Magnetic survey on Izu-Osima Island after volcanic eruption in November 1986.

Hydrographic Department, M. S. A. Japan conducted airborne/land magnetic survey on Osima island to investigate magnetic structure and their magnetic change accompanied by volcanic activity after eruption on November, 1986. In airborne magnetic survey, total intensity was measured by proton precession magnetometer every two seconds. Positions just over island were fixed using video image, and that over sea were determined by VLF /OMEGA positioning system. Positioning error over island was assumed less than 100 m in the usual condition. Detailed magnetic anomaly maps were made based on above survey data as shown in this report. These magnetic anomaly maps may play an important role for future study of magnetism of Osima volcano accompanied by volcanic activity. Using the above survey data, three dimensional magnetic analysis were conducted by Talwanis method. These results show that magnetization vector of Mt. Mihara become nearly half of that of Osima volcanic edifice, which may imply thermal demagnetization process of Mt. Mihara. The total intensity field derived by magnetic survey in December 1986 were also compared with that in September 1964, which were reported by Utashiro et al. (1972) to obtain magnetic change for these 22 years. This comparison shows drastic magnetic decrease amounting more than 1000 nT near Mt. Mihara, although there still remain some problems to confirm such a large amount of magnetic change. In addition, some model calculations on thermal demagnetization were shown for interpretation of magnetic change accompanied by volcanic activity. Twice of Land magnetic survey were conducted, First one was from Jan 26 to Feb 2, 1987 and second from Mar 2 to Mar 6, 1986. Vector component of magnetic field were measured by GSI first order magnetometer and proton magnetometer at three station, SO (near weather observatory), FU (near Mt. Futago), UR (eastern slope of Osima island), besides total intensity were measured at five points within caldera-rim. No distinctive magnetic change were observed between two land magnetic surveys, however comparison with magnetic contour map made by Yokoyama I. (1954) show a large amount of change at FU station.

Bibliographic Details

Main Authors:	Ueda, Yoshio, Nakagawa, Hisaho, Onodera, Kenei, Nagaya, Yoshiharu, Kumagawa, Koichi, Asao, Toshiyuki
Format:	Journal Contribution biblioteca
Language:	Japanese
Published:	1987
Online Access:	http://hdl.handle.net/1834/16110
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