HR: 0800h
AN: V11B-1419    [Abstracts]
TI: Small inflation source producing seismic and infrasonic pulses without eruptions, during the 2000 Miyake-jima volcanic activity, Japan
AU: * Kobayashi, T
EM: tkoba@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Univ. of Kyoto, Gokasho, Uji, Kyo 611-0011 Japan
AU: Ida, Y
EM: yida@sci.u-hyogo.ac.jp
AF: Graduate School of Life Science, Univ. of Hyogo, 2167 Shosha, Himeji, Hyo 671-2201 Japan
AU: Ohminato, T
EM: takao@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tok 113-0032 Japan
AB: We detected low frequency seismic events and infrasonic pulses that repeated during the 2000 activity of Miyake-jima volcano, Japan. The signals are interpreted to have been excited by underground small inflation beneath a summit crater without remarkable eruptions. The low frequency seismic signals were intermittently observed with gradual increase of the magnitude toward the explosive eruption on 18 August. Small infrasonic pulses (IS pulses) occurred associated with the seismic events without eruptions, and had the pulse width of 1 to 2 seconds. The sources of IS pulses were located in the caldera area. The initial motions of low frequency events are of compression at all seismic stations, and their particle motions are outward from the region beneath the summit crater. Grid search by waveform inversion method shows that the initial motions are excited by isotropic inflation source beneath the south edge of the caldera at 1.4 km in depth. There were clear differences of the origin times between seismic events and IS pulses which emitted on the surface: 3.5 to 4.5 sec. The time difference suggests that a small isotropic inflation at the depth of 1.4 km excited a seismic pulses, simultaneously a wave that travels as a sound wave in the conduit and produce an IS pulses on the surface. In this scheme, the conduit should have been filled with a mixture gas composed of vapor and something, e.g. SO2 gas or basaltic ash, in order to explain the time difference. Our analyzing low frequency seismic signals have similar features often observed in volcanic explosions at various volcanoes; they are of compression in initial motions, and the sources are at the depth of a few km beneath the crater bottom. Repetitive events have high similarity. and infrasonic pulses were observed just after the seismic signal. However, the distinctive difference is that the seismic/airborne events did occur without remarkable eruptions. We think that we detected small underground eruptions not leading to volcanic explosions on ground surface, so to speak, seeds of eruptions.
DE: 8419 Eruption monitoring (7280)
DE: 7280 Volcano seismology (8419)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting