Abstract
The Nishiyama fault zone is composed of a series of NW-trending left-lateral strike-slip faults in northern Kyushu Island for a length of 110 km. The central segment, Nishiyama segment, has been studied extensively in the past 30 years. However, its northern offshore segment, Ohshima-oki segment, and its southern segment, Kamatoge segment, were recently identified as active faults and their geologic characteristics and past activities were poorly known. In order to evaluate the seismic potential of the Kamatoge segment, we conducted aerial photograph interpretation, field mapping, trenching, shallow seismic reflection surveys and borehole drilling. We found that the Kamatoge segment consists of a conjugate set of the left-lateral Kamatoge-Koishiwara fault and the right-lateral Haki fault. These faults exhibit systematic deflections of stream channels and fault scarps on late Quaternary terraces. A shallow S-wave seismic reflection survey and borehole drilling across a fault scarp on the Haki fault suggested that the most recent faulting event on the fault occurred at around 5,500 yBP. Our study demonstrated that the Kamatoge segment has been active in the late Quaternary and is capable of producing a M7.4 earthquake if the entire length of the segment ruptures simultaneously.
