Hitoshi YAMASATO, Keiichi FUKUI, Kohichi UHIRA, Tetsuo HASHIMOTO and Hirokazu MORI
(appeared in Bulletin of Volcanological Society of Japan, 38, 79-90, 1993)
Small pyroclastic flows have been generated by collapses of lava domes at Unzendake volcano, Japan. Seismic and acoustic waves were analyzed that were excited by the pyroclastic flows. Minor seismic waves are excited at the time of lava dome collapse and low frequency seismic signals with larger amplitude follow them as the blocks of lava fall onto the slope. The low frequency seismic waves are interpreted as a result of the dome collapses and their amplitudes are correlative to the volumes of the collapsed lava blocks. While the pyroclastic flows descend along the mountain slope, high frequency seismic waves are emitted and they die out as the pyroclastics ceased to flow. The variation of amplitude ratio among the seismic waves recorded at different stations indicates that the source of seismic signals runs down along the slope. By analyzing the acoustic waves observed by low frequency microphones, the precise locations of the dome collapses were determined. Acoustic signals intermittently observed during the flow of pyroclastics are inferred to be excited from sources moving with the pyroclastic flow front. Excitation of seismic and acoustic waves are sensitive to topographic singu-larities and their amplitudes and frequencies vary with time.
Hitoshi YAMASATO, Sadayuki KITAGAWA, and Manabu KOMIYA
(appeared in Papers in Meteorology and Geophysics, 48, 1997)
At Unzen volcano, rainfall caused lava dome collapses and pyroclastic flow in some cases. Heavier precipitation increases the probability of dome collapses and pyroclastic flows, and increased pyroclastic flows are correlated with precipitation for certain periods, but not others. Dome collapses and pyroclastic flows were clearly triggered on fresh, and not yet cooled lava, and presumable originates in the instability of the lava dome cracked due to rapid cooling by rainwater.
Hitoshi YAMASATO
(appeared in Journal of Physics of the Earth, 45, 1997)
The process of the collapse of the dacitic lava dome and the development of pyroclastic flows at Unzen volcano, Japan, were studied by 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 onto the mountain slope and that the infrasonic waves are excited by small fracture 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 the 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 energy ratio of infrasonic to seismic becomes larger for larger and more mobile pyroclastic flows. This means that the development of pyroclastic flows is controlled not only by volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava.
Hitoshi YAMASATO
(appeared in Bulletin of Volcanological Society of Japan, 43, 1998
High frequency and low frequency earthquakes were observed associated with the intrusion of dacitic magma beneath the summit of Unzen volcano, Japan. The low frequency earthquakes were accompanied by small impulsive infrasonic signals, whereas the high frequency earthquakes were not. From the records of a low frequency microphone network, the infrasonic pulses are inferred to be excited at almost the same times as the earthquake occurrences. Source location of the infrasonic pulses could be accurately identified from the records of the low frequency microphone network. The sources were distributed around areas from which a new lava was emerging. The infrasonic pulse is inferred to result from an emission of volcanic gases through a fracture of an existing lava dome due to the intrusion of new magma.