AbstractMultichannel seismic reflection profiling in the Ulleung back‐arc Basin reveals that the acoustic basement largely comprises volcanic materials. The volcanics are interlayered with sediment sequences, forming an anomalously thick layer. The volcanic activities resulted in a zonation of the acoustic basement, trending northeast‐southwest. In the southeastern part, the acoustic basement is deep and obscure, whereas in the northwestern part it is shallow and forms mounds and seamounts. The volcanic activities were probably initiated in the Early Miocene (ca20Ma). The volcanism was time‐transgressive northward, associated with the possible southward drift of the Japanese is