Abstract

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 σ1 during extension to σ3 during flex-slip folding of the detachment in a restraining bend of the San Felipe fault zone (where σ1, σ2, and σ3 are the maximum, intermediate, and minimum principal stress directions, respectively). Regionally (10–100 km scale), we infer that the vertical stress alternates from σ1 during extension to σ2 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.

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