Location of Magma Reservoir beneath Bandai Volcano Deduced from Fan-Shooting Seismic Survey

Abstract

Velocity structure beneath Bandai volcano, the southern part of the Tohoku District, is studied by the use of amplitude data of the P-waves from local earthquakes occurring around the volcano and by the use of the data obtained from a fan-shooting seismic experiment around the volcano using a large capacity marine airgun. Instead of estimating amplitude change of the P-wave in accordance with the difference in event size, focal mechanism, attenuation by Q, site effect beneath observation stations, and geometrical spreading, amplitude of the initial part of the P-wave is normalized by the maximum amplitude of its coda wave to estimate attenuation along the ray path for the initial part of the P-waves. By arranging normalized seismograms, observed at a station near Bandai volcano from local earthquakes around the volcano, by back azimuths measured from the station to epicenters of events, it is found that initial parts of the P-waves become small in case when ray paths, connecting events to the station, pass t rough a zone within about 2.5km from the summit of the volcano. Anomalous attenuation zone found by the fan-shooting seismic experiment is also coincident with the zone determined by the local earthquake data. Theoretical amplitudes of the diffracted P-waves are calculated, and the results indicate that the existence of a melting zone with a diameter of 5km just beneath Bandai volcano explains observations of both the amplitude and the travel time data.