Study of the caldera collapse process and magma plumbing system of Miyakejima volcano

点击翻译按钮获取中文摘要

The manner of the Miyakejima 2000 eruption was largely different from what islanders had experienced these hundred years. Lateral migration of magma outside Miyakejima, which occurred passively due to tectonic reasons, generated the summit subsidence, being associated with explosive eruptions from the summit in the summer of 2000. There are two kinds of juvenile materials in the 2000 eruption; aphyric glassy basaltic andesite erupted in the 27 June spatter and phyric basalt erupted as volcanic bomb of the 18 August eruption. It is likely that juvenile materials was replaced with from basaltic andesite to basalt in the middle August.Very long period seismic pulses (VLP pulses) were observed a few times a day from July 8 to Aug 18, 2000, synchronized with the step-like tilt increase. The pulse width is 40 to 50 sec and is almost constant regardless their amplitude. The occurrence of VLP pulses completely ceased after the summit eruption on Aug 18. Waveform analyzes of these signals show … More that the source mechanism of these pulses is characterized by large volume expansion of 10 m . Several models have been proposed. One is an intermittent subsidence of a piston in the volcanic conduit. Another is an underground vapor expansion model. An analysis of the seismic swarms prior to the VLP pulses also revealed the details of precursors to the piston-like collapse of the summit.We conducted a dense seismic observation during the period of May to September, and could found that the seismic waves were anomalously attenuated when crossing beneath the south part of the island.Based on analyzes of GPS data during the period of active degassing from the summit, we could propose a plausible model that the volume of the magma reservoir decreased corresponding to the amount of the volcanic gas components released from the magma in the reservoir.In order to detect the magma movements and temporal changes of hydrothermal system around the summit conduit, we have conducted electromagnetic and continuous absolute gravity observation, and observed systematic variations possibly caused by a temporal up rise of magma in the conduit in October to November 2001. Less