霧島-やや張力的応力場に生成した火山群

抄録

The volcanoes of Kirishima, located in southern Kyushu, consist more than 20 volcanoes occupying an area of about 20 x 30 km elongated northwest-southeast. At least three volcanoes have historic records of eruptions ; Shinmoe-dake, Ohachi and Iwo-yama, and more than 10 volcanoes were active in the past 22, 000 years. This indicates that Kirishima is a multi-active volcanic group. Intense geothermal activity and frequent earthquake swarms are also characteristic features of Kirishima. Through seismological and electromagnetic observations, the nature of the Kirishima 'multi-active volcanic system' has been revealed.
According to the seismometrical observation by the Kirishima Volcano Observatory, the majority of the hypocenters are distributed along several trends that are elongate either NE-SW or NW-SE. The dominant focal mechanism solutions indicate normal faulting with a NW-SE tensile axis in the NE-SW segments and of strike slip in the NW-SE segments. Active volcanoes are located along these lines, and extremely shallow earthquakes are distributed just beneath the craters of these volcanoes. The seismicity and focal mechanism indicate that the Kirishima area is 3-subject to NW-SE extensional stress. The slight extensional stress is favorable for a fault system that allows magma to ascend at various points. This may be the reason why a multi-active volcanic group was generated in this area instead of a large stratovolcano.
According to the VLF-MT survey, the southwestern side of Kirishima is covered with low resistive surface layer, while the northeastern side is covered with high resistive layer. It is consistent with the fact that hot springs, fumaroles and steaming ground are scarce on the northeastern side of Kirishima, but are abundant on the southwestern side. It is also found that low resistive zone distributed along the trends of hypocenters ; from Shinmoe-dake to Takaharu. In the Iwo-yama region, which has the most active geothermal features at Kirishima, precise geothermal and electromagnetic surveys have been carried out. Through an inversion analysis using VLF, ELF and ULF-MT, a low resistivity second layer is found throughout the Kirishima area beneath the more resistive surface layer, which is about 100 m thick. The resistivity of the second layer decreases dramatically just around the summit of Iwo-yama. This is interpreted as being caused by a porous layer saturated with water which has wide distribution around Kirishima ; high temperature volcanic gas supplied from beneath Iwo-yama creates the extremely low resistive hydrothermal zone around the summit. The depth of the heat an dgas source is estimated to be about 10 km. Similar result was obtained at Shinmoe-dake, while no such low resistive zone was found beneath Ohachi. This result and other seismological and petrological evidences indicate that magma is stored at about 10km depth and rises several kilometers beneath northern volcanoes in Kirishima, such as Iwo-yama and Shinmoe-deke, while beneath southern volcanoes such as Ohachi, magma is supplied directly from the deeper part. Precise model of magma supply system in Kirishima as a multi-active volcanic system will be presented in the near future.