Abstract

The Cache Valley, Utah, earthquake of 30 August 1962 (Ms 5.7) has been reexamined. Our revised focal mechanism, with dip 43°, strike 193° and rake −101°, confirms oblique normal faulting with nodal planes striking approximately north and south. This mechanism is based mainly on polarities from records at U.S. Air Force LRSM stations, which are a valuable scientific resource. Inversion of long-period teleseismic body waveforms indicates centroid focal depth 10 km and seismic moment 3.1 × 1024 dyne cm (0.31 × 1018 Nm) corresponding to Mw 5.6.

The Cache Valley, like other north-south-trending extensional features in the epicentral region, is bounded on its east side by a major active normal fault, the East Cache fault, which has several km of Neogene throw. However, our revised source coordinates for the Cache Valley earthquake indicate it occurred too far east to have involved slip on this fault. Its hypocenter was near a downdip projection of the subparallel Temple Ridge fault, a less prominent feature with only ∼500 m of Neogene throw, oriented subparallel to the west-dipping nodal plane of our focal mechanism. We suggest that this fault is continuous and approximately planar between ∼10 km depth and the earth's surface with ∼40° dip, and moved in the Cache Valley earthquake with a component of right-lateral strike-slip, with slip vector azimuth ∼N 65°W.

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