Earth, Planets and Space

, Volume 58, Issue 1, pp 23–31 | Cite as

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

  • Ayako OkuboEmail author
  • Tadashi Nakatsuka
  • Yoshikazu Tanaka
  • Tsuneomi Kagiyama
  • Mitsuru Utsugi
Open Access


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 


  1. Baranov, V. and H. Naudy, Numerical calculation of the formula of reduction to the magnetic pole, Geophysics, 29, 67–79, 1964.CrossRefGoogle Scholar
  2. Bhattacharyya, B. K., Magnetic anomalies due to prism-shaped bodies with arbitrary polarization, Geophysics, 29, 517–531, 1964.CrossRefGoogle Scholar
  3. Finn, A. C. and L. A. Morgan., High-Resolution aeromagnetic mapping of volcanic terrai], Yellowstone National Park, J. Volcanol. Geotherm. Res., 115, 207–231, 2002.CrossRefGoogle Scholar
  4. Hashimoto, T. and Joint Research Group of Universities for Unzen Volcano, Searching the conduit of Unzen Volcano using selfpotential anomalies, Report of Grant-in-ai] for Scientific Research (No. 06306011; Ohta, K.), 80–86, 1995a (in Japanese).Google Scholar
  5. Hoshizumi, H., K. Uto, and K. Watanabe, Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan, J. Volcanol. Geotherm. Res., 89, 81–94, 1999.CrossRefGoogle Scholar
  6. Hoshizumi, H., K. Uto, A. Matsumoto, S. Shu, A. Kurihara, and T. Sumii, History of formation of Unzen volcano, Chikyu Monthly, 24(12), 828–834, 2002 (in Japanese).Google Scholar
  7. Hoshizumi, H., K. Uto, and A. Matsumoto, Geology and petrology of Unzen volcano. Field Guidebook, A3: Unzen and Aso Volcanoes, XXIII General Assembly of the International Union of Geodesy and Geophysics, p. 11–19, 2003.6.Google Scholar
  8. Inoue, N. and K. Takemura, Subsurface structure around Unzen volcano based on gravity and geological data, Institute for Geothermal Sciences, Kyoto University, Annual Report FY 2002, 15–16, 2002.Google Scholar
  9. Kagiyama, T., H. Utada, F. Masutani, T. Yamamoto, H. Murakami, Y. Tanaka, H. Masuda, T. Hashimoto, Y. Honkura, M. Mishina, H. Matsuwo, and H. Shimizu, MT observation and the estimated process of magma ascent, Report of Grant-in-ai] for Scientific Research (No. 03306009; Ohta, K.), 73–86, 1992 (in Japanese).Google Scholar
  10. Kagiyama, T., T. Hashimoto, S. Wahyu, W. Kanda, Y. Tanaka, and M. Utsugi, Interaction of magma and water in Unzen volcano inferred from electromagnetic observation, 2002, Chikyu Monthly, 24(12), 858–865, 2002 (in Japanese).Google Scholar
  11. Makino, M., T. Nakatsuka, R. Morijiri, Y. Okubo, S. Okuma, and Y. Honkura, Derivation of three-dimensional distribution of geomagnetic anomalies from magnetic values at various elevations, Proc. 88th SEGJ Conf., Soc. Explor. Geophys. Japan, 502–507, 1993 (in Japanese).Google Scholar
  12. Mogi, T., Y. Tanaka, T. Morikawa, K. Kusakabe, M. Tanahashi, T. Nakatsuka, K. Tanaka, and H. Utada, Subsurface structure of Unzen- Fugen and Mayu-yama Volcano inferred from ai]borne electromagnetic method and magnetic survey, Bull. Volcanol. Soc. Japan, 40, 263–276, 1995 (in Japanese with English abstract).Google Scholar
  13. Nakatsuka, T., Reduction of magnetic anomalies to and from an arbitrary surface, Butsuri-Tanko (Geophys. Explor.), 34, 332–339, 1981.Google Scholar
  14. Nakatsuka, T., Aeromagnetic anomalies over the area of Unzendake volcano, J. Geomag. Geoelectr., 46, 529–540, 1994.CrossRefGoogle Scholar
  15. Nakatsuka, T., Minimum norm inversion of magnetic anomalies with application to aeromagnetic data in the Tanna area, Central Japan, J. Geomag. Geoelectr., 47, 295–311, 1995.CrossRefGoogle Scholar
  16. NEDO (New Energy Development Organization), Western district of Unzen, Rep. Promot. Dev. Geotherm., 15, 1060, 1988 (in Japanese).Google Scholar
  17. Okubo, A., Y. Tanaka, M. Utsugi, N. Kitada, H. Shimizu, and T. Matsushima, Magnetization intensity mapping on Unzen Volcano, Japan, determined from high-resolution, low-altitude helicopter-borne aeromagnetic survey, Earth Planets Space, 57, 743–753, 2005.CrossRefGoogle Scholar
  18. Okuma, S., Magnetic constrai]ts on the subsurface structure of Akita- Yakeyama volcano, northeast Japan, Earth Planets Space, 50, 153–163, 1998.CrossRefGoogle Scholar
  19. Okuma, S., M. Makino, and T. Nakatsuka, Magnetization intensity mapping in and arround Izu-Oshima Volcano, Japan, J. Geomag. Geoelectr., 46, 541–556, 1994.CrossRefGoogle Scholar
  20. Tada, T., Spreading of the Okinawa trough and its relation to the crustal deformation in Kyushu, J. Seismol. Soc. Japan, 37, 407–415, 1984 (in Japanese with English abstract).Google Scholar
  21. Tanaka, H., H. Hoshizumi, Y. Iwasaki, and H. Shibuya, Applications of paleomagnetism in the volcanic field: A case study of the Unzen Volcano, Japan, Earth Planets Space, 56, 635–647, 2004.CrossRefGoogle Scholar
  22. Utada, H., T. Kagiyama, T. Yamamoto, and Joint Research Group of Universities for Unzen Volcano, Deep resistivity structure in Unzen volcano by MT observation, Report of Grant-in-ai] for Scientific Research (No. 04302030; Ohta, K.), 64–71, 1994 (in Japanese).Google Scholar
  23. Uto, K., N. Hoang, K. Oguri, H. Hoshizumi, and S. Uchiumi, History of volcano development and magma evolution in Shimabara peninsula, Chikyu Monthly, 24(12), 835–842, 2002. (in Japanese).Google Scholar
  24. Webring, M.W., SAKI: A Fortran program for generalized linear inversion of gravity and magnetic profiles, U.S. Geol. Surv. Open File Rep., 85–122, 104 pp., 1985.Google Scholar

Copyright information

© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2006

Authors and Affiliations

  • Ayako Okubo
    • 1
    Email author
  • Tadashi Nakatsuka
    • 2
  • Yoshikazu Tanaka
    • 3
  • Tsuneomi Kagiyama
    • 3
  • Mitsuru Utsugi
    • 3
  1. 1.Disaster Prevention Research InstituteKyoto UniversityKyotoJapan
  2. 2.Geological Survey of JapanAISTTsukubaJapan
  3. 3.Institute for Geothermal Sciences, Graduate School of ScienceKyoto UniversityKumamotoJapan

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