Stress orientations and magnitudes in the SAFOD pilot hole

Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, S-Hmax, but show a generalized increase in the angle between S-Hmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 +/- 10degrees at 1000-1150 m to a maximum of 69 +/- 14degrees at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust.