The Northeast Japan arc as an island arc is morophotectonically characterized by a north-south trending chain of inter-mountain basins relatively subsided by Quaternary reverse faulting. The Nagai Basin among them has been produced by the fault movement of western marginal fault zone, set between granitic mountains. Our precise airphoto reading improved geomorphic fault traces and their continuity, and our field investigation using Quaternary method and leveling survey clarified the late Quaternary activity of the fault zone, ca.22km long. Geomorphic fluvial terraces distributed in the basin are classified into 5 levels: H, Ml, M2, L1, and L2 terraces. These terraces have been displaced with the vertical accumulation of faulting that shows the mean rate of about 0.5 m/ka. This western marginal fault zone of Nagai Basin is probably originated from the Tanakura tectonic line truncating the late Cretaceous granitic rock. Its subsurface structure is estimated to be simple, dipping west with relatively high angle. Vertical displacement of the fault in Quaternary reaches more than 1400m. The last seismic event generated from this fault zone undoubtedly occurred since ca.2000 years ago, and a vertical slip at an event is around 2.5m. The earthquake time recurrence possibly is between 3,000 and 7,000 years.
