Abstract
The Kuromatsunai Lowland fault zone is a 32 km-long, reverse fault zone which develops through the Kuromatsunai Lowland, southwest Hokkaido. This is a complexed fault zone consists of a series of reverse faults trending approximately north–south. Individual faults are 3-4 km and displace middle to late Quaternary terrace surfaces and deposit, the west side raising up. We observed a fault outcrop of one of the reverse faults in the Kuromatsunai Lowland. The outcrop on the Babasawa River exposes Lower Pleistocene sediments that are thrusting over the youngest sediment along the F1 fault. Tephra analyses and radiocarbon dates indicate at least two seismic events. One is between 10 C and 17 C AD, and another is about 20 ka. The younger event is not associated with any of the known paleoseismic events from the Warabitai fault in south. Therefore, the F1 fault and Warabitai fault did not move simultaneously. The layer III of the Late Glacial age is displaced vertically 1.6 m by the F1 fault yielding the vertical component of net slip to be about 0.08 mm/yr.
