Analysis of Volcanic “Shallow Earthquakes” Observed near the Active Crater

Abstract

A great number of volcanic microearthquakes so called B-type occur at shallow depths beneath the active crater preceding volcanic explosions. The particle motions and apparent phase velocities are examined. The arrival times are read more precisely from the waveforms enhanced with a polarization filter. The following results are obtained. 1) The initial part of waves is composed of longitudinal waves, because the direction of polarization coincides with the azimuth of propagation and the apparent velocities are 2.0-3.2km/s. 2) Transvers waves are identified, because the direction of polarization is perpendicular to the azimuth of propagation and the apparent velocities are 1.0-1.5km/s. 3) The S-P times are ranging from 0.7 to 1.2 seconds at the station which is 1.7km apart from the active crater. The hypocenters of “Shallow earthquake” are determined by the tripartite method. “Shallow earthquakes” occur at the depths less than 2 km.