"The San Andreas Fault Observatory at Depth: Testing fundamental theories of faulting and earthquake mechanics through drilling"

Stephen Hickman, Earthquake Hazards Team, United States Geological Survey, Menlo Park, CA

The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physics of faulting and earthquake generation and determine the composition, physical properties, and mechanical behavior of an active, plate-bounding fault at depth. SAFOD is located near Parkfield, California, and penetrates a section of the fault that is moving through a combination of repeating microearthquakes and fault creep. In 2004 and 2005, SAFOD was drilled vertically to a depth of 1.5 km and then deviated across the entire San Andreas Fault Zone to a vertical depth of 3.1 km. In 2007, cores were acquired from holes branching off the main hole to directly sample the country rock and actively deforming traces of the fault.

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 3194 m and 3301 m (corresponding to vertical depths of 2.6 - 2.7 km), indicating that they are actively creeping shear zones. The 3194 m casing deformation zone lies ~100 m above a cluster of repeating M2 earthquakes that form the southwestern boundary of the active fault zone.

Stress measurements in SAFOD confirm previous inferences that the San Andreas Fault is a weak fault in an otherwise strong crust. The direction of the maximum horizontal principal stress rotates with increasing depth to maintain a high angle to the fault, even as the active fault zone is approached. Hydraulic fracturing stress measurements indicate an increase in the magnitude of the least principal stress in proximity to the fault, as theoretically predicted. There are no indications of anomalous pore pressure within the fault; instead, the fault separates distinct hydrologic regimes, with elevated pore pressure and different geochemical signatures on the northeast side of the fault.

Core was obtained in 2007 from just outside the geologically defined San Andreas Fault Zone and the active deformation zones at 3194 and 3301 m. Cores from both deformation zones contain 1-2 m of a pervasively sheared, cohesionless, foliated fault gouge. This fault gouge has an anomalous mineralogy including serpentine and an ordered chlorite/smectite phase, which may be responsible for fault creep and low strength. These cores will be extensively tested in the laboratory to study their composition, deformation mechanisms, physical properties and rheological behavior. In 2008, an array of seismic, tilt and fluid pressure sensors will be deployed in SAFOD near the repeating M2 earthquakes to test hypotheses related to earthquake rupture initiation and propagation and the possible role of fluid pressure in controlling fault strength and earthquake periodicity.

 Ver Resumo