Open-File Report 01-111
Images and Fault Relations of the Santa Monica Thrust Fault,
West Los Angeles, California
R. D. Catchings, G. Gandhok, M. R. Goldman, and D. Okaya
In May 1997,
the US Geological Survey (USGS) and the University of Southern California
(USC) acquired high-resolution seismic reflection and refraction images
on the grounds of the Wadsworth Veterans Administration
Monica fault is one of the several northeast-southwest-trending, north-dipping,
reverse faults that extend through the Los Angeles metropolitan area (Fig.
1a). Through much of area, the Santa Monica fault trends subparallel to
the Hollywood fault, but the two faults apparently join into a single
fault zone to the southwest and to the northeast (Dolan et al., 1995).
The Santa Monica and Hollywood faults may be part of a larger fault system
that extends from the Pacific Ocean to the Transverse Ranges. Crook et
al. (1983) refer to this fault system as the Malibu Coast-Santa Monica-Raymond-Cucamonga
fault system. They suggest that these faults have not formed a contiguous
zone since the Pleistocene and conclude that each of the faults should
be treated as a separate fault with respect to seismic hazards. However,
Dolan et al. (1995) suggest that the Hollywood and Santa Monica faults
are capable of generating Mw 6.8 and Mw 7.0 earthquakes, respectively.
Thus, regardless of whether the overall fault system is connected and
capable of rupturing in one event, individually, each of the faults present
a sizable earthquake hazard to the Los Angeles metropolitan area. If,
however, these faults are connected, and they were to rupture along a
continuous fault rupture, the resulting hazard would be even greater.
the Santa Monica fault represents a hazard to millions of people, its
lateral extent and rupture history are not well known, due largely to
limited knowledge of the fault location, geometry, and relationship to
other faults. The Santa Monica fault has been obscured at the surface
by alluvium and urbanization. For example, Dolan et al. (1995) could find
only one 200-m-long stretch of the Santa Monica fault that was not covered
by either streets or buildings. Of the 19-km length onshore section of
the Santa Monica fault, its apparent location has been delineated largely
on the basis of geomorphic features and oil-well drilling. Seismic imaging
efforts, in combination with other investigative methods, may be the best
approach in locating and understanding the Santa Monica fault in the Los
This investigation and another recent seismic imaging investigation (Pratt et al., 1998) were undertaken to resolve the near-surface location, fault geometry, and faulting relations associated with the Santa Monica fault.
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Last modified: 10-31-01 (cad)