Quantitative Analysis of Pyroclastic Flows Using Infrasonic and Seismic Data at Unzen Volcano, Japan

Abstract

The process of the collapse of the dacitic lava dome and the development of pyroclastic flows at Unzen volcano, Japan, were studied using infrasonic, seismic and video records. Characteristic infrasonic and seismic signals were recorded corresponding to the collapse of lava blocks from the dome, the drop of blocks on the slope and the migration of pyroclastic flow on the mountain slope. Small infrasonic and seismic waves are excited when the lava dome starts to collapse. When the lava blocks fall onto the mountain slope and are fragmented, larger waves are excited. This suggests that the seismic waves are generated by the collision of pyroclastics on the mountain slope and that the infrasonic waves are excited by small fractures of the dome and the fragmentation of pyroclastics. Some of the infrasonic signals show an obvious Doppler effect, indicating that the pyroclastic flows emit infrasonic signals during their propagation. The location of dome collapse and the path of pyroclastic flows can be identified and traced by a network of low-frequency microphones. The migrating source of infrasonic signals and probably seismic signals is inferred to be located near the front of pyroclastic flows by comparison with video images. This suggests that the fragmentation of pyroclastics occurs mainly near the front of pyroclastic flows. The speed of pyroclastic flows is estimated as 10-30 m/s from the infrasonic records. The excitation of infrasonic and seismic signals is affected by the topography of the mountain slope. The infrasonic energy is almost the same order as the seismic energy but the ratio of infrasonic to seismic energies increases for larger and more mobile pyroclastic flows. This means that the development of pyroclastic flows is controlled not only by the volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava.