Conductivity structure of the San Andreas fault, Parkfield, revisited

A previous interpretation of a high resolution magnetotelluric profile across the San Andreas fault at Parkfield attributed an anomalously conductive region just west of the fault and extending to depths of 3 km to fractured rock saturated with brines. Laboratory measured electrical resistivities of samples of sedimentary rocks known to border the fault to the east are comparable to resistivities of the anomalous zone. We suggest that the anomalous region results from conductive sedimentary rocks in a plunging syncline that is adjacent to the fault, rather than caused by fractured rock. If correct, the current surface trace of the San Andreas fault at Middle Mountain does not form the boundary between the Salinian block and the North American plate. Instead, that boundary is similar to1 km west and collocated with current seismicity.