GEOLOGIC CHARACTERIZATION OF RANGE-FRONT THRUST FAULTS, WESTERN MARGIN OF SANTA CLARA VALLEY, CALIFORNIA

A complex zone of southwest-dipping thrust faults is present along the western margin of Santa Clara Valley, California. The segment of range-front faults extending from Los Gatos through Los Altos Hills occupies a desirable area for continued residential development. Local faults include the Berrocal, Shannon, Monta Vista, and Altamont. Fault investigations completed for residential development in this area have yielded a wealth of information about the near-surface character of these faults.

Completed investigations and geomorphic expression strongly suggest relatively young (<10,000 years) displacement along some of these faults. Extensive ground cracks and/or areas of concentrated structural damage were also recorded along some of these faults in response to the 1989 Loma Prieta earthquake. Differentiation of sympathetic offsets from primary rupture has not yet been adequately resolved by exploratory trenching, and paleoseismic event history is essentially undefined.

Multiple, sub-parallel fault traces are not uncommon. Data show the presence of imbricate thrusting in some cases, with individual fault traces separated by up to 100 meters. Faults exposed near the ground surface often display dips from 45 degrees to near vertical. In many cases, fault plane striations are oblique suggesting a component of right-lateral displacement that may possibly replace a past history of reverse displacement. Where fault planes are parallel to the San Andreas fault and display near vertical dips, horizontal striations have occasionally been observed suggesting accommodation of right-lateral displacement. Fault offsets have been observed at formation contacts and within individual formations. Collection of near surface rupture data should assist with the development of 3D models depicting geologic structure of the Santa Clara Valley.