THREE-DIMENSIONAL RESISTIVITY STRUCTURE IN TOYA CALDERA REGION, SOUTHWEST HOKKAIDO, JAPAN - CONSTRAINTS ON MAGMATIC AND GEOTHERMAL ACTIVITIES

Abstract

Southwestern Hokkaido, Japan, is characterized by numerous Quaternary volcanoes and geothermal areas. At the same time, the region hosts various critical infrastructures, and there is a need to assess the impact of volcanic hazards on them. Geophysics could provide scientific clues for the hazard assessment by elucidating the abundance of subsurface magma. To clarify the resistivity structure from the crust to uppermost mantle of the Toya caldera, a representative Quaternary volcanic area, a wideband magnetotellurics survey of 117 points over land, sea, and lake areas, as well as 3D inversion was conducted. In combination with petrological and seismological findings, quantitative interpretation of the inverted model found that conductive bodies in the uppermost mantle (14–68 Ωm) suggest the presence of melts (0.25 vol%–3.4 vol%) or fluids (0.068 vol%–0.45 vol%). An extremely conductive body (<10 Ωm) at a depth of approximately 3–14 km in the eastern geothermal area could be interpreted as a hydrothermal reservoir; below this body, the conductive column (1.8–15 Ωm), rising from the uppermost mantle, suggested fluid upwelling. In contrast, high resistivity (>100 Ωm) beneath Usu Volcano, the center of active volcanism, suggested that no mobile magma was present. A columnar-shaped region of slightly low resistivity (44 Ωm at minimum) was observed below the Toya caldera, which was inferred as cooling magma, or an altered or heated upper crust attributed to past magma intrusion. A resistivity structure observed below the volcanic edifice was considered to reflect the steady state of the dormant volcanic system in this area, and there was likely no large amount of melt that would be deemed imminent for a caldera-forming eruption. This information could be a valuable scientific contribution to the volcanic hazard risk assessments currently being conducted in Japan.