Crustal structure of the southwestern margin of the Kuril arc sited in the eastern part of Hokkaido, Japan, inferred from seismic refraction/reflection experiments

Hokkaido Island in northern Japan is located at the intersection of the Kuril and Northeast Japan arcs. Eastern Hokkaido represents the southwestern margin of the Kuril arc that evolved as an oceanic island arc, but its crustal structure has remained unclarified. In the summer of 2000, a highly dense onshore-offshore integrated seismic experiment was conducted in order to reveal the entire crustal section covering from the Kuril Trench to the back-arc basin of the Okhotsk Sea crossing eastern Hokkaido. High-quality refraction/wide-angle reflection data collected for the onshore survey line delineated a detailed crustal structure of easternmost part of Hokkaido Island. Our seismic velocity model shows a good correlation with the surface geology along the profile. The notable feature of the velocity model is the existence of a 6.0-km/s layer beneath the onshore survey line. A reflective middle to lower crust is also found beneath eastern Hokkaido. These results indicate that the southwestern margin of the Kuril arc is in a matured state of oceanic island-arc crust.