We describe the distribution and Holocene activity of the Nikaho active fault zone based on geomorphic features and surface geology. We performed aerial photo interpretation, drilling surveys of the sediments composing the topographic surfaces using soil auger and driven type core sampler, drew topographic profiles of the deformed terraces, and performed analysis of boring data by public works in the study area. This fault zone is composed of the Iseiji fault, the Ishida fault, and the Koide fault distributed in the western part of the Nikao hills, trending NNE to SSE with a length of about 11 km. Based on the displacement of the topographic surfaces and chronology of the sediments composing the topographic surfaces, the average vertical slip rate and the amount of displacement accompanying an earthquake are estimated to be 1.3-1.8m/ky, and c.a.2-3m, respectively. We estimated that the latest faulting event of the active fault zone occurred after c.a. 600 cal.y.B.P. and the average recurrence interval of the active fault zone is about 1,400-1,900 years. In addition, this fault zone is presumed to extend more than 15 km to the northern sea area.
