Cross‐fault triggering in the November 1987 Superstition Hills Earthquake Sequence, southern California 论文

摘要

Two large strike‐slip ruptures 11.4 hours apart occurred on intersecting, nearly orthogonal, vertical faults during the November 1987 Superstition Hills earthquake sequence in southern California. This sequence is the latest in a northwestward progression of earthquakes (1979, 1981, and 1987) rupturing a set of parallel left‐lateral cross‐faults that trend northeast between the Brawley seismic zone and Superstition Hills fault, a northwest trending main strand of the San Jacinto fault zone. The first large event (M S =6.2) in the 1987 sequence ruptured the Elmore Ranch fault, a cross‐fault that strikes northeasterly between the Brawley seismic zone and the Superstition Hills main fault. The second event (M S =6.6) initiated its rupture at the intersection of the cross‐fault and main fault and propagated towards the southeast along the main fault. The following hypotheses are advanced; (1) slip on the cross‐fault locally decreased normal stress on the main fault, and triggered the main fault rupture after a delay; and (2) the delay was caused by fluid diffusion. It is inferred that the observed northwestward progression of ruptures on cross‐faults may continue. The next cross‐fault expected to rupture intersects both the San Andreas fault and the San Jacinto fault zone. We hypothesize that rupture of this cross‐fault may trigger rupture on either of these main faults by a mechanism similar to that which occurred in the Superstition Hills earthquake sequence.