Oshima-Oshima, a small, 4-km-wide Japan Sea island 55 km west of the SW tip of Hokkaido, is the emergent summit of two coalescing basaltic-to-andesitic stratovolcanoes.  Higashi-yama, at the east end of the island, is cut by a 2-km-wide caldera covered on its west side by Nishi-yama volcano.  The western cone failed during an eruption in 1741, creating a large horseshoe-shaped caldera breached to the north that extends from the summit down to the sea floor at the base of the volcano and producing a mostly submarine debris avalanche that traveled 16 km.  A tsunami associated with the collapse swept the coasts of Hokkaido, western Honshu, and Korea, and caused nearly 1500 fatalities.  The 1741 eruption, the largest in historical time at Oshima-Oshima, concluded with the construction of a basaltic pyroclastic cone at the head of the breached caldera.  No eruptions have occurred since the late-18th century.