Structure and Properties of the San Andreas Fault in Central California: Recent Results from the SAFOD Experiment

The San Andreas Fault Observatory at Depth (SAFOD) is a 3.2-km-deep borehole observatory drilled into the San Andreas fault zone at seismogenic depths to study directly the physical and chemical processes controlling faulting and earthquake generation. SAFOD forms one component of EarthScope, a major Earth Science facility program of the U.S. National Science Foundation; the other two funded components are U.S. Array, a continental-scale seismic network, and the Plate Boundry Observatory, an extensive network of GOS receivers and strainmeters deployed acorss the western U.S. SAFOD is being conducted in collaboration with the U.S. Geological Survey and the International Continental Scientific Drilling Program (ICDP). A detailed scientific rationale for drilling into active faults (including an overview of past and ongoing fault zone drilling projects) is presented in Zoback et al. (2007). SAFOD is located in central California within transition between the creeping and locked sections of the San Andreas Fault, 9 km NW of Parkfield, Calif. and just NW of the rupture zone of the 2004 M6.0 Parkfield earthquake. The fault displays a range of behaviors at this site. At the surface, it is creeping at a rate of 1.8 cm yr, with most of the fault displacement localized to a zone no more than 10 m wide (Burford and Harsh, 1980; Schulz, 1989). Numerous earthquakes occur directly on the San Andreas fault at this location, and they define the fault to be a narrow, near vertical seismically active zone with a top at about 3-km depth (Fig. 1)