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

Legrand, D., S. Kaneshima, H. Kawakatsu
submitted to J. Vol. Geothermal Res., 101, 155-169, 2000.

Abstract

Locations and focal mechanisms of long period volcanic events observed at Aso volcano, Japan, are determined by waveform inversion. Near field broadband 3-component seismograms of 4 to 7 stations are simultaneously inverted in the time domain in order to find the 6-component seismic moment tensor. A linear inversion is performed at each point of a 3-D 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 is taken into account. 43 long period tremors and 6 phreatic eruptions inverted have mainly an isotropic mechanism, part of which may originate from a north-south trending crack. They are all located in a small region at about 1.3\,km depth beneath the active crater. The high accuracy of locations (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.