Reexamination of moment tensors for initial motion of explosion earthquakes using borehole seismograms at Sakurajima volcano, Japan

Abstract

We estimated the seismic moment tensor for the initial phase of explosion earthquakes associated with Vulcanian explosions at the andesitic volcano Sakurajima. We applied a waveform inversion method to 24 events observed with borehole seismometers at 6 stations assuming a single triangular source time function. The three diagonal components representing volumetric change were dominant in the estimated moment tensors. The three diagonal components were positive and had almost the same values with the difference among them less than 10% for most of the explosion earthquakes, while non-diagonal components were negligibly small compared with the diagonal ones. Durations of source time functions had a range of 0.2–0.5 s and seismic moments were 1011 ∼ 1012 Nm. We conclude that the explosion earthquakes are initiated by an isotropic expansion.