Reflection Profiles of the Active Seismic and Volcanic Region off the East Coast of Izu Peninsula.

Abstract

VIBROSEIS reflection explorations profiled the crustal structure under the east coast of Izu Peninsula where a seabed volcano erupted on July 13, 1989. Profiles along two exploration lines show a common feature that consists of three layers. The uppermost layer is floored with relatively intense reflections from shallow depths within 0.5s of two-way travel time, the next one is rather transparent layer, and the last is the basement ceiled with weaker reflections around 1.0s. The shallower reflectors are seen to be broken here and there. The depth of deeper reflector correctly coinsides with the upper limit of the earthquake swarm which is concentrated to the depth range of 2-4km. It is the most interesting that narrow spot-like high velocity bodies are found in the shallowest crust near the seismic swarm area. We suppose that those bodies may be interpreted as aged magma intrusions, and may create an asperity of the major earthquake of M5.5 prior to the submarine volcanic eruption.