Parkfield is the site of characteristic earthquakes which repeat every 22¿5 years, so that the next one is due between 1983 and 1993. We have installed a telluric monitoring array to detect possible changes of resistivity prior to the next characteristic earthquake. However, the array detects only changes in the telluric response and an independent estimate of the geoelectrical structure is needed in order to locate the perturbed regions and to determine the magnitude of the change in resistivity. We have developed a three-dimensional electrical model for Parkfield based on 12 shallow (<1500 m) Schlumberger soundings and 31 deep soundings using our array. The model is complex but shows that rock adjacent to and southwest of the San Andreas fault is conductive (less than 10 ohm m) to depths of at least 10 km. We infer that these low resistivities are due to brecciation from intense shearing along the fault.

We have also used this geoelectric structure to predict the sensitivity of our telluric array to changes in resistivity along the fault. Changes in resistivity beneath the array at depths from 1.5 to 6 km produce the largest effects at the surface (1% change for a perturbation of 1% at depth). Changes of resistivity from depths of 6--12 km yielded lower sensitivities but are still detectable. Because the top of the seismogenic zone in Parkfield is at 5 km, the array should be sensitive to changes of resistivity prior to the next earthquake. ¿ American Geophysical Union 1990