Tectonics, structure, and resurgence of the largest quaternary caldera in Japan: Kutcharo, Hokkaido

Kutcharo is the largest Quaternary caldera in Japan and one of the youngest worldwide. It is subcircular, with a diameter of 20-26 km, and was formed by repeated rhyolitic explosive eruptions from 340 to 35 ka. Kutcharo has a previously unrecognized resurgent dome (10-12 km across, 360 m high) that was produced by reinflation of the underlying magma chamber between 28 and 23 ka. The resurgent dome consists of uplifted intracaldera lacustrine deposits and overlying dacite lavas and is transected by an L-shaped graben 3 km wide. A rhyolite lava and 11 rhyolite lava domes occur around the resurgent dome and within the graben. The architecture of Kutcharo caldera closely resembles that of well-understood resurgent calderas such as Valles caldera and Long Valley caldera. Geochronological data indicate that caldera resurgence occurred within similar to 10 k.y. after caldera collapse, i.e., much faster than other calderas where the timing of resurgence is known. Kutcharo has shown signs of unrest (earthquakes and ground deformation) since A.D. 1938. Seismic, geodetic, geothermal, and resistivity data all suggest that magma occurs at depth beneath the resurgent dome. Kutcharo is located at the intersection of two regional faults that accommodate the ongoing movement of the Kuril forearc sliver, caused by oblique subduction of the Pacific plate. The structure and activity of the resurgent dome are linked to this tectonic setting. Kutcharo has the potential for future volcanic eruptions. Our new view of the architecture of the Kutcharo caldera provides invaluable information for hazard assessment of resurgent calderas.