Three-Dimensional Fault Zone Structure and Earthquake Locations at Parkfield, CA: Preparing for the San Andreas Fault Observatory at Depth (SAFOD)

Arrival-time data from about 1000 local earthquakes and a number of shots are inverted for earthquake locations and three-dimensional Vp and Vp/Vs structure of the San Andreas fault zone at Parkfield, CA. Included are data from a temporary array of surface seismic stations installed around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole drilled this summer. The primary features of the velocity models are generally consistent with those from previous tomography studies, but with much finer detail. The structure is dominated by the velocity contrast across the SAF, with the southwest side about 20-25% faster than the northeast side. A low-Vp zone adjacent to the fault trace penetrates to a depth of as much as 9 km. Nearly all the earthquakes occur almost directly beneath the surface trace of the San Andreas Fault on the edge of a zone of high horizontal Vp gradient. Regions of anomalously high Vp/Vs (> 2.0) are found at shallow depths along and northeast of the SAF. These high-Vp/Vs anomalies can be associated with features in an existing resistivity model that are interpreted to represent fluid-saturated zones. Based on along-strike profiles through the model, we find that earthquakes are restricted to areas with Vp > 4 km/s and Vp/Vs < 1.9, with slightly higher Vp and lower Vp/Vs limits occurring where the surface fault slip rate is lower. We locate several magnitude 1 to 2 earthquakes at depths < 4 km and locations that are less than 100 meters southwest of the surface fault trace that are potential target events for drilling of the main SAFOD hole.