[S-CG61] Dynamics in mobile belts

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

*Yoshiya Usui¹, Makoto Uyeshima¹, Shin'ya Sakanaka², Yusuke Yamaya³, Yasuo Ogawa^4,5, Masahiro Ichiki⁵, Yoshimori Honkura⁶, Hidekuni Kuroki⁷, Tasuku Hashimoto¹, Hayata Sato⁸, Takao Koyama¹, Koji Miyakawa¹, Akira Watanabe¹, Hiroko Tendo⁸, Masataka Masuda¹, Dieno Diba¹, Han Song^1,9, Kanju Bahati Haji², Kelym Reynald Martinez Mena², Riho Takahashi², Hayato Tazawa², Takuro Kanetsuki⁸, Toshiki Kaida⁵, Yuki Tomioka⁸, Kei Nakayauchi⁸, Takafumi Murakita⁸, Takao Miura⁸, Momoka Yamakawa⁸, Shogo Masuda⁸, Masamichi Ikeda⁸, Yoshiaki Harashima⁸, Taro Nishimoto¹, Shunsuke Yaegashi⁸, Kota Suzuki⁸, Junichi Kitadokoro² (1.Earthquake Research Institute, the University of Tokyo, 2.Graduate School of International Resource Sciences, Akita University, 3.National Institute of Advanced Industrial Science and Technology, 4.Institute of Integrated Research, Institute of Science Tokyo, 5.Graduate School of Science, Tohoku University, 6.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 7.Seismology and Volcanology Department, Japan Meteorological Agency, 8.Japan Organization for Metals and Energy Security, 9.State Key Laboratory of Marine Geology, Tongji University)

Keywords:Magnetotellurics, Electrical resistivity, Volcano, Earthquake, Crustal fluid

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).