The structure of the crust and distribution of earthquakes in southern California

The lithologically and tectonically complex crust of southern California and the current broad deformation zone accommodating the relative motion between the Pacific and North American plates, result in significant variations in style, depth distribution, and rate of earthquakes, and thus also in the seismic hazard across southern California. Although the thickness of the seismogenic crust is an important parameter in seismic hazard analysis, it has never been determined systematically for southern California. Seismogenic thickness can be predicted by the depth distribution of the moment release of regional seismicity. The seismogenic thickness of southern California is highly variable, ranging from less than 10 km in the Salton Trough to greater than 25 km at the southwestern edge of the San Joaquin Valley. On average, the seismogenic thickness of southern California is 15.0 km. Seismogenic thickness along the major strike slip systems of southern California can vary significantly along strike. Fault segmentation based upon surface features does not correspond to the variation in seismogenic thickness and thus the potential down-dip width of the fault. A model of the broad scale features of the crust and upper mantle structure of the borderland-continent transition zone adjacent to Los Angeles constrains the crustal thickness and the location and width of the transition zone. The data require the Moho to deepen significantly to the north, dramatically increasing the crustal thickness over a relatively short distance of 20–25 km. The Moho is coherent and laterally continuous beneath the Inner California Borderland and transition zone. The Inner Borderland seems to be modified and thickened oceanic crust, with the oceanic upper mantle intact beneath it. The static stress change triggering model has some validity and can be useful in explaining apparently triggered seismicity within one fault length of a large mainshock. However, because its applicability varies between different sequences, its general application to seismic hazard evaluation requires more refinement and the inclusion of parameters such as tectonic regime, regional stress state, and fault strength.