SOURCE MECHANISM OF SEISMIC-WAVES EXCITED BY PYROCLASTIC FLOWS OBSERVED AT UNZEN VOLCANO, JAPAN

A number of pyroclastic flows have occurred at Unzen volcano (Unzendake) in Kyushu, Japan, since the end of May 1991. Seismic waves, which are excited by the pyroclastic flows, contain low-frequency components in their initial phases. They were recorded by two long-period seismographs in the seismic network deployed around the volcano. This paper is an attempt to clarify the source mechanism of the seismic waves triggered by the pyroclastic flows. Visual data show that the pyroclastic flow, which excites these seismic waves, occurs in three successive steps: first, fall of a lava dome front (dome collapse), then its collision with the mountain slope and its fragmentation into small blocks, and, finally, descent of the blocks along the slope. We can build a simple mechanical model which gives theoretical time histories of vertical and horizontal components of a force acting on the spot of dome collapse on the mountain slope. The model of a mechanism generating these seismic waves was constructed through analysis of the low-frequency components with the aid of inversion methods which can estimate source time function of the force acting on the source. The estimated time histories are: close to those predicted by the simple mechanical model. We conclude that the three steps mentioned above are essential in exciting the low-frequency seismic signals. This idea is supported by the fact that the volume of fallen material estimated from the visual data is proportional to the maximum vertical amplitude of seismic waves triggered by the pyroclastic flow. This relation holds for the two pyroclastic flows of June 22, 1991, and May 28, 1992, which were studied in this paper. These findings seismically confirm a visual observation that the pyroclastic flow is a result of dome collapse.