[S-CG61] 変動帯ダイナミクス

[SCG61-03] Three-dimensional electrical resistivity structure beneath the Nikko-Ashio area, Northeastern Japan

*臼井 嘉哉¹、上嶋 誠¹、坂中 伸也²、山谷 祐介³、小川 康雄^4,5、市來 雅啓⁵、本蔵 義守⁶、黒木 英州⁷、橋本 匡¹、佐藤 颯太⁸、小山 崇夫¹、宮川 幸治¹、渡部 熙¹、天藤 寛子⁸、増田 正孝¹、ディバ ディエノ¹、ソン ハン^1,9、カンジュ バハティ ハジ²、ケリム レイナルド マルチネス メナ²、髙橋 莉穂²、田澤 勇人²、金築 拓郎⁸、海田 俊輝⁵、冨岡 優貴⁸、中谷内 奎⁸、村北 貴郁⁸、三浦 貴生⁸、山川 桃佳⁸、増田 章吾⁸、池田 柾道⁸、原島 義明⁸、西本 太郎¹、八重樫 俊介⁸、鈴木 康太⁸、北所 純一² (1.東京大学地震研究所、2.秋田大学工学資源学部、3.産業技術総合研究所、4.東京科学大学総合研究院、5.東北大学大学院理学研究科、6.東京工業大学理学部、7.気象庁地震火山部、8.独立行政法人エネルギー・金属鉱物資源機構、9.State Key Laboratory of Marine Geology, Tongji University)

キーワード：MT法、電気比抵抗、火山、地震、地殻流体

The Nikko-Ashio area is one of the best areas worldwide for elucidating the mechanisms of volcanic and seismic activities and the linkage between them. A number of quaternary volcanoes (e.g., Nikko-Shirane and Nantai volcanoes) exist in this area. Swarm earthquakes and M6- and M7-class inland earthquakes have occurred around the area. For example, in 2013, an M6.3 earthquake occurred in the north of the Nikko area. Both volcanic and seismic activities are associated with subsurface fluids. To deepen the understanding of the relationship between those crustal activities and subsurface fluids, we revealed the three-dimensional electrical resistivity structure in the crust beneath the Nikko-Ashio area by deploying broadband magnetotelluric surveys. The station array comprised over 40 stations and covered the Nikko-Ashio area. As a result, a conductive anomaly was imaged to the north of the Nikko-Shirane and Nantai volcanoes. In depths greater than 15 km, this conductive area is rooted in the northwest of the Nikko-Ashio area. The conductive area is spatially consistent with the low-velocity and low-Q area found by previous seismological studies. The conductive area is considered to be fluid-rich and likely corresponds to the trans-crustal magmatic systems under the Nikko-Shirane and Nantai volcanoes. The hypocenters of the 2013 M6.3 earthquake and its aftershocks were located around the top edge of the conductive area. Therefore, the crust fluid in the conductive area might have contributed to the generation of this large inland earthquake (e.g., by causing local stress accumulation in the upper brittle parts or by reducing the shear strength of faults).