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University of Wisconsin
Madison, Wisconsin 53711
(W.J.L., N.I.C., C.H.T.)
U.S. Geological Survey
Menlo Park, California 94025
(G.S.F., J.M.M., V.E.L.)
GeoForschungsZentrum
Potsdam, 14473 Germany
(T.R.)
Southern California Earthquake Center (SCEC)
University of Southern California
Los Angeles, California 90089
(D.A.O., M.L.B., N.J.G.)
SCEC
California Institute of Technology
Pasadena, California 90025
(R.W.C.)
SCEC
University of California at Los Angeles
Los Angeles, California 90024
(P.M.D.)
Geophysikalisches Institut
Universitaet Karlsruhe
Karlsruhe D-76187, Germany
(C.P.)
University of Copenhagen
Denmark 1350
(K.T.)
California State University at Northridge
Northridge, California 91330
(G.S.)
Department of Geological Sciences
University of Texas at El Paso
El Paso, Texas 79968
(G.R.K.)
Manuscript received 24 March 2003.
In 1999, the U.S. Geological Survey and the Southern California Earthquake Center (SCEC) collected refraction and low-fold reflection data along a 150-km-long corridor extending from the Santa Monica Mountains northward to the Sierra Nevada. This profile was part of the second phase of the Los Angeles Region Seismic Experiment (LARSE II). Chief imaging targets included sedimentary basins beneath the San Fernando and Santa Clarita Valleys and the deep structure of major faults along the transect, including causative faults for the 1971 M 6.7 San Fernando and 1994 M 6.7 Northridge earthquakes, the San Gabriel Fault, and the San Andreas Fault. Tomographic modeling of first arrivals using the methods of Hole (1992) and Lutter et al. (1999) produces velocity models that are similar to each other and are well resolved to depths of 5-7.5 km. These models, together with oil-test well data and independent forward modeling of LARSE II refraction data, suggest that regions of relatively low velocity and high velocity gradient in the San Fernando Valley and the northern Santa Clarita Valley (north of the San Gabriel Fault) correspond to Cenozoic sedimentary basin fill and reach maximum depths along the profile of 
4.3 km and >3 km, respectively. The Antelope Valley, within the western Mojave Desert, is also underlain by low-velocity, high-gradient sedimentary fill to an interpreted maximum depth of 2.4 km. Below depths of 2 km, velocities of basement rocks in the Santa Monica Mountains and the central Transverse Ranges vary between 5.5 and 6.0 km/sec, but in the Mojave Desert, basement rocks vary in velocity between 5.25 and 6.25 km/sec. The San Andreas Fault separates differing velocity structures of the central Transverse Ranges and Mojave Desert. A weak low-velocity zone is centered approximately on the north-dipping aftershock zone of the 1971 San Fernando earthquake and possibly along the deep projection of the San Gabriel Fault. Modeling of gravity data, using densities inferred from the velocity model, indicates that different velocity-density relationships hold for both sedimentary and basement rocks as one crosses the San Andreas Fault. The LARSE II velocity model can now be used to improve the SCEC Community Velocity Model, which is used to calculate seismic amplitudes for large scenario earthquakes.
This article has been cited by other articles:
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  J. Pujol, K. Mueller, P. Shen, and V. Chitupolu High-Resolution 3D P-Wave Velocity Model for the East Ventura-San Fernando Basin, California, and Relocation of Events in the Northridge and San Fernando Aftershock Sequences Bulletin of the Seismological Society of America, December 1, 2006; 96(6): 2269 - 2280. [Abstract] [Full Text] [PDF]
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