The largest earthquake swarm ever recorded in Japan began off the islands of Miyake-jima, Kozu-shima and Nii-jima, and was linked to volcanic activity on Miyake-jima, at the northern end of the volcanic front of the Izu-Ogasawara (Bonin) arc. Ocean bottom seismographic observations made directly above the swarm region gave clear indication of a magmatic intrusion. The hypocenter distribution shows a vertical sheet, 2 km wide and at a depth of 12--20 km, that broadens markedly at depths up to 12 km. The P wavespeed structure obtained from a seismic refraction survey shows no significant difference in the characteristics of the shallow crust between the strong swarm region and inactive region. However, a low wavespeed region was detected 10--15 km northwest of Miyake-jima.