Earth Planets Space, Vol. 57 (No. 8), pp. 743-753, 2005
Ayako Okubo1, Yoshikazu Tanaka2, Mitsuru Utsugi2, Naoto Kitada2, Hiroshi Shimizu3, and Takeshi Matsushima3
1Disaster Prevention Research Institute, Kyoto University, Kyoto 611-0011, Japan
2Institute for Geothermal Sciences Graduate School of Science, Kyoto University, Kumamoto 869-1404, Japan
3Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Nagasaki 855-0843, Japan
(Received January 29, 2004; Revised June 30, 2004; Accepted July 12, 2004)
On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-borne aeromagnetic survey at two flight altitudes, using spiral trajectories for the first time, over Unzen Volcano in the framework of the Unzen Scientific Drilling Project (USDP). This study obtained more detailed and new information than the previous aeromagnetic studies in Unzen volcano about the geological features, for understanding the history and eruption mechanism of the Unzen volcano. Therefore, we conducted a magnetization intensity mapping on the volcano, on the assumption that the magnetic anomalies are caused by the terrain magnetized in the same direction as the present Earth's magnetic field and the magnetization intensity varies only laterally. This map shows good agreement with the geologic features, especially the hydrothermal alteration zone and the collapsed pyroclastic deposits. In addition, even in the area covered by lavas, the magnetization intensities show various values corresponding to each eruption event. It may be considered that the differences in magnetic properties reflect different oxygen fugacity in rocks during their cooling time period. Local magnetization lows on Heisei-Shinzan suggest that the Heisei lava produced by the 1991-1995 eruption has not yet been cooled enough.
Key words: Aeromagnetic survey, Unzen volcano, magnetization intensity mapping.