3-DIMENSIONAL P-WAVE AND S-WAVE VELOCITY STRUCTURE OF REDOUBT VOLCANO, ALASKA

The three-dimensional P and S wave structure of Redoubt Volcano, Alask a, and the underlying crust to depths of 7-8 km is determined from 621 9 P wave and 4008 S wave first-arrival times recorded,by a 30-station seismograph network deployed bn and around the volcano. First-ai rival times are calculated using a finite-difference. technique, which allo ws for flexible parameterization of the slowness model and easy inclus ion of topography and source-receiver geometry. The three-dimensional P wave velocity structure and hypocenters are determined simultaneousl y, while the three-dimensional S wave velocity model is determined usi ng the relocated seismicity and an initial S wave velocity model deriv ed 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 mod els derived for both P and S waves is a relative low-velocity, near-ve rtical, pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level. This' feature aligns axially with t he bulk of seismicity and is interpreted as a highly fractured and alt ered zone encompassing a magma conduit. The velocity structure beneath the north flank Of the volcano between depths of 1 and 6 lan is chara cterized by large lateral velocity variations High velocities within t his region are interpreted as remnant dikes and bills and low velociti es as regions along which magma migrates. No large low-velocity body s uggestive of a magma chamber is resolved in the upper 7-8 lan of th cr ust.