Discussion

The subsurface structure and sedimentary facies in the Beppu Bay basin were studied using 2D seismic and 3D high-resolution seismic (3D-HRS) surveys. A series of high-angle faults extend near the southern bay coast and are interpreted as active traces of the laterally moving Median Tectonic Line (MTL). A releasing bend of the MTL forms a pull-apart sag around the bay bottom accompanied by numerous normal fractures, whereas a compressional bulge is emerging on the southern side of the bay, suggesting complicated and transient stress states in the late Quaternary strata. Based on the general development history of the Hohi Volcanic Zone (HVZ), the lower, middle, and upper seismic horizons are assigned to 5–6 Ma (initial stage of the HVZ), 0.7 Ma, and 0.3 Ma, respectively. We identified three auxiliary reflectors within the upper part of the sediment pile and correlated them with oxygen isotope stages 15–11. These stratigraphic constraints are used to discuss paleoenvironments in the latest Pleistocene by means of an amplitude root mean square attribute analysis of the 3D-HRS data, which successfully delineated a buried river channel that was active during the last glacial period