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Downdip segmentation of strike-slip fault zones in the brittle crust

¹ Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA

Outcrop-scale field evidence from the southern San Jacinto fault zone in southern California indicates that bedding-parallel slip may be an important mechanism for transferring and accommodating slip within strike-slip fault zones that cut across heterogeneous stratigraphy. Steeply dipping strike-slip fault segments exposed in conglomeratic sandstones abruptly terminate at weaker shale layers where slip occurs on bedding-parallel planes. Cross sections of fault-related seismicity within the southern San Jacinto fault zone reveal that events generally cluster within tabular, steeply dipping fault-parallel streaks that are limited in extent in the downdip direction. The depths of the downdip terminations of shallow streaks tend to be roughly coincident with the depths of the updip terminations of deeper streaks; these configurations define stepover geometries that range in character from sharp to diffuse. The examples described herein provide evidence that downdip segmentation within strike-slip fault zones may be common over a wide range of scales.