Abstract

On 20 August 1999, a magnitude 5.3 earthquake occurred in southwestern Montana, ending a 25-year hiatus for magnitude 5+ seismicity in Montana. This earthquake occurred in the central part of the Red Rock Valley, a northwest-trending graben bounded by late Pleistocene and Holocene faults. A focal depth of 12.4 km and a normal-faulting focal mechanism suggest that this earthquake resulted from continued graben development, although not along graben-bounding faults mapped at the surface. The 1999 Red Rock Valley earthquake occurred near the northern end and within the footwall block of the northeast-dipping Red Rock fault. We deployed a temporary network close to the mainshock epicenter and located 65 aftershocks over 3 days, including a magnitude 4.0 aftershock on 26 August that allowed determination of P-wave travel-time delays for regional seismograph stations. Using these station delays to improve relative hypocenter locations, we recomputed hypocenter locations for >1000 Red Rock Valley area earthquakes that occurred since 1989. Relocated hypocenters avoid the Holocene portion of the Red Rock fault but do surround it. The mainshock epicenter location is close to that of a magnitude 5.0 earthquake in 1965.

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