Detailed investigations of geological structures, particularly active faults, were performed in the area surrounding the Onagawa Nuclear Power Plant (NPP) site located on the Pacific Ocean side of Northeast Japan. As a result of investigations, we detected no active faults on land, but several NW trending submarine active faults were evaluated for the aseismic design of the Onagawa NPP. Supposed earthquakes and ground motions caused by these active faults were estimated by some geotechnical engineering methods, and the estimated ground motions were lower than that determined in the Design Basis Earthquake of the Onagawa NPP based on past earthquakes and seismo-tectonic structure. In viewing the differences in the thickness of stratum between the hanging wall and foot wall of these faults, it was considered that some of these faults were west-down faults in the Miocene, and, from the Pliocene to the Pleistocene, upheaval of the west side began with the same shear zone. Seismogenic "active" faults should be observed by geologic and geomorphologic evidence of repeated displacement throughout the late Quaternary. The characteristics of several NW-trending submarine faults, which had undeniable activity throughout the late Quaternary, were confirmed.
In Japan, electric power companies should perform detailed investigations on geological structures, particularly active faults in the area surrounding a nuclear power plant (NPP) site, for the aseismic design of NPP facilities. The characteristics of active faults around a NPP site, e.g. their location, length and activity, should be determined to satisfy the ÒExamination Guide for Aseismic Design of Nuclear Power Reactor FacilitiesÓ (1978). For active faults, their activity shall be evaluated and the ground motion caused by the active faults taken into account according to the degree of their activity in identifying a Design Basis Earthquake (DBE) for a NPP.
