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Source mechanisms of vulcanian eruptions at Mt. Asama, Japan, inferred from volcano seismic signals

Abstract During the 2004 Asama volcanic activity, five summit eruptions, accompanied by the broadband seismic signals, were observed. We re-analyse the broadband waveform data analysed by Ohminato et al. (2006) using relaxed restrictions. The results are essentially the same as those shown in the previous study. The results of the waveform inversions that assume a point source show that the force system is dominated by vertical single-force components. The source depths with dominant single-force components are 200–300 m beneath the summit crater. In the source-time history of the vertical single-force component, two downward forces separated by an upward force lasting for 5–6 s are clearly seen. We conduct a grid search for the best combination of two point sources, each consisting of a single-force component. The best waveform-match solution was obtained when one of them is positioned near the top of the conduit, and the other source is positioned 2000 m below the upper source. When a combination of single-force and moment-tensor components is assumed for the two-point source model, the moment source is located out of the vertical hypocentre distribution, suggesting a steeply inclined conduit.