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The Hebgen Lake, Montana, earthquake of August 18, 1959: P waves

SEISMOGRAPHIC STATION UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA

Abstract

The instrumental epicenter of the Hebgen Lake earthquake is found to lie within the region of surface faulting. The depth of focus had a maximum value of 25 kilometers.

Times of P are studied in detail for epicentral distances less than 13 degrees. The apparent scatter of arrival times from 700 to 1400 kilometers can be explained by variations of the velocity of P_n between the physiographic provinces of the western United States. A comparison of observations for the Hebgen Lake earthquake with published times for blasts in Nevada and Utah indicates that the velocity of P_n in the central and eastern Basin and Range is about 7.5 km/sec, and that the crust in that region thickens toward the east and thins toward the south. A comparison of apparent velocities in northern California, in directions parallel and transverse to the structure, indicates that the crust thins by about 19 kilometers, from the edge of the Sierra Nevada to the Pacific Ocean.

A discontinuity is observed in the travel-time curve at a distance of 24–25 degrees. Arrivals of P waves in the range 65–128 degrees fall on two parallel travel-time branches; this multiplicity in the travel-time curve may be related to repeated motion at the source. Travel-times of PKIKP appear to deviate from published curves.

The fault-plane solution for the Hebgen Lake earthquake, together with a consideration of the first motion at Bozeman, Montana, indicates a focal mechanism of the dipole, or fault, type. The strike and dip of the instrumental fault plane agree well with observed ruptures at the surface.

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