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Glacier-generated earthquakes from Prince William Sound, Alaska

GEOPHYSICAL INSTITUTE UNIVERSITY OF ALASKA, FAIRBANKS, ALASKA 99775-0800

Abstract

Analysis of seismic records from stations near the Prince William Sound supports the association of monochromatic, low-frequency signals with tidewater glaciers in the area. Thirty-three of the larger, better-recorded signals, with corresponding earthquake magnitudes of M_L 1.5 to 2.5, were located using routine seismological techniques. Epicenters for these "glacierquakes" clustered in the vicinity of the Harvard, Columbia, Yale, and Barry glaciers. Monthly counts from one station for 1975, 1979, and part of 1976 range from less than 4 to 248 events. A sudden decrease in the number of events per month in mid-1979 raises the possibility of a single source or a common source mechanism for these glacier-related signals. The monochromatic, nondispersive waveforms which characterize these 1- to 2-Hz signals may result from harmonic resonance of the glaciers in the epicentral region. We speculate that a variety of source mechanisms within or near the glaciers may provide the initial impulse for these events. However, the characteristic observed waveform is most likely the result of seismic energy which is internally reflected within the ice at fundamental resonant frequencies. Boundary conditions associated with tidewater glaciers may also allow the ice body to act as a wave guide for shear or surface waves. The signal we observe, therefore, appears to be a source-site phenomenon.

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