Earthquake locations and three‐dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD

Abstract

Arrival‐time data from 453 local earthquakes and 6 explosions at Parkfield, CA, are inverted for earthquake locations and three‐dimensional Vp and Vp/Vs structure. The structure is dominated by the velocity contrast across the SAF, with the southwest side about 20–25% faster, consistent with previous studies. Nearly all the earthquakes occur almost directly beneath the fault trace. We find high‐Vp/Vs anomalies that correlate with low‐resistivity features in a magnetotelluric model that are interpreted to represent fluids. We locate a magnitude 2 earthquake that is a potential target event for the final stage of SAFOD drilling, yielding a depth of 3.1 km below surface and an epicenter 100 m southwest of the fault trace. Nonlinear analyses indicate 95%‐confidence relative and absolute uncertainties on the order of 500–700 m vertically and 200–300 m horizontally for this target earthquake.