Description
New gravity, DC resistivity, and seismic refraction data were combined to image the damage zone surrounding the San Andreas Fault (SAF) near Little Rock, California, using fault-normal survey lines 76 to 380 m long. Analysis of these new data shows a decrease in electrical resistivity and seismic velocity in granitoids surrounding the fault, which is interpreted as increased damage, with more disruption on the northeast side. Immediately surrounding the fault, a zone less than 10 m wide of increased resistivity and seismic velocity suggests the presence of calcite cementation around gouge zones. Secondary faults also appear to be evident from the data. Comparisons with core samples, outcrop samples, and a trench wall log confirm the presence and interpretation of the features that are inferred from geophysics. Damaged rock extends across the entire length of the survey lines, with greater intensity near the principal slip zone of the San Andreas fault, as well as adjacent to secondary faults. The scale of the geophysically defined damage zone and structures agree well with field observations from earlier studies of core and outcrop samples and support the presence of an asymmetric zone of damage that may reflect a preferred direction of rupture.
