Electrical conductivity structure of the San Andreas Fault in central California

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Abstract
The electrical conductivity structure of a section of the San Andreas fault is investigated by means of an electromagnetic induction experiment that utilizes a grounded electric dipole source. Location of the study area is 18 km southeast of Hollister, where the fault runs adjacent to the Gabilan Mountains. The rerults indicate a sharp contrast in conductivity across the fault zone. The resistivity of the sediments on the northeast side of the fault is ≈ 20 Ω m compared to ≈ 1000 Ω m for the Gabilan granites on the southwest side. There is evidence for a zone of high conductivity beneath the Gabilans. The high conductivities beneath the Gabilans correlate with the results of earlier seismic and gravity studies. A gravity survey in the area indicates the presence of a pocket of low‐density rocks located ≈ 1–9 km beneath the Gabilans and ≈ 2 km southwest of the surface trace of the fault. The depth to the conducting layer there correlates reasonably well with the upper bound of these rocks. Farther away from the fault there is a discontinuity in seismic velocity at a depth of 12 km. The depth to the conducting layer in this area 13 km. The low‐density, highly conducting rocks adjacent to the fault zone are interpreted as sedimentary rocks like those on the northeast side of the fault, or fault gouge. Farther from the fault zone the seismic velocities and electrical conductivity at depth are consistent with the hypothesis that the Gabilans are underlain by a slab of serpentinized oceanic crust.