HOLOCENE FAULT ACTIVITY ALONG THE SANTA CLARA VALLEY MARGIN, CALIFORNIA

Paleoseismic trenching across a trace of the Shannon fault, recently named the Blossom Hill fault (BHf), has revealed repeated activity within approximately the last 1 ky with at least two events rupturing the ground surface. The BHf is one of several thrust and oblique-slip faults, sub-parallel to and northeast of the San Andreas fault (SAf), which have accommodated uplift of the Santa Cruz Mountains since the Late Miocene. These faults occupy an area of increasing residential development in hillsides adjacent to the densely populated Santa Clara Valley (a.k.a., Silicon Valley). Recent geomorphic studies by others have indicated Pleistocene activity, and extensive damage during the 1989 M 6.9 Loma Prieta Earthquake (SAf) occurred along mapped traces of the faults. Nevertheless, convincing paleoseismic evidence of Holocene activity had eluded investigators prior to this study.

Three distinct faulting events within the trench exposure are indicated by: a 60 cm thick sub-vertical fissure, bedrock steps, displaced and missing Holocene soil horizons, and folded soil horizons. Structures associated with two of the events are coincident with a well-developed gouge zone within underlying Plio-Pleistocene bedrock. Slickensides on discrete shear surfaces indicate both dip-slip and oblique-slip events have occurred along this fault.

Single-event vertical displacements are on the order of 30 to 45 cm. Total vertical displacement of the bedrock/soil contact is 150 cm, implying possible additional events to account for missing slip. Soil ages were interpreted by analyzing the soil profile development, as a lack of dateable carbon material precluded obtaining absolute ages. However, interpreted late Holocene ages of the soils are within the uncertainty limits of this dating technique. Our data suggest a Holocene vertical slip rate of approximately 0.15 mm/yr for the BHf. This rate is consistent with long-term estimates by others for associated faults.

Whether or not the BHf or associated faults are independent seismic sources remains uncertain. However, from a seismic hazard perspective, the fact that the BHf and associated faults experienced triggered slip during the Loma Prieta Earthquake suggests they may be of greater concern than if they only ruptured independently.