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Observations and analysis of B-type earthquakes, explosions, and volcanic tremor at Pavlof Volcano, Alaska

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY AND DEPARTMENT OF GEOLOGICAL SCIENCES, PALISADES, NEW YORK 10964

Abstract

An eight-station network of short-period seismometers has been operated near Pavlof Volcano (55°25'N, 161°54'W) Alaska Peninsula since 1976. High-quality analog-to-digital and direct digital data were recorded during eruptions in 1980 through 1983. Data are analyzed from explosions with distinct air phases, B-type volcanic earthquakes (shallow, low-frequency events lacking clear S phases), and volcanic tremor to determine source and propagation effects. Between zero and several hundred B-type events and explosions per day are recorded, with higher numbers during eruptions. Magnitudes range between –0.5 and 1.3, and b-values range between 1.9 ± 0.1 and 2.6 ± 0.1 with no systematic relation to eruptive activity.

Six results are most important: (1) data from a recently installed three-component station reveal that the highest-amplitude waves of the B-type events show retrograde elliptical particle motion and normal dispersion, and are hence probably Rayleigh waves; (2) stacking of the events enhances the P wave, which has a velocity of 3.1 ± 0.1 km sec^–1; (3) the explosions and B-type events have similar hypocenters, as evidenced by signal characteristics and wave arrival times; (4) the B-type events contain virtually no high-frequency energy, even when allowing for attenuation effects. In particular, we estimate the anelastic attenuation coefficient Q to have a value of 45 ± 20 for Rayleigh waves in the shallow part of the volcano; (5) volcanic tremor spectra exhibit evenly spaced narrow peaks, which we model as eigenvalues of organ-pipe resonance in the magma conduit; we estimate the length of the conduit to be a maximum of 1.6 km; (6) the volcano shows evidence of extreme lateral variation of velocity and attenuation in its shallow structure.

Footnotes

^* Present address: State of California, Department of Conservation, Division of Mines and Geology, 630 Bercut Drive, Sacramento, California 95814.

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