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Masters Thesis

Seismicity of the Western Transverse Ranges of southern California : January 1981 to January 1995

The purpose of this thesis is to evaluate the spatial distribution of earthquakes with respect to fault zones in the Western Transverse Ranges of Southern California. The Western Transverse Ranges trend west-northwest to east-southeast. Some of the faults in the Transverse Ranges, of particular interest in this study, are the Oakridge, San Cayetano, Santa Susana, Simi, Northridge Hills, and the newly discovered thrust ramp in the San Fernando Valley. I used earthquake data gathered by computer from California Institute of Technology and compared my findings to those of reports by others. A select set of earthquakes was relocated to analyze the mechanisms of faulting and stress orientations during the time period from January 1981 to January 1995. The study concentrates on the seismicity in Ventura basin, the northern San Fernando Valley, and the south west Los Angeles basin. Earthquake studies by myself and G. W. Simila (CSUN) from 1981 to 1993, found increasing seismicity in the region of the newly found south-dipping thrust ramp associated with the January 17, 1994, Northridge Mw = 6.7, earthquake. Up to the time of the Northridge earthquake, however, there were not enough correlative data to show a definite south-dipping fault plane. Seismicity across the Ventura basin during the time period (1981-1995) did not show a marked increase until the 1994 Northridge event. Seismicity increases were noted as clusters of small earthquakes along known faults propagating west and northwest from the 1994 Northridge earthquake hypocenter. The orientation of the principal compressional (?1), and intermediate (?2) stress axes both before and after the 1994 Northridge earthquake are similar. The extensional axis (?3) shifted from about 45� east plunge to 90� plunge. This shift to vertical of the extensional (?3) axis is one indication of an increase in compressional strain across the Western Transverse Ranges during the study period.

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