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P-velocity models and earthquake locations in the Livermore Valley Region, California

LAWRENCE LIVERMORE NATIONAL LABORATORY UNIVERSITY OF CALIFORNIA, LIVERMORE, CALIFORNIA 94550

Abstract

P-wave travel times from 223 well-recorded earthquakes and five timed explosions recorded by the U.S. Geological Survey-Lawrence Livermore National Laboratory seismic network are inverted simultaneously for one- and three-dimensional velocity structures and hypocenter locations. The stability of earthquake locations between the one- and three-dimensional inversions demonstrates the adequacy of a layered earth model combined with station corrections for routine hypocenter locations in the Livermore Valley.

Initial one-dimensional velocity inversions indicate a fairly strong positive velocity gradient in the upper crust to depths of about 6 km. Calculated station delays show a good correlation with geologic structure. Arrivals at stations within the Livermore Valley are delayed by about 0.4 sec relative to those located to the south in the Diablo Range where relatively high-velocity rocks of the Franciscan formation are exposed. The Franciscan basement appears to occur at a depth of about 5 to 8 km beneath the valley.

Three-dimensional models show that the strongest velocity contrasts occur along the eastern and southern edges of the valley where lower velocity down-dropped Tertiary sediments and basin-full alluvium are found in structural contact with higher velocity Late Mesozoic and Tertiary sediments of the Altamont anticline and the Diablo Range. The low-velocity basin structure is truncated along the western margin of the Livermore Valley by the Calaveras fault across which higher velocity Cretaceous and Tertiary formations are exposed.

Some of the relocated aftershocks from the Livermore earthquake sequence show a good correlation with the surface trace of the northwest-trending Greenville fault. In the southeast corner of the Livermore Valley, a number of short (5 km or less), linear, and discontinuous epicentral zones are observed. These seismic lineations appear to define a conjugate set of right-stepping faults in a right-lateral shear zone. Focal mechanisms for these aftershocks show many complexities, but are generally consistent with this interpretation.

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