1996 Fiscal Year Final Research Report Summary
CHARACTERISTICS OF GEOMAGNETIC TOTAL INTENSITY VARIATIONS ON MIYAKE-JIMA VOLCANO AND ITS DEEP RESISTIVITY STRUCTURE
| Project/Area Number |
07640549
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
固体地球物理学
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| Research Institution | The University of Tokyo |
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Principal Investigator |
SASAI Yoichi The University of Tokyo, Earthquake Research Institute, Associate Professor, 地震研究所, 助教授 (20012924)
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| Co-Investigator(Kenkyū-buntansha) |
TOH Hiroaki The University of Tokyo, Ocean Research Institute, Research Associate, 海洋研究所, 助手 (40207519)
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| Project Period (FY) |
1995 – 1996
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| Keywords | Miyake-jima volcano / Meanders of Kuroshio current / Motinally-induced magnetic field / Transfer functions of short-period geomagnetic disturbances / Island effect / Electro-magnetic watching system for active volcanoes |
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| Research Abstract |
We conducted continuous measurements of geomagnetic total force intensity at 8 stations and self electric potential (SP) at 2 stations on Miyake-jima island, a basaltic volcano about 150 km south of Tokyo, since October 1995 till February 1997. In late November of 1995, Kuroshio, the most dominant ocean current in the northwestern Pacific, changed its flow direction from west-east to south-north near around Miyake-jima island. The meanders recovered to normal west-east flow in June 1996. On Miyake-jima island, the total intensity increased by 5nT or more. SP variations were also observed in accordance with the meanders. This is the first example that electric and magnetic field variations associated with the meandering ocean flow were simultaneously observed at many stations on an isolated island. The motionally-induced electric and magnetic fields turned out to be the most serious noise of natural origin for the purpose of watching for the activity of Miyake-jima volcano by electromagnetic (EM) observations. However, its effect is dominant only when the main stream of Kuroshio current flows around the island.
We obtained the transfer functions of short-period geomagnetic disturbances at each magnetometer site as sources of information of the resistivity structure beneath Miyakejima island. The island effect, i. e. local magnetic field produced by distortion of induced electric currents in the ocean due to the land-sea conductivity contrast is rather weak. This implies that there exists a very conducting layr at a shallow depth beneath the island. A more quantitaive modeling study is now in progress.
We presented a possible eruption mechanism of Miyake-jima volcano based on EM observations during and after the 1983 eruption, combined with other geophysical and geological data. The present study gives the optimum design as well as its reliability assessment of the EM watching system for Miyake-jima volcano, which would most probably erupt early in the next century.
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