11:30 〜 11:45
[SEM12-09] ヒクランギ沈み込み帯のマグマシステム解明に向けて - ３次元比抵抗分布を基にした地下の不均質性 -
キーワード：ヒクランギ沈み込み帯、マグマシステム、地下の不均質性、３次元比抵抗分布、長周期MT法データ
In subduction zones, the distribution and movement of fluids brought into the Earth's interior with the subducting plate induces igneous/volcanic and seismic activity. When the fluid brought into the mantle reaches a certain temperature-pressure condition, it is released from the plate/slab due to a dehydration reaction. The added fluid to the mantle causes partial melting of the mantle and creates magma sources of island arc volcanoes. This process leads to the forming of a volcanic chain/belt along the depth contour lines of the subducting plate. On the other hand, some volcanic chains of island arcs are not formed to completely traverse the island arcs, as seen in the island of Kyushu (the southernmost and third largest of the four main islands of Japan) and the North Island of New Zealand, there is a non-volcanic region with no active Quaternary volcanoes for ~100 km. It is not completely 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 by using the electromagnetic method to aid in understanding the mechanism behind the formation of island-arc volcanoes.
In this study, we conducted long-period Magnetotelluric (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 New Zealand in the Hikurangi subduction zone, during the period from July 2023 to January 2024. The main 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. For approximately 45 days at 28 sites, three components of the magnetic field and two components of the electric field were recorded at a sampling interval of 1 second using the LEMI-417M system manufactured by the Laboratory of Electromagnetic Innovations. Then, the MT response functions (four components of impedance tensors and two components of tipper vectors) based on time-series data of the magnetic and electric fields, which are used to obtain underground electrical resistivity information/distribution, were determined in 32 periods (5–245,760 seconds) by applying remote reference processing [Gamble et al., 1979]. Furthermore, phase tensor analysis [Caldwell et al., 2004] was performed on the impedance tensors to understand the heterogeneity of the underlying 3-D resistivity distribution in the study area. In this presentation, we introduce a detailed discussion for the subsurface heterogeneity beneath the transition area between the TVZ and the non-volcanic region in the Hikurangi subduction zone, which are inferred from the 3-D resistivity distribution.
In this study, we conducted long-period Magnetotelluric (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 New Zealand in the Hikurangi subduction zone, during the period from July 2023 to January 2024. The main 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. For approximately 45 days at 28 sites, three components of the magnetic field and two components of the electric field were recorded at a sampling interval of 1 second using the LEMI-417M system manufactured by the Laboratory of Electromagnetic Innovations. Then, the MT response functions (four components of impedance tensors and two components of tipper vectors) based on time-series data of the magnetic and electric fields, which are used to obtain underground electrical resistivity information/distribution, were determined in 32 periods (5–245,760 seconds) by applying remote reference processing [Gamble et al., 1979]. Furthermore, phase tensor analysis [Caldwell et al., 2004] was performed on the impedance tensors to understand the heterogeneity of the underlying 3-D resistivity distribution in the study area. In this presentation, we introduce a detailed discussion for the subsurface heterogeneity beneath the transition area between the TVZ and the non-volcanic region in the Hikurangi subduction zone, which are inferred from the 3-D resistivity distribution.