Article,
Crustal imaging in southern California using earthquake sequences
Tectonophysics, 286 (1-4): 223--236 (Mar 10, 1998)
DOI: 10.1016/S0040-1951(97)00267-9
Abstract
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.
