Moment tensor analysis of near-field broadband waveforms observed at Aso Volcano, Japan

Locations and focal mechanisms of long-period volcanic events observed at Aso volcano, Japan, are determined by waveform inversion. Near-field broadband three-component seismograms of four to seven stations are simultaneously inverted in the time domain in order to find the six-component seismic moment tensor. A linear inversion is performed at each point of a 3D grid located under the volcano in order to constrain the centroid position and the focal mechanism of the source. The complete displacements, including near-field waves, in a homogeneous half-space for a general point source are taken into account. Inversions of 43 long-period tremors and six phreatic eruptions exhibit mainly an isotropic mechanism, with a minor deviatoric component that may originate from a north–south-trending crack. These events are all located in a small region at about 1.3 km depth beneath the active crater. The high accuracy, both of locations (a few hundred meters) and of focal mechanisms given by the inversion, is partly attributed to the rapid variation of the amplitude of the static displacement seen in the long-period part of the near-field seismograms.