Abstract: Geological and Seismological Investigations of the Evergreen Fault, Southeastern San Francisco Bay Area, California

Clark H. Fenton, Ivan G. Wong, Janet E. Sawyer

The eastern part of Santa Clara Valley is a region of compressive strain created by the left-stepover between the Calaveras and Hayward faults. Strain is accommodated by both vertical deformation, as evidenced by the East Bay Hills, and by transpressive movement on east-dipping faults, placing Paleozoic and younger rocks over Plio-Pleistocene gravels. Several of these east-dipping faults, including the Evergreen, Silver Creek, and Quimby faults, exhibit evidence for late Quaternary reverse or oblique movement.

Recent geologic investigations along the southern Evergreen fault, on the campus of Evergreen Valley College, indicate that it has been active in middle to late Pleistocene, forming a prominent southwest-facing topographic scarp and offsetting late Quaternary drainage. The most recent faulting event did not rupture to the surface, but caused warping and folding of overlying deposits that are possibly greater than 100,000 years old. Based on the offset of late Quaternary geomorphic surfaces, the estimated slip rate of less than 0.4 mm/yr on the Evergreen fault is an order of magnitude less than that reported for either the southern part of the Hayward fault or the Calaveras fault.

Relocation of over 350 earthquakes, recorded by the U.S. Geological Survey between 1968 and 1994, in the area surrounding the Evergreen fault shows a concentration of events located to the east of the southern portion of the fault, consistent with an east-dipping structure. Cross sections, however, show a diffuse distribution of seismicity with no clear alignment of hypocenters along an east-dipping fault plane. P-wave focal mechanisms indicate the presence of northwest-striking reverse faulting in response to a northeast-southwest maximum principal stress. Despite a relatively low slip rate and the absence of Holocene surface rupture, the structural association of the Evergreen fault with the Hayward fault suggests that it is a significant structure in terms of crustal deformation in this portion of the East Bay although its seismogenic potential remains unclear.

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