Evidence for Contemporary Block Rotation in Strike-Slip Environments: Examples from the San Andreas Fault System, Southern California

Abstract

Recent moderate size earthquakes in southern California indicate seismic slip on both right- lateral faults trending north to northwest, and left-lateral faults trending northeast to east. These sets of faults are most prevalent in areas of local extension parallel to the regional strike of the San Andreas system, and are consistent with a model in which contemporary crustal deformation is partially accommodated by block rotation in response to distributed right-lateral shear. Block rotation by strike-slip faulting is inferred from the seismicity based on: sets of small crustal blocks defined by domains of active, nearly-parallel left-slip cross-faults; the unusual kinematic pattern of faulting exhibited by the seismicity around rotating block edges; and the identification at depth of seismic slip on low-angle detachments to decouple the blocks and permit rotational movement. As block corners rotate into or away from edge- bounding master faults, they increase or decrease normal stress and can therefore generate time-dependent asperities that control fault behavior, thus modifying the rupture patterns of large earthquakes.Where reliable data exists, there is often good agreement between seismicity and seismic reflection profiles, structural controls from geologic evidence, paleomagnetic results, and geodetic constraints for support of a rotating block model associated with the observed interacting left- and right-slip faults, distributed regional right-lateral shear, and basal detachments. Since edge-bounding secondary faults rotate with the adjacent block material, new faults must form as old faults rotate into positions unfavorable for further slip. Observations suggest that block rotation by strike-slip faulting is highly non-uniform in space and time, can occur on several scale lengths from millimeters to several tens of kilometers (requiring multiple zones of decoupling within the upper crust), and can accommodate both elastic and non-elastic strain accumulation. Identifying the conditions whereby block rotation is preferred over simple linear translation will be a critical element in understanding the processes controlling contemporary crustal deformation in strike-slip environments.