Abstract: Holocene Earthquakes on the Southern Sargent Fault, San Benito County, California

Erik N. Zinn, Jeffrey M. Nolan, Gerald E. Weber

The Sargent fault extends for 45 kilometers across San Benito and Santa Clara Counties, California. At its northern terminus, the fault appears to merge with the San Andreas fault near Lake Ellsman north of Loma Prieta mountain. The fault may connect with the Calaveras fault south of Hollister, California at its southern terminus. The Sargent fault appears to provide the primary control of the geomorphic development of Lomerias Muertas and the Flint Hills northwest of Hollister. Previous estimates of fault movement inferred from geomorphic expression are right-lateral reverse-oblique with the southwest side up.

Two trenches were excavated across the fault in Holocene sediments underlying the flood plain of the Pajaro River, north of Lomerias Muertas. The exposed fault dips 70 degrees to the northeast, contrary to previous characterizations of the fault, and displays down to the northeast, normal separation, although slickensides on the fault surface indicate a primarily horizontal slip vector. At least two Holocene rupture events are suggested by offsets of the stratigraphic section. Dating of C₁₄ samples presently in progress is expected to yield a Holocene slip rate for the fault. Funding for this project was provided by USGS National Earthquake Hazards Reduction Program award #1434-94-G2466.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California