Alternating extensional and shortening stress fields on the west salton detachment fault, southern california

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Stress field perturbations are known to follow earthquakes, but the length scales over which the stress field varies are still poorly understood. Most focal mechanism inversions are assumed to represent the bulk regional stresses, yet fault irregularity and observed small-scale stress changes in boreholes suggest a strong degree of local heterogeneity. We present geologic paleostress inversion results from a segment of the folded West Salton detachment fault located in a left stepover in the San Felipe dextral fault zone of Southern California (United States). Extension and dextral slip in this region were synchronous during latest detachment slip. The local (1-10 km scale) vertical stress field alternated from s1 during extension to s3 during fl ex-slip folding of the detachment in a restraining bend of the San Felipe fault zone (where s1, s2, and s3 are the maximum, intermediate, and minimum principal stress directions, respectively). Regionally (10-100 km scale), we infer that the vertical stress alternates from s1 during extension to s2 during dextral slip. Both are consistent with nearly complete earthquake stress drops. If the fault-slip data set records paleoseismic strains, the magnitudes of differential stress and stress drop must be similar. Both may be relatively small during tectonic transitions. © 2013 Geological Society of America.

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