17:15 〜 19:15
[SCG52-P05] 島弧火山の形成メカニズム解明に向けた沈み込み帯の電気比抵抗構造における不均質性の比較
キーワード：マグマ（流体）供給系、島弧火山、沈み込み帯、火山・非火山地域の変遷域、電気比抵抗構造モデル、地磁気地電流法
In subduction zones, the movement and distribution of fluids brought into the Earth's interior by the subducting oceanic plate (slab) are crucial in driving igneous/volcanic activity and seismic events. As these fluids reach specific temperature-pressure conditions during the subducting process, they are released from the oceanic plate through a dehydration reaction. The released fluids in the mantle lead to partial melting of the mantle, resulting in the formation of magma sources for island arc volcanoes. This process establishes volcanic chains/regions (on island arcs) that align with the depth contour lines of the subducting plate. On the other hand, some island arcs, such as the island of Kyushu in Japan and the North Island of New Zealand (NZ), exhibit non-volcanic regions devoid of active Quaternary volcanoes for approximately 100 kilometers. It is not entirely understood why volcanic and non-volcanic regions form or why volcanic chains are discontinuous in a single-island arc despite being under the same tectonic conditions. Thus, a key objective of our research is to obtain and compare subsurface heterogeneity information in different island arcs using the electromagnetic method to aid in understanding the mechanism behind the formation of island-arc volcanoes.
Besides, due to the subduction of the oceanic plates, various types of earthquakes have repeatedly occurred in and around the island of Kyushu and the North Island of NZ, respectively, such as large thrust earthquakes in respective offshore and historic earthquakes along the tectonic lines in the land area. We have imaged three-dimensional (3-D) electrical resistivity structures by inverting magnetotelluric (MT) data, which were acquired on the whole of Kyushu by various surveys, to reveal the fluid/magma distribution beneath Kyushu [e.g., Hata et al., 2015; 2017; 2020]. The 3-D resistivity models indicate magma and fluid systems relating to slab-derived fluid as significant electrical resistivity features/anomalies. In addition, we conducted long-period MT surveys in a 300 km x 150 km square area, including the southernmost part of the Taupo volcanic zone (TVZ) and a non-volcanic region on the North Island of NZ in the Hikurangi subduction zone, during the period from July 2023 to January 2024. The primary purpose of the surveys is to extract subsurface heterogeneity information, which covers the depths of the crust and mantle in the transition area between the TVZ and the non-volcanic region, as a 3-D electrical resistivity model. In this presentation, we especially introduce a detailed discussion of the subsurface heterogeneity beneath the transition area between the volcanic and non-volcanic regions of the two island arcs, inferred from the 3-D resistivity distribution.
Besides, due to the subduction of the oceanic plates, various types of earthquakes have repeatedly occurred in and around the island of Kyushu and the North Island of NZ, respectively, such as large thrust earthquakes in respective offshore and historic earthquakes along the tectonic lines in the land area. We have imaged three-dimensional (3-D) electrical resistivity structures by inverting magnetotelluric (MT) data, which were acquired on the whole of Kyushu by various surveys, to reveal the fluid/magma distribution beneath Kyushu [e.g., Hata et al., 2015; 2017; 2020]. The 3-D resistivity models indicate magma and fluid systems relating to slab-derived fluid as significant electrical resistivity features/anomalies. In addition, we conducted long-period MT surveys in a 300 km x 150 km square area, including the southernmost part of the Taupo volcanic zone (TVZ) and a non-volcanic region on the North Island of NZ in the Hikurangi subduction zone, during the period from July 2023 to January 2024. The primary purpose of the surveys is to extract subsurface heterogeneity information, which covers the depths of the crust and mantle in the transition area between the TVZ and the non-volcanic region, as a 3-D electrical resistivity model. In this presentation, we especially introduce a detailed discussion of the subsurface heterogeneity beneath the transition area between the volcanic and non-volcanic regions of the two island arcs, inferred from the 3-D resistivity distribution.