Complexities of the San Andreas fault near San Gorgonio Pass: Implications for large earthquakes

Geologic relationships and patterns of crustal seismicity constrain the three-dimensional geometry of the active portions of San Andreas fault zone near San Gorgonio Pass, southern California. Within a 20-km-wide contractional stepover between two segments of the fault zone, the San Bernardino and Coachella Valley segments, folds, and dextral-reverse and dextral-normal faults form an east-west belt of active structures. The dominant active structure within the stepover is the San Gorgonio Pass-Garnet Hill faults, a dextral-reverse fault system that dips moderately northward. Within the hanging wall block of the San Gorgonio Pass-Garnet Hill fault system are subsidiary active dextral and dextral-normal faults. These faults relate in complex but understandable ways to the strike-slip faults that bound the stepover. The pattern of crustal seismicity beneath these structures includes a 5-8 km high east-west striking step in the base of crustal seismicity, which corresponds to the downdip limit of rupture of the 1986 North Palm Springs earthquake. We infer that this step has been produced by slip on the linked San Gorgonio Pass-Garnet Hill-Coachella Valley Banning (SGP-GH-CVB) fault. This association enables us to construct a structure contour map of the fault plane. The large step in the base of seismicity downdip from the SGP-GH-CVB fault system probably reflects a several kilometers offset of the midcrustal brittle-plastic transition. (U/Th)/He thermochronometry supports our interpretation that this south-under-north thickening of the crust has created the region's 3 km of topographic relief. We conclude that future large earthquakes generated along the San Andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1-20 km. Two tasks in seismic hazard evaluation may now be attempted with greater confidence: first, the construction of synthetic seismograms that make useful predictions of ground shaking, and second, theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures.