| Co-Investigator(Kenkyū-buntansha) |
UTADA Hisashi ERI, Univ. Tokyo, Professor., 地震研究所, 教授 (70134632)
WATANABE Hidefumi ERI, Univ. Tokyo, Professor., 地震研究所, 教授 (20113649)
RIKIISHI Kunio Fac. Sci. Tech., Hirosaki Univ., Professor., 理工学部, 教授 (70038561)
UYESHIMA Makoto ERI, Univ. Tokyo, Research Associate., 地震研究所, 助手 (70242154)
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| Research Abstract |
We conducted observations of geomagnetic total force intensity on Miyake-jima, Kozu-shima. Mikura-jima and Hachijo-jima islands, electric field via ocean-bottom telephone cables among Izu-Oshima, Miyake-jima and Hachijo-jima islands, and long-distance self potential (SP), using telephone cables on Miyake-jima island. The observation is targeted at the behavior of EM fields induced by the dynamo action of conducting sea water under the main geomagnetic field in the oceanic area of southern Izu Islands region centered at Miyake-jima. In an open sea, no qontinl40us data are available to, monitor the overall distribution of ocean currents Instead of them, we presented a theoretical model of the tide in a flat ocean with a circular island and its, associated EM fields We also obtained the tidal constituents in the total intensity and ocean-bottom-cable electric field with the aid of the tidal analysis program BAYTAP-G. The model well explains the spatial dependence of the tide-induced total intensity on Miyake-jima By analogy, we inferred that the total intensity changes due to the meander of Kuroshio current should be mostly produced by electric currents in SE - NW direction, which was verified by the long-distance SP measurements across the island We carried out the resistivity soundings on Miyake-jima volcano by MT method. We found the low resistivity layer in the southern part of the island, where the amplitude of Kuroshio-induced magnetic field is large owing to the penetration of induced electric currents. A localized anomalous change was observed during the period from June 1996 to Sep. 1998, which suggests the thermal demagnetization within the volcano, i.e. the first observation of temperature rise since the 1983 eruption. The total intensity on Miyake-jima island has been slightly decreasing for the past 5 years, which is probably related to the inflation of the magma reservoir at depth through piezomagnetic effect.
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