Three‐dimensional P and S wave velocity structure of Redoubt Volcano, Alaska

Abstract

The three-dimensional P and S wave structure of Redoubt Volcano, Alaska, and the underlying crust to depths of 7-8 km is determined from 6219 P wave and 4008 S wave first-arrival times recorded by a 30-station seismograph network deployed on and around the volcano. First-arrival times are calculated using a finite-difference technique, which allows for flexible parameterization of the slowness model and easy inclusion of topography and source-receiver geometry. The three-dimensional P wave velocity structure and hypocenters are determined simultaneously, while the three-dimensional S wave velocity model is determined using the relocated seismicity and an initial S wave velocity model derived from the P wave velocity model assuming an average Vp/Vs ratio of 1.78. Convergence is steady with approximately 73% and 52% reduction in P and S wavearrival time RMS, respectively, after 10 iterations. The most prominent feature observed in the three-dimensional velocity models derived for both P and S waves is a relative lowvelocity, near-vertical, pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level. This feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit. The velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations. High velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates. No large low-velocity body suggestive of a magma chamber is resolved inthe upper 7-8 km of the crust. Introduction Redoubt Volcano, Alaska, one of several active Quaternary volcanoes lying in south-central Alaska at the eastern end of the Aleutian volcanic arc (Figure la), erupted more than 20 times between December 13, 1989, and April 21, 1990. As part of its investigation of this eruption sequence the United States Geological Survey (USGS) deployed a network of 21 portable three-component seismographs in the vicinity of Redoubt Volcano for 3 weeks during July 1991 (Figure lb). The portable seismic network, complemented by a nine-station regional seismic network operated by the Alaskan Volcano Observatory (AVO), recorded thousands of volcano-tectonic (VT) earthquakes that are used to determine the three-dimensional P and S wave velocity structure of Redoubt Volcano and to gain a better understanding of the dynamics of magma migration, emplacement, and eruption.

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