Abstract

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling faulting and earthquake generation along an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California and penetrates a section of the fault that is moving due to a combination of repeating microearthquakes and fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2–3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3192 m and 3302 m. Cores from both deforming zones contain a pervasively sheared, cohesionless, foliated fault gouge that coincides with casing deformation and explains the observed extremely low seismic velocities and resistivity. These cores are being now extensively tested in laboratories around the world, and their composition, deformation mechanisms, physical properties, and rheological behavior are studied. Downhole measurements show that within 200 m (maximum) of the active fault trace, the direction of maximum horizontal stress remains at a high angle to the San Andreas Fault, consistent with other measurements. The results from the SAFOD Main Hole, together with the stress state determined in the Pilot Hole, are consistent with a strong crust/weak fault model of the San Andreas. Seismic instrumentation has been deployed to study physics of faulting – earthquake nucleation, propagation, and arrest – in order to test how laboratory-derived concepts scale up to earthquakes occurring in nature. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.11.02.2011" target="_blank">10.2204/iodp.sd.11.02.2011</a>

Highlights

  • Introduction and GoalsSan Andreas Fault Observatory at Depth (SAFOD) is a scientific drilling project intended to directly study the physical and chemical processes occurring within the San Andreas Fault Zone at seismogenic depth

  • SAFOD is a scientific drilling project intended to directly study the physical and chemical processes occurring within the San Andreas Fault Zone at seismogenic depth

  • Detailed planning of a research experiment focused on drilling, sampling, and downhole measurements directly within the San Andreas Fault Zone began with an international workshop held in Asilomar, California in December 1992

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Summary

Introduction and Goals

SAFOD (the San Andreas Fault Observatory at Depth) is a scientific drilling project intended to directly study the physical and chemical processes occurring within the San Andreas Fault Zone at seismogenic depth. After drilling and downhole measurements were completed, the Pilot Hole was used for deployment of a vertical seismic array to record naturally occurring microearthquakes and to image some of the large-scale structures at depth in the vicinity of the San Andreas (Chavarria et al, 2003; Oye and Ellsworth, 2007) This array was used to record surface explosions as an important part of the effort to constrain seismic velocities in the vicinity of the borehole for achieving the best possible locations of the target earthquakes (Roecker et al, 2004). Phase 1, carried out during the summer of 2004, involved rotary drilling vertically to a depth of ~1.5 km, steering the well at an angle ~60° from vertical toward the repeating microearthquakes described above (Fig. 3) Note that these earthquakes occur to the southwest of the surface trace of the San Andreas, which indicates that at this location the fault dips steeply to the southwest. A number of other important downhole measurements were made during Phases 1 and 2. Boness and Zoback (2006) reported that to within

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June ‘07
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Findings
Summary
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