Investigation of temporal variations in stress orientations before and after four major earthquakes in California

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Orientations of the principal stresses before and after four major earthquakes in the greater San Francisco Bay Area were determined by inversions of 34 suites of focal mechanisms of about 1500 small earthquakes recorded by the Northern California Seismic Network over three decades. Stress orientations are expected to rotate due to the release of shear stress in a major earthquake. The degree of rotation can place some constraints on the ambient level of stress in the crust surrounding the mainshock. For the four earthquakes studied here, the 1986 Mt. Lewis, 1984 Morgan Hill, 1979 Coyote Lake, and 1989 Loma Prieta events, modest rotations of the maximum compressive stress S H to a higher angle (i.e., an orientation closer to fault-normal) appear to occur at the time of the mainshock. In some cases, S H eventually rotates back towards its original orientation. However, due to relatively large uncertainties obtained on the stress orientations, the constraints that can be inferred on the absolute levels of stress surrounding the mainshock regions are rather weak. By considering the largest stress change permitted by the confidence limits, we obtain lower bounds on the background deviatoric stress of 3–12 MPa, levels only slightly greater than the mainshock static stress drops.