INTERNAL STRUCTURE OF THE SAN-ANDREAS FAULT AT PARKFIELD, CALIFORNIA

Magnetotelluric and seismic reflection surveys at Parkfield, Californi a, show that the San Andreas fault zone is characterized by a vertical zone of low electrical resistivity. This zone is approximate to 500 m wide and extends to a depth of approximate to 4000 m. The low electri cal resistivity is attributed to high porosity of saline fluids presen t in the highly fractured fault zone. The occurrence of microearthquak es and creep in the low resistivity zone is consistent with suggestion s that seismicity at Parkfield is fluid driven.