STRESS AND PORE PRESSURE WITHIN AND NEAR THE SAN ANDREAS FAULT IN CENTRAL CALIFORNIA: OBSERVATIONS FROM SAFOD AND THE SURROUNDING CRUST RELATED TO THE MECHANICS OF FAULTING

The SAFOD drillsite is located 1.8 km from the San Andreas fault in central California at the transition between the creeping and locked sections of the fault, ~ 10 km northwest of the town of Parkfield. Observations of stress orientation and magnitude have been made to ~ 2 km depth in the vertical SAFOD pilot hole as well as in the SAFOD main hole, which cuts across the San Andreas Fault Zone at 3.1 km depth in the vicinity of clusters of repeating microearthquakes. Both in the pilot hole and in the main hole at distances as close as ~100 m from the active fault trace at depth, the direction of maximum horizontal stress is nearly normal to the San Andreas Fault. These observations, when combined with stress magnitude measurements in the pilot hole, strongly support the hypothesis that the San Andreas is a weak fault in an otherwise strong crust and confirms implications about fault strength based upon three decades of heat flow and stress measurements at shallower depth and greater distance from the fault. In addition, measurements of stress magnitude in the SAFOD main hole in the vicinity of the active fault trace at depth indicate an increase in the magnitude of all three principal stresses to values well above the lithostatic stress. High mean stress magnitudes (with low differential stress) within the fault zone are predicted by a variety of theoretical models of a weak transform fault embedded in an otherwise strong crust.

Anomalously high pore pressure within the San Andreas Fault Zone is one of the principal hypotheses called upon to explain how fault slip might occur at low levels of shear stress. To be an effective mechanism for lowering fault strength, pore pressure within the fault zone must be high relative to that in the adjacent country rock. However, observations during SAFOD drilling have so far revealed no indications of anomalously high pore pressure within the fault zone. Rather, the fault zone appears to be a permeability barrier that separates distinct hydrologic and geochemical regimes on either side of the fault. On the southwest side of the fault in the Salinian block, pore pressures are slightly subhydrostatic and normal 3He/4He ratios are observed. In contrast, elevated pore pressure, high 3He/4He ratios and distinct geochemical signatures are observed on the north-east side of the fault in the Great Valley Formation.