Seismicity and Forecasting of the 1992 Eruptions of Crater Peak Vent, Mt. Spurr, Alaska: An Overview

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Seismicity associated with the 1992 eruptions of the Crater Peak vent of Mount Spurr was monitored by a network of 6 to 10 seismometers within 15 km of the mountain's summit. Precursory seismicity be - gan in August of 1991. It was marked by a conspicu - ous swarm of shallow volcano - tectonic (VT) earth - quakes beneath Crater Peak, the first such swarm in a decade of monitoring, and by the gradual onset of seismicity between depths of 5 and 40 km, in a vol - ume of rock that had been essentially aseismic for nearly a decade. The volcano erupted three times during 1992: on June 27, August 18, and September 16 - 17. The June eruption was preceded by about 14 hours of cantinuous tremor and a vigorous 4 - hour swarm of shallow VT earthquakes. Following the June eruption, shallow seismicity decreased to normal background levels, and the August eruption occurred without any detectable precursory seismicity. A few hours of shallow seismicity preceded the September eruption, after which seismicity in the depth range of 5 to 40 km peaked. From an initial analysis of the seismic data, we sug - gest the 1992 eruption sequence resulted from the in - trusion of magma into the mid - crust that began in mid- 1991. The magma had migrated to shallower depths by early June of 1992, when the distribution of hy - pocenters changed and volcanic tremor began. The character of seismograms suggests the deep seismic - ity included a variety of source processes ranging from brittIe failure of the country rock in response to in - jection and withdrawal of magma to the vibration of fluid - filled cracks associated with magma transport. Close monitoring of Mount Spurr seismicity al - lowed the Alaska Volcano Observatory to issue pub - lic statements describing the increased activity begin - ning in August of 1991. Forecasts were issued prior to the June 27 and September 16 - 17 eruptions, and notifications were issued shortly after all three events. Two additional forecasts were issued on the basis of observed increases in seismicity that did not culmi - nate in


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U.S. Geological Survey Bulletin, v. 2139, p. 149-159