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High frequency teleseismic energy from Aleutian sources

UNIVERSITY OF MICHIGAN WILLOW RUN LABORATORIES GEOPHYSICS LABORATORY, ANN ARBOR, MICHIGAN 48107

Abstract

The Long Shot event, detonated at Amchitka in the Aleutian Islands, was recorded at three sites in Michigan at distances of 57 to 63 degrees. Analysis of the data recorded on magnetic tape shows significant high frequency (4-5 Hz) energy in the first compressional wave arrivals at all three stations, although the most prominent spectral peaks are in the range 1.0 to 1.6 Hz. Initial P phases of aftershocks of the Good Friday earthquake of 1964 recorded at the Ann Arbor station do not contain appreciable energy above about 2 Hz. However, the P-wave spectrum of an earthquake of comparable magnitude whose epicenter is near Long Shot is very similar to that of Long Shot, and indicates a favorable signal-to-noise ratio to 5 Hz.

The Long Shot spectra and apparent angle of incidence at the Michigan stations appear to be influenced by the presence of _pP and by crustal layering. The effective response of an intermediate-depth well system is shown to be strongly affected by surface reflections.