Using the hydraulic fracturing technique, we have made a systematic series of in situ stress measurements in wells drilled near the San Andreas fault. In an attempt to provide constraints for the magnitude of shear stress on the San Andreas fault at depth we have measured both the variation of stress with distance from the falt in relatively shallow (ca.230 m) wells and the variation of stress with depth in a ca. 1-km-deep well located 4 km from the fault. The shallow wells are located along profiles roughly perpendicular to the fault in the western Mojave desert near Plamdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45o from the local trend of the San Andreas. The two stress profiles show very similar results: 1) shear stress (on planes parallel to the San Andreas) increases with distance from the fault, more markedely in the western Mojave, 2) the far-field shear stress at ca. 200 m depth is ca. 50 bars ,and 3) the horizontal principal stresses as well as shear stress increase with depth more rapidly in the wells farthest from the fault. -Authors
Additional Publication Details
Stress measurements at depth in the vicinity of the San Andreas fault. Implications for the magnitude of shear stress at depth.