Crustal fluids and their relation to seismic activities in the Nikko-Ashio area of Northeastern Japan inferred by electrical resistivity imaging

Subsurface heterogeneities in temperature, materials, and rheology are expected beneath volcanoes. Such heterogeneities have been suggested to play a crucial role in the generation of earthquakes in volcanic areas. Therefore, to deepen our understanding of seismic activities and their relationships with volcanic systems, it is essential to elucidate subsurface heterogeneity in seismically and volcanically active areas. Hence, we delineated the crustal heterogeneity beneath the Nikko-Ashio area of Northeastern Japan, where several volcanoes exist and M6- and M7-class intraplate earthquakes as well as swarm earthquakes have occurred. One of the most effective approaches for imaging subsurface heterogeneity is to probe the electrical resistivity structure, which is sensitive to temperature and the abundance of fluids. Therefore, we estimated the three-dimensional electrical resistivity structure in the Nikko-Ashio area after deploying broadband magnetotelluric measurements. The resulting electrical resistivity structure contained a conductive anomaly in the crust to the north of Nikko-Shirane and Nantai volcanoes. The conductive area is considered to be fluid-rich and likely corresponds to trans-crustal magmatic systems under these active volcanoes, supplying magmatic fluids to these volcanoes. The hypocenter of an M6.3 earthquake was located near the edge of the conductive area. The lateral difference in the pore-fluid pressure and/or temperature gradients near the rim of the conductive area possibly contributed to the generation of the large intraplate earthquake. • We delineated the electrical resistivity structure beneath the Nikko-Ashio area • Conductors under active volcanoes likely correspond to a magmatic system • Hypocenter of a large inland earthquake is located near the rim of the conductor • Pore-fluid pressure gradient might contribute to the nucleation of the earthquake