Study area in the northeastern Japan arc (inset map) and magnetotelluric sites around Naruko volcano. Purple dots denote new magnetotelluric sites used in this study. Yellow and blue dots denote magnetotelluric sites from previous studies by Mishina () and Asamori et al. (), respectively. Major Quaternary volcanoes are shown by triangles. Late Miocene collapse calderas are outlined by dotted lines following Geological Survey of Japan ().