Stress measurements at depth in the vicinity of the San Andreas Fault: Implications for the magnitude of shear stress at depth

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 fault in relatively shallow (∼230 m) wells and the variation of stress with depth in a ∼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 Palmdale and in central California where the fault is creeping. In both areas the direction of maximum compression was found to be approximately 45° 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 markedly in the western Mojave, (2) the far-field shear stress at ∼200 m depth is ∼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. The ∼1-km-deep well, also located in the western Mojave desert, shows increases of both horizontal principal stresses and shear stress with depth. Shear stress increases from about 25 bars at 150–300 m to about 80 bars at 750–850 m. Although this rapid increase of shear stress with depth suggests that the mean shear stress on the fault at seismogenic depths exceeds several hundred bars, the principal stresses increase with depth in a steplike manner. As this may be a near-surface effect, extrapolation of the measurements to much greater depths may not be warranted.