faults in central California to determine which faults exhibit creep and the spatiotemporal distribution of this creep. At the juncture of the San Andreas and southern Calaveras–Paicines faults, both faults as well as a smaller secondary fault, the Quien Sabe fault, are observed to produce REs over the observation period of March 1984 through May 2005. REs in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time. Cumulative slip over the observation period at individual sequence locations is determined to range from 5.5–58.2 cm on the San Andreas fault, from 4.8–14.1 cm on the southern Calaveras–Paicines fault, and from 4.9–24.8 cm on the Quien Sabe fault. Creep at depth appears to mimic the behaviors seen for creep on the surface in that evidence of steady slip, triggered slip, and episodic slip phenomena are also observed in the RE sequences. For comparison, we investigate the occurrence of REs west of the San Andreas fault within the southern Coast Range. Events within these RE sequences occurred only minutes to weeks apart from each other and then did not repeat again over the observation period, suggesting that REs in this area are not produced by steady aseismic creep of the surrounding fault surface.