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松永 佳大*; 相澤 広記*; 浅森 浩一; 小川 大輝*; 宇津木 充*; 吉村 令慧*; 山崎 健一*; 内田 和也*; 松島 健*; 井上 智裕*; et al.
Tectonophysics, 915, p.230880_1 - 230880_11, 2025/11
被引用回数：1 パーセンタイル：48.81(Geochemistry & Geophysics)We estimated the resistivity structure in the region of the 1997 Northwestern Kagoshima earthquake doublet (March 26, Mw6.1; May 13, Mw6.0) by dense magnetotelluric (MT) observations to investigate the influence of low-resistivity zones on the initiation and termination of large inland earthquake ruptures. In addition to MT data from previous studies, we acquired new broadband MT and telluric data at 42 sites. With a total of 86 sites in approximately 50 km 
50 km area, we constructed a high-resolution resistivity structure through 3-D inversion. A comparison between the slip distribution and resistivity structure showed that the rupture of the mainshock in March initiated at the western edge of a low-resistivity zone, propagated westward (with a maximum slip of 0.8 m), and was ultimately arrested by another low-resistivity zone. Based on geological structures and 
He/
He ratios in hot springs, we suggest that magma-derived fluids, located beneath a shallow, low-permeability smectite-rich zone and granodiorite, contributed to the initiation of the mainshock rupture. Another smectite-rich zone, along with underlying high pore pressure fluids, played a role in arresting the rupture. The rupture of the mainshock in May initiated at the edge of another low-resistivity zone and propagated in two directions (with a maximum slip of 0.4 m): one along the edge of the low-resistivity zone and the other toward the low-resistivity zone, before finally stopping within the low-resistivity zone. Our findings are consistent with those from the 2016 Kumamoto earthquake (M7.3), supporting the idea that resistivity structures can help assess the spatial potential of large inland earthquakes.
松永 佳大*; 相澤 広記*; 浅森 浩一; 小川 大輝*; 宇津木 充*; 吉村 令慧*; 山崎 健一*; 内田 和也*; 山口 雅弘*; 井上 智裕*; et al.
no journal, ,
We utilized broadband magneto-telluric (MT) data to analyze the 3-D resistivity structure around the epicenters of the 1997 Kagoshima earthquake doublet, Japan. We reanalyzed previous MT data and further obtained additional MT data. Our findings reveal low-resistivity zones near the eastern and western edges of the aftershock region at depths of 5-10 km. The hypocenters of the M6 earthquakes are located near the edge of the low-resistivity zones. A distinct high-resistivity zone is sandwiched between two low-resistivity zones at depths of 0-3 km, corresponding to the distribution of a granodiorite body. The slip distribution of the March M6 earthquake is located beneath this shallow granodiorite body and is sandwiched by the low-resistivity zones.
中村 謙佑*; 相澤 広記*; 浅森 浩一; 大志万 直人*; 井上 智裕*; 臼井 嘉哉*; 市原 寛*; 山口 雅弘*; 宇都 智史*; 畑岡 寛*; et al.
no journal, ,
We present the resistivity structure around the focal region of the 2000 Western Tottori Earthquake (M7.3) to investigate the relationship between the mainshock rupture and the resistivity stricture. The Western Tottori Earthquake occurred on October 6, 2000, in an area without surface faulting. Deep low-frequency earthquakes were observed approximately 8 km west of the epicenter at depths of around 30 km. To obtain a detailed resistivity structure, we conducted additional observations at 35 sites between October and December 2024, including 21 telluric-only sites. For our three-dimensional resistivity analysis, we utilized MT data from 52 sites used in previous study, 12 sites observed in 2001 by Kyoto University and Tottori University, and the newly added 35 sites. As a result, we identified a low-resistivity zone directly beneath the epicenter. We discuss the relationship between this resistivity structure, the slip distribution of the mainshock, and the occurrence of aftershocks.
松永 佳大*; 相澤 広記*; 浅森 浩一; 小川 大輝*; 宇津木 充*; 吉村 令慧*; 山崎 健一*; 内田 和也*; 松島 健*; 井上 智裕*; et al.
no journal, ,
1997年鹿児島県北西部地震では、3月26日と5月1日にM6.3、M6.2の地震が発生し、F字型の余震域を形成した。この地域では、既往研究によって比抵抗構造が既に推定されているが、地震の破壊と比抵抗構造の関係を詳細に議論するためにはMagnetotelluric (MT)観測点数が少なく、空間解像度が十分ではなかった。本研究では、観測データを加えることで、既往研究より高解像度の比抵抗構造を推定し、地震の破壊と比抵抗構造の関係を検討した。その結果、本震震源付近には、2つの低比抵抗帯とそれらに挟まれた高比抵抗帯が推定された。3月に発生した本震のすべり分布とこの比抵抗構造を比べると、本震は東側の低比抵抗帯の西縁で始まり、西側の低比抵抗帯の東縁で破壊が止まったことが明らかになった。また、この地域では第四紀の火山活動を引き起こした流体が深部から上昇し、それが深さ10km付近での鹿児島県北西部地震の破壊開始に寄与したと考えられる。
