著者

笹井 洋一 上嶋 誠 歌田 久司 鍵山 恒臣 Jacques ZLOTNICKI 橋本 武志 高橋 優志

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.2, pp.226-244, 2001-04-25 (Released:2009-11-12)

参考文献数

31

被引用文献数

8 10

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Electric and magnetic field observations have been extensively carried out since 1995. A precursory magnetic anomaly was detected in July 1996, which was ascribed to thermal demagnetization at a depth of several hundreds of meters beneath the southern periphery of the summit Hatcho-taira caldera. Magnetic data revealed that the large depression at the summit associated with the steam explosion on July 8, 2000 had been completed within four minutes. Since the beginning of July, anomalous magnetic changes were observed at several magnetometer sites along the central N-S line of Miyake-jima volcano, which indicated the rise of a demagnetized area from depth to the summit. On July 4, a few days before the steam explosion, an area survey of SP in the summit caldera was conducted, discovering an extremely negative zone around the forthcoming depression, which suggested the intense absorption of ground water. Tilt-step events; i.e., abrupt uplifts around the summit area, were accompanied by electric field variations, which were very similar to the velocity waveform of the ground motion, as well as magnetic variations with step-like changes. An electric field can be interpreted as being due to electric currents generated by the forced injection of steam and/or water from the pressure source (electrokinetic phenomena). Magnetic changes are attributed to the piezomagnetic effect of rocks due to increased stresses. The geomagnetic total intensity showed large variations after the July 8 eruption, the typical feature of which was positive at the east and west sides and negative along the central north-south line of the volcano. They are ascribed to 1) the loss of magnetic mass from the summit and 2) the thermal demagnetization at depth. After the August 18 eruption, which was the largest, the steep changes in total intensity became flat, which suggested that the temperature rise at depth had weakened. At the time of the August 18 eruption, a large increase in self-potential was observed around the southwestern foot of the central cone Oyama : This implies that a definite change occurred in the hydrothermal system of the volcano.

著者

笹井 洋一 上嶋 誠 歌田 久司 鍵山 恒臣 Jacques ZLOTNICKI 橋本 武志 高橋 優志

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.110, no.2, pp.Plate7-Plate8, 2001-04-25 (Released:2009-11-12)