S‐wave polarization inversion of the 15 October 1979, 23:19 Imperial Valley Aftershock: Evidence for anisotropy and a simple source mechanism

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Abstract
S polarizations from the 16 near source strong motion stations of the 15 October 1979, 23:19 Imperial Valley aftershock have been analyzed for constraining the source mechanism. They show a significant delay of the East component relative to the North component at most stations. Assuming that this is an effect of anisotropy with a horizontal symmetry axis in the upper 4 km of sediments (where the rays are dominantly vertical), the measured fast S polarizations are mainly oriented between N0°E and N25°E, and show a mean delay of 0.25 s between slow and fast S. This is consistent with an hexagonal anisotropy due to vertical cracks induced by the regional compressional stress field and parallel to it, with a mean fluid filled crack density of 0.1 to 0.2. The records, once corrected for the splitting effect, show S polarizations stable in time, which have been used to estimate the fault mechanism parameters. This is done by applying a new inversion method which is based on a model probability estimate. The error function is related to the polarization misfits, and the associated joint probability function of the data and parameter is computed in the whole model space. The inversion of the corrected S polarizations provides a unique and well constrained solution for the Imperial Valley aftershock mechanism (strike 145±5°, dip 85±10°, slip −155±15°), which is compatible with the available P polarities and is quite similar to the main shock mechanism.