Using mechanical models, interseismic focal mechanisms, GPS velocities, and long-term slip rates to constrain the geometry of the southern Big Bend of San Andreas fault, California

Abstract:

Within regions of complex 3D faulting we can look beyond geologic and GPS data sets to constrain active fault configuration. GPS data that can resolve slip rates or fault geometry along relatively isolated faults can struggle to produce reliable inversions for very closely spaced networks of faults. For example, the southern Big Bend of the San Andreas fault has multiple nearby active strands for which both subsurface active configuration and slip rate are under constrained. Mechanical models reveal the long-term kinematic incompatibility of interpreted fault geometry and geologic slip rates in this region. It is not surprising then that inversions of GPS data fail to recover geologic slip rates along close parallel strands of the San Andreas. For additional insight, we compare stress state off of faults from the interseismic model, based on long-term slip rates, to focal mechanisms from microseismicity within the interseismic period. Because focal mechanisms occur at depth and closer to the slipping portions of faults within the interseismic period, they can, in some instances, better resolve details of active faulting than interseismic surface GPS velocities.

The slip sense of focal mechanisms around the southern Big Bend shows 3D variability of interseismic loading that corresponds with complex active fault configuration. Mechanical models that are consistent with geologic slip rates also show that lateral variation in deep creep rate along the northern San Jacinto fault associated with transfer of strain to the San Andreas fault can account for normal slip focal mechanisms within the San Bernardino basin. These enigmatic focal mechanisms occur predominantly below 10 km, consistent with deep interseismic creep. Within the San Gorgonio Pass, reverse focal mechanisms are consistent with contraction between the southern and northern branches of the southern San Andreas fault. Because the crustal stress state is sensitive to changes in active fault geometry, we can use focal mechanism stress inversions to differentiate amongst interseismic stress states predicted by mechanical models of plausible fault configurations. We show that mechanical models can link interseismic geophysical data sets and long-term geologic slip rates and provide a way to evaluate the consistency of data across time periods.