霧島火山群の構造とマグマ供給系

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The Kirishima Volcanic Region, located in southern Kyushu, is a group of more than 20 volcanoes. At least 3 volcanoes have historic records of eruptions, and more than 10 volcanoes were active within the past 22,000 years. This suggests that Kirishima is a multi-active volcanic group, which might have a complex system of magma supply. Therefore, it is interesting to reveal the structure for the further understanding of the magma-supplying system; for example, locations of magma reservoirs, magma rising process before eruptions and causes of related phenomena. In 1993 and 1994, a joint experiment was conducted on the Kirishima Volcanic Region by a group of scientists from national universities. The experiment was carried out by seismological, electromagnetic and other geophysical and geochemical methods. This paper briefly presents a structure of Kirishima revealed by magnetotelluric (MT) survey in the ULF, ELF and VLF bands and by explosion seismological survey, and discusses a magma-supplying system. The survey revealed that the Kirishima volcanic area is divided into two sub-areas; the northwestern volcanic area and the southeastern volcanic area. Volcanoes in the northwestern volcanic area such as Iwo -Yama and Shinmoe-Dake are characterized by a simple four-layer structure; relatively resistive overburden, very conducting second layer at the depth of a few hundred meters, resistive basement, and deep conductor. The low-resistivity second layer is interpreted as a water-saturated porous layer which is widely distributed throughout Kirishima. This layer plays an important role in controlling types of eruption and in generating precursory phenomena of volcanic eruptions through interaction of the water with ascending magma. The deep conductor appears about 10 km below the surface in average, but shallower beneath the presently active volcanoes, such as Iwo-Yama or Shinmoe-Dake, up to several km in depth. This is probably related to the seismic reflectors detected by the seismic explosion experiment at 10 km beneath northern sub-area and at several km beneath Iwo-Yama, respectively. Precise seismological and other geophysical observations indicate that an outbreak of earthquake swarm at the top of the deep conductor beneath the crater in Shinmoe-Dake, is followed by anomalous volcanic phenomena; upward migration of seismic activity and dilatational crustal deformation in the resistive third layer, volcanic tremor and thermal demagnetization within the low resistivity second layer, and finally steam and ash emission from the crater. On the other hand, volcanoes in the southeastern volcanic area such as Ohachi and Mi-Ike are characterized by a rather resistive second layer and lack of deep conductor.
特定非営利活動法人日本火山学会の論文
1997-04-25

霧島火山群の構造とマグマ供給系

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