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The San Andreas Fault Observatory at Depth (SAFOD), drilled near the town of Parkfield, California (Fig. 1) (Hickmen et al., 2004) as part of the U.S. National Science Foundation's EarthScope Project (see http://www.earthscope.org), provieds a continuous set of samples through the active San Andreas Fault (SAF) zone. These samples can help address decades-old questions such as apparent weakness of the SAF (Zoback, 2000) providing the unparalleled opportunity to contrain the parameters that control the behavior of plate counding faults like the San Andreas.
Samples collected form SAFOD also complement studies of exhumed fault zones. While studies of exhumed fault zones have benefit from regionally extensive exposures that allow for detailed mapping and analyses (Chester and Chester, 1998; Evans and Chester, 1995), such studies also have inherent limitations including uncertainty about stress states and styles of deformation (i.e., seisomgenic vs. creeping). Moreover, mineral assemblages in exhumed fault zones can be altered during exhumation, obscuring fault-related mineral assemblages and textures (Solum et al., 2003; Solum and can der Pluijm, 2004).
The SAFOD target zone contains two parallel strands that generate repeating M2 earthquakes in addition to having a component of creep (Nadeau et al., 2004). Therefore, SAFOD also offeres the opportunity to obtain samples from seismogenic and aseismic faults and to constrain the rheology of the active SAF system.