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

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 hydrothemally 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 hydrothemally altered zones caused by the upflow of geothermal convection that exists in the central part in the graben.