Crustal imaging in southern California using earthquake sequences

An inexpensive means to further understand the geometry of active faults in southern California arises from the use of aftershock recordings to image crustal structures. The advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active-source seismic data. We show that it is feasible to image fault structure using three-dimensional, wide-angle prestack Kirchhoff migration. We achieve this with the use of aftershock traces recorded on the short-period vertical stations of the Southern California Seismic Network. This work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross-sections rather than having to associate events with certain faults. Further, it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths. We demonstrate the validity of these images as showing reflective structures, and the ability to use clipped high-gain seismograms as sign-bit data to yield valid geometric imaging. Work with data from the 1991 Sierra Madre earthquake sequence images the prominent lower-crustal reflective zone observed beneath most of the San Gabriel Mountains by the Los Angeles Region Seismic Experiment Line 1. Aftershocks of the 1994 Northridge earthquake allow us to image a north-dipping structure that may represent the fault plane of a crustal-penetrating blind thrust. The images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced-section reconstructions of shallow-crustal profiles and borehole data. Our results are more consistent with a thick-skinned tectonic regime in the vicinity of the Northridge earthquake, rather than a thin-skinned model.