Three-dimensional P and S wave velocity structure of Redoubt Volcano,
Alaska
Journal of Geophysical Research, 101 (B4):
8111--8128 (1996)DOI: 10.1029/95JB03046
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 wave arrival 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 low-velocity,
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
in the upper 7-8 km of the crust.