Near-surface in situ stress: 1. Strain relaxation measurements along the San Andreas Fault in southern California

Strain relaxation stress measurements were made in the Mojave Desert southeast of Palmdale, California, at two sites during the summers of 1979 and 1980, using the U.S. Bureau of Mines technique to depths of about 30 m. The field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m. At depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is N22°W±9° at 2 km south of the San Andreas fault and N13°W±2° at 20 km north of the fault. These azimuths compare favorably with the average of N21°W determined with nearby hydrofracture stress measurements (Zoback et al, 1980). Savage et al. (1981) also found a NNW orientation for the maximum shortening from a geodetic network with a 15-km aperature in the Palmdale area. The fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress. Finite element models of the San Andreas fault in southern California develop a stress field similar to that observed regionally (∼15°E for the maximum compressive stress) away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models. Near the fault, however, the model principal stresses are rotated counterclockwise similar to those measured near Palmdale, demonstrating the influence of the faults on the principal stress orientations.