Norcal Geophysical Consultants INC

 

Case Studies

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1. MAPPING THE SAN ANDREAS FAULT, SANTA CLARA COUNTY, CALIFORNIA

NORCAL conducted a geophysical investigation at a site located in the foothills west of Saratoga, in southwestern Santa Clara County, California. The focus of the investigation is a small concrete bridge that spans the northwest trending Saratoga Creek Canyon. Since this bridge is located within the San Andreas fault zone it will need to be seismically retrofitted. This will require information on the proximity of fault traces relative to the bridge footings.

A trace of the San Andreas Fault has been mapped trending northwest along the canyon but the position has not been precisely determined within the study area. Exploratory borings and test pits completed by others have indicated differing types of bedrock on opposite sides of the canyon as well as two fault traces southeast of the bridge. The location of these structures, labeled East Trace and West Trace, are shown on the site map below. However, the exact locations of these fault traces at the bridge crossing are unknown. To aid in this endeavor NORCAL collected high resolution electrical resistivity and seismic refraction data from a 250-ft long geophysical traverse crossing Saratoga Creek Canyon about 50-ft southeast of the bridge, as shown.

fault investigation using electrical resistivity profiling

The results of the electrical resistivity survey are illustrated by the profile shown below. Variations in electrical resistivity with depth and distance beneath the geophysical traverse are indicated by colored contours according to the scale shown below the profile. There is a significant lateral change in resistivity at about the 100-ft Station, from moderate and moderately high resistivity on the west to low resistivity on the east. We interpret this change as indicative of faulting. The interpreted fault zone is indicated by the hachured area.

fault investigation using electrical resistivity profiling

The results of the seismic refraction survey are indicated by the profile shown below. Variations in seismic velocity versus depth and distance are indicated by colored contours according to the scale shown below the profile. Here, we see a lateral change in velocity from 6,000- to 6,500-ft/sec on the west to 5,000- to 6,000-ft/sec on the east. What makes the change notable is that the 6,000-ft/sec contour is vertical at about the 100-ft station. We interpret this as indicative of faulting and have denoted the interpreted fault zone by the hachured area.

fault investigation using electrical resistivity profiling

The points where the surface projections of the two fault traces delineated by trenching intercept the geophysical traverse are indicated on both images. It can be seen the surface projection of the west trace correlates with the interpreted fault zone based on the electrical and seismic data but the east trace does not. This suggests that either the east trace has no electrical or seismic expression or that it terminates south of the geophysical traverse.