Three-dimensional Seismic Velocity Structure beneath Unzen Volcano, Kyushu, Japan Inferred by Tomography from Experimental Explosion Data

Abstract

Seismic travel time inversion is widely used to estimate the velocity structure within a target area. For a given velocity structure a suitable ray tracer is needed to calculate the ray path and travel time between points. The velocity structure of volcanic areas varies considerably. Any seismic ray tracer used in volcanic areas should therefore be robust to velocity variation. A seismic ray tracer developed by the author to be effective with strongly varying velocity fields is used to calculate the travel time in an inversion scheme, and is shown to work well in volcanic regions (Nishi, 2001). The present paper gives the results of this travel time inversion method applied to data obtained from experimental explosions in Unzen volcano, Kyushu Japan. Seismic velocity in the center of the volcanic edifice is in general higher than peripheral area, as was already known from an analysis of the travel time by the refraction method (Explosion Seismic Research Group of Unzen Volcano and Shimizu, 1997). In detail, however seismic velocity structure in this region is laterally heterogeneous. Among the high velocity zone, some low velocity zones and also still higher velocity zone are found. Low velocity zone beneath the Fugendake peak (near to the lava dome generated in the 1991-1995 activity) is seemed to be related to the pressure source beneath the Fugendake peak estimated from leveling survey.