Magnetotelluric imaging of the creeping segment of the San Andreas Fault near Hollister

[1] Fluids may play a significant role in the earthquake rupture process. Since fluids can dramatically change the electrical resistivity of crustal rocks, electromagnetic exploration methods can image in situ fault-zone fluids. Magnetotelluric (MT) data has been collected on segments of the San Andreas Fault (SAF) in Central California exhibiting a range of seismic behavior from locked to creeping. This paper describes MT data from the creeping segment of the SAF near Hollister, California. Inversion models image a zone of enhanced conductivity beneath the surface trace of the SAF that has similar geometry and greater depth extent than the fault zone conductor at Parkfield. Comparison with geologic and seismic tomographic studies show this zone is bounded by the San Andreas and Calaveras Faults, and coincident with a zone of low seismic P-wave velocities. Combined with previous MT studies, this indicates a correlation between seismicity and the presence of fault-zone fluids.