The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system.
The San Andreas Fault Observatory at Depth (SAFOD) was conceived to closely study and monitor the earthquake dynamics and structure of the San Andreas Fault (SAF) at Parkfield, CA. Characterizing the structure and dynamics of the SAF strike-slip system is crucial for understanding the geodynamics of transform plate boundaries and their associated seismicity. Consisting of a vertical borehole, the Pilot Hole (PH), and of a deviated well that intersects the SAF, the Main Hole (MH), SAFOD is designed to sample and monitor the SAF system from within the subsurface at Parkfield. Figure 1a is a scaled schematic cartoon that summarizes and connects results from several publications that analyze data from the SAFOD site. Much of the information on the surface geology and on the basement and sedimentary structures at Parkfield comes from geologic mapping (Rymer et al., 2003) and from surface refraction (Catchings et al., 2003) and reflection (Hole et al., 2001; Catchings et al., 2003) seismic data. The lateral delineation of the Salinian granite to the SW of the SAF has also been inferred from magnetotelluric (Unsworth and Bedrosian, 2004) measurements and from joint inversion of gravity and surface seismic data (Figure 1b; Thurber et al., 2004). Recent studies of rock samples from drilling (Solum et al., 2006) and well-logs (Boness and Zoback, 2006) from the MH have shed light on the subsurface geology along the SAFODMH. It was not until 2006 that the SAFOD MH first intersected the SAF (Figure 1a), and the upcoming coring of the SAF system during the Phase 3 drilling of the MH (summer 2007) promises to bring important information on the internal composition of the SAF. Previous to the drilling of the SAFOD MH, microseismic events along with surface active-shots recorded at the PH seismometer array were used to make some of the first images of the SAF at depth.