Abstract
We conducted seismic reflection survey in the coastal sea area of the Ishikari lowland, southern Hokkaido to reveal the offshore extension of the southern part of the active fault along the eastern margin of the Ishikari lowland. As a result, five depositional sequences (1 to 5 in descending order) and two anticlines (A and B) were recognized. The depositional sequences 1, 3 and 5 show downlap patterns and depositional sequences 2 and 4 show onlap patterns. So these deposits show cyclic process of regression and transgression. Therefore they are considered as the upper Pleistocene to the Holocene deposited under the influence of glacial sea-level change. The anticlines deform the upper Pleistocene and the scale of the deformation rate is 10-1 m/ky. Below the scarp which considered as the offshore extension of the active fault by the previous report, there are no deformation nor fault. The scarp is sedimentary structure formed after the last glacial maximum. These anticlines are comparable to the anticlines reported by previous survey along the coastline and offshore area. Then the southern part of the active fault extends to the Mukawa-oki Anticline and the length is extended to 86 km.
