S38 Bulletin of the Seismological Society of America, Vol. 96, No. 4B, pp. S38–S49, September 2006, doi: 10.1785/0120050825 E Three-Dimensional Compressional Wavespeed Model, Earthquake Relocations, and Focal Mechanisms for the Parkfield, California, Region by Clifford Thurber, Haijiang Zhang, Felix Waldhauser, Jeanne Hardebeck, Andrew Michael, and Donna Eberhart-Phillips Abstract We present a new three-dimensional (3D) compressional wavespeed (V p ) model for the Parkfield region, taking advantage of the recent seismicity asso- ciated with the 2003 San Simeon and 2004 Parkfield earthquake sequences to provide increased model resolution compared to the work of Eberhart-Phillips and Michael (1993) (EPM93). Taking the EPM93 3D model as our starting model, we invert the arrival-time data from about 2100 earthquakes and 250 shots recorded on both per- manent network and temporary stations in a region 130 km northeast–southwest by 120 km northwest–southeast. We include catalog picks and cross-correlation and catalog differential times in the inversion, using the double-difference tomography method of Zhang and Thurber (2003). The principal V p features reported by EPM93 and Michelini and McEvilly (1991) are recovered, but with locally improved reso- lution along the San Andreas Fault (SAF) and near the active-source profiles. We image the previously identified strong wavespeed contrast (faster on the southwest side) across most of the length of the SAF, and we also improve the image of a high V p body on the northeast side of the fault reported by EPM93. This narrow body is at about 5- to 12-km depth and extends approximately from the locked section of the SAF to the town of Parkfield. The footwall of the thrust fault responsible for the 1983 Coalinga earthquake is imaged as a northeast-dipping high wavespeed body. In between, relatively low wavespeeds (5 km/sec) extend to as much as 10-km depth. We use this model to derive absolute locations for about 16,000 earthquakes from 1966 to 2005 and high-precision double-difference locations for 9,000 earth- quakes from 1984 to 2005, and also to determine focal mechanisms for 446 earth- quakes. These earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure. The aftershock sequence of the 2004 mainshock con- centrates into the same structures defined by the pre-2004 seismicity, confirming earlier observations (Waldhauser et al., 2004) that the seismicity pattern at Parkfield is long lived and persists through multiple cycles of mainshocks. Online material: 3D V p model and earthquake relocations. Introduction The long-anticipated occurrence of an M 6.0 earthquake at Parkfield, California, took place on 28 September 2004 (Bakun et al., 2005; Langbein et al., 2005). Due to the on- going Parkfield Earthquake Prediction Experiment (Bakun and Lindh, 1985) the earthquake was recorded by a sizable array of seismic (and other) instrumentation that allows us to develop a new, higher resolution, three-dimensional (3D) tomographic model of the compressional wavespeed (V p ) of the crust surrounding Parkfield, from San Simeon on the 0southwest to Coalinga on the northeast (Fig. 1). We use this new 3D model to derive absolute locations for about 16,000 earthquakes from 1966 to 2005 and high-precision relative locations for 9,000 earthquakes from 1984 to 2005 and determine focal mechanisms for 446 earthquakes. In ad- dition to refining our knowledge of the crustal structure and seismotectonics of this region, we anticipate that these re- sults will facilitate the development of high-quality rupture models for the 2004 event. Not only is this region well instrumented, but it has also been previously studied with 3D seismic wavespeed models