Fine-scale structure of the San Andreas fault zone and location of the SAFOD target earthquakes

We present results from the tomographic analysis of seismic data from the Parkfield area using three different inversion codes. The models provide a consistent view of the complex velocity structure in the vicinity of the San Andreas, including a sharp velocity contrast across the fault. We use the inversion results to assess our confidence in the absolute location accuracy of a potential target earthquake. We derive two types of accuracy estimates, one based on a consideration of the location differences from the three inversion methods, and the other based on the absolute location accuracy of “virtual earthquakes.” Location differences are on the order of 100–200 m horizontally and up to 500 m vertically. Bounds on the absolute location errors based on the “virtual earthquake” relocations are ∼50 m horizontally and vertically. The average of our locations places the target event epicenter within about 100 m of the SAF surface trace.