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The Mt. Rainier earthquake of July 18, 1973, and its tectonic significance

GEOPHYSICS PROGRAM UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON 98195
U.S. GEOLOGICAL SURVEY, TACOMA, WASHINGTON 98402

Abstract

On July 18, 1973, a magnitude 3.9 earthquake was strongly felt at Longmire and surrounding areas near Mt. Rainier, Washington. Network analysis permitted an accurate hypocenter to be located at 46°49.29'N and 121°49.86'W at a depth of 10.9 km, about 7 km southwest of the summit of Mt. Rainier. No prolonged aftershock sequence was generated, although two small aftershocks were recorded and located. Aerial photographs of the epicentral region reveal several northwest-trending lineaments which may be related to active faults in the region, although no surface ground breakage was discovered. The focal mechanism obtained for the main shock is well constrained and consistent with right-lateral strike-slip motion along a northwest-trending fracture, in general agreement with northwest-trending surface lineaments. The nature of the relationship of the earthquake occurrence to Mt. Rainier is uncertain. The principal compressive axis direction is in agreement with that found in the central Puget Sound basin. However, the shallow depths, the strike-slip mode of faulting, and the past evidence of earthquakes near Mt. Rainier suggest a direct relationship between faulting, earthquake generation, and the volcano location.

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