雲仙火山における人工地震探査 : 観測および初動の読みとり

In 1995, the second joint seismic experiment for investigations on the structures of volcanoes was conducted on Unzen Volcano, Western Kyushu, by a group of scientists from national universities and under the National Research Project for the Prediction of Volcanic Eruptions. Mt. Fugen, the main peak of Unzen volcano, abruptly started eruption in November, 1990, and the eruption continued till early 1995. A new lava dome was formed on the east side of Mt. Fugen and pyroclastic flows were generated from the lava dome. The estimated volume of magma erupted in this activity was 2xl08m3. To reveal the structure and the magma supply system under the volcano is one of the major aim of this experiment. A set of extensive explosion seismic experiments was conducted on November 30, 1995. Observations were made along a 26-km line and a 12-km line lying in the WNW-ESE and NNE-SSW directions respectively, and 5 small aperture arrays. The two lines cross on the western flank of the volcano. Six shots by chemical explosives with a charge size of 200-250 kg and 295 temporary seismic observations were arranged. All 6 shots were successfully fired, and clear onset and significant phases of seismic waves were recorded on compact data-loggers with precise GPS clock at most observation sites. The travel time curves of the first arrivals suggest that higher velocity region exists beneath the western flank of the volcano. In the observed records, we also found a number of remarkable later phases which are interpreted as reflected and scattered waves from the structure under the volcano. An outline of this experiments is given briefly in this paper, and detailed results of the experiments will be published later in separate papers