Limited data from a long-period vertical seismometer and the prototype of a new mercury tiltmeter at the station, Harvard, Massachusetts, tend to confirm that the Rayleigh-wave spectral ratio of short- to long-period energy provides a useful discriminant for underground explosions in the Western United States at distances of from 28° to 38° in the magnitude range of 3.5 < Ms < 5.6. This corresponds to discrimination in the range of 5.0 < mb < 6.3. The same spectral ratio, however, does not discriminate the large Milrow event in the Aleutians from earthquakes in the same region, possibly because of differences in earthquake source mechanisms in different regions and because this explosion released significant tectonic strain. The Rayleigh spectral ratio complements the Mgmb discriminant by providing a clear separation of earthquakes and explosions in some geographical regions where earthquakes are of low stress drop and generally strike-slip in mechanism.

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