STRUCTURE AND COMPOSITION OF THE SAN ANDREAS FAULT AT SEISMOGENIC DEPTHS: RECENT RESULTS FROM THE SAFOD EXPERIMENT

The San Andreas Fault Observatory at Depth (SAFOD) is being drilled into the San Andreas Fault Zone to study the physics of earthquake nucleation and rupture and determine the composition, physical properties, and mechanical behavior of an active, plate-bounding fault at depth. SAFOD is located 10 km NW of Parkfield, California, and penetrates a section of the fault that is moving through a combination of repeating microearthquakes and fault creep. During Phases 1 and 2 in the summers of 2004 and 2005, SAFOD was drilled vertically to a depth of 1.5 km and then deviated through the entire San Andreas Fault Zone to a depth of 3.1 km. During Phase 3 of SAFOD (being carried out in the summer of 2007), continuous cores are being acquired from holes branching off the main SAFOD borehole to directly sample both creeping and seismically active traces of the fault at depth.

Borehole geophysical logs and drill cuttings analyses conducted during SAFOD Phases 1 and 2 reveal that the San Andreas Fault Zone consists of a relatively broad (~200 m) damage zone of anomalous geophysical properties, which contains several discrete zones with even more anomalous geophysical properties (and mineralogies) that may represent active shear zones. Two of these shear zones are now actively deforming the cemented casing. One of these zones is also associated with the sudden appearance of serpentine and talc, minerals thought to be important in controlling frictional strength and the stability of sliding.

The Phase 3 core samples will be extensively studied in the laboratory to compare and contrast the composition, deformation mechanisms, physical properties and rheological behavior of creeping and seismogenic fault rocks at realistic in-situ conditions. At the conclusion of Phase 3, an array of seismometers, accelerometers, tiltmeters and fluid pressure sensors will be deployed in the cased borehole in the immediate vicinity of repeating M2 earthquakes. Acting in concert with studies on recovered samples, SAFOD long-term monitoring will be used to test hypotheses related to earthquake rupture nucleation, propagation and arrest; the possible role of fluid pressure in controlling fault strength and earthquake periodicity; and the manner in which earthquake energy is partitioned among seismic radiation, frictional dissipation, grain-size reduction, and chemical reactions.