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

A number of pyroclastic flows have occurred at Unzen volcano (Unzendak e) in Kyushu, Japan, since the end of May 1991. Seismic waves, which a re excited by the pyroclastic flows, contain low-frequency components in their initial phases. They were recorded by two long-period seismog raphs in the seismic network deployed around the volcano. This paper i s an attempt to clarify the source mechanism of the seismic waves trig gered by the pyroclastic flows. Visual data show that the pyroclastic flow, which excites these seismic waves, occurs in three successive st eps: first, fall of a lava dome front (dome collapse), then its collis ion with the mountain slope and its fragmentation into small blocks, a nd, finally, descent of the blocks along the slope. We can build a sim ple mechanical model which gives theoretical time histories of vertica l and horizontal components of a force acting on the spot of dome coll apse on the mountain slope. The model of a mechanism generating these seismic waves was constructed through analysis of the low-frequency co mponents with the aid of inversion methods which can estimate source t ime function of the force acting on the source. The estimated time his tories are: close to those predicted by the simple mechanical model. W e conclude that the three steps mentioned above are essential in excit ing the low-frequency seismic signals. This idea is supported by the f act that the volume of fallen material estimated from the visual data is proportional to the maximum vertical amplitude of seismic waves tri ggered by the pyroclastic flow. This relation holds for the two pyrocl astic flows of June 22, 1991, and May 28, 1992, which were studied in this paper. These findings seismically confirm a visual observation th at the pyroclastic flow is a result of dome collapse.