In the survey for the offshore active faults, seismic explorations are generally used. However, the past seismic explorations have focused on grasping widely geological structures with hundreds meters or several kilometers survey lines interval. Thus, accurate mapping of faults and reconstruction of recent faulting history have not yet done at offshore area. In this study, we aim to reveal the shallow subsurface deformation history associated with the offshore Hinagu Fault Zone in the Yatsushiro Sea, which is located in the western part of central Kyushu, using high-resolution seismic survey with 20-50 meters interval survey lines. Furthermore, we carried out core sampling to obtain the shallow subsurface geological information.
The high-resolution seismic survey allows us to identify seven depositional sequences (layer A1, A2, A3, B1, B2, C and D from top to bottom) based on reflection patterns. The piston coring enables to estimate formation period of some unconformities; 1) reflector R1 (between layer D and layer C) is Last Glacial Maximum erosion surface (about 20,000yBP), 2) reflector R2 (between layer C and layer B2) is Younger Dryas cold interval erosion surface (about 13,000yBP), and 3) reflector R5 (between layer B1 and layer A3) is formed in 8,990-3,840 cal BP. Between reflector R5 and Layer A3, there is a hiatus. So, it is difficult to reveal the activity of this period.
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.