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Earth Planets Space, Vol. 58 (No. 1), pp. 23-31, 2006

Aeromagnetic constraints on the subsurface structure of the Unzen Graben, Kyushu, Japan

Ayako Okubo1, Tadashi Nakatsuka2, Yoshikazu Tanaka3, Tsuneomi Kagiyama3, and Mitsuru Utsugi3

1Disaster Prevention Research Institute, Kyoto University, Kyoto 611-0011, Japan
2Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan
3 Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Kumamoto 869-1404, Japan

(Received March 17, 2005; Revised August 16, 2005; Accepted August 16, 2005; Online published January 27, 2006)

Abstract: Aeromagnetic analyses have been conducted in and around the Unzen Volcano, Kyushu, Japan, in order to reveal the subsurface structure of the Unzen graben. First, we applied a magnetization intensity mapping method to analyze the aeromagnetic anomalies of the central part of the Shimabara peninsula. Magnetization highs and lows correspond to the Older Unzen (0.15-0.5 Ma) and the Younger Unzen (<0.15 Ma), respectively. However, the Mayu-Yama volcano is exceptionally high in the Younger Unzen. Moreover, it turns out that the Pre-Unzen (>0.5 Ma) or localized hydrothermally altered areas show magnetization lows. Next, magnetic models were constructed from aeromagnetic anomalies, drilling data and the result of magnetization intensity mapping. Finally, similar to the results of other geophysical data, it turns out that the Unzen graben has the features of a half-graben, with the northern fault (the Chijiwa fault) down in the western Unzen region and the southern fault (the Futsu and Fukae fault) down in the eastern Unzen region. Moreover, it clarified that the layers of low magnetization extend to the near-surface beneath Shimo-Dake, Kami-Dake, and the Unzen hot spring. These layers of low magnetization reflect the fractured or hydrothermally altered zones caused by the upflow of geothermal convection that exists in the central part in the graben.
Key words: Aeromagnetic analyses, the Unzen graben, and magnetic structure.


Corresponding author E-mail: ayako@svo.dpri.kyoto-u.ac.jp


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