Abstract

Histograms of fault dips have been compiled for moderate to large (M > 5.5) reverse-slip intracontinental earthquakes with the slip-vector raking 90 ± 30° in the fault plane. The principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane; the reverse fault dips are bracketed within the range 12° < δ < 60° with a prominent peak in the 25° to 35° interval and a subsidiary peak in the 45° to 55° interval. Assuming horizontal trajectories for maximum compressive stress (σ1), the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within Byerlee's (1978) range (0.85 > μs > 0.6), undergoing frictional lockup at dips approaching 60°. The broad 25° to 35° peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening. Paucity of very low-angle thrusts implies that it is generally the steeper ramps within ramp-flat assemblages that fail in moderate to large earthquakes. The subsidiary peak at 45° to 55° likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion, requiring some degree of fluid overpressuring. The results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60°; together, these analyses suggest that “Byerlee” friction coefficients apply to faults with displacements of up to a few kilometers.

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