On the basis of a profile obtained through a multi-channel seismic-reflection survey in southern part of the Yoro Mountains, subsurface structure and activities of the Yoro and Ichinohara Faults are discussed together with the geological and geographical data published previously.
The Nobi Plain and Yoro Mountains (500-900 meters in average elevation) are separated by the Yoro Fault.
It is known from deep drillings that the sediments under the Nobi Plain consist of five principal units; pre-Tokai Group, Kono Formation, Kuragari Formation, Oizumi Formation, and middle Pleistocene to Holocene sediments. The seismic profile revealed two faults; the Yoro Fault in the central part of the profile and the Ichinohara Fault in the western part of the profile. The eastern part of the depth section is divided into six principal units. The structure along the Yoro Fault is disclosed for the first time by the present survey to be a very large flexure caused by reverse faulting. The total vertical displacement is about 500m at the bottom of the Tokai Group. The Yoro Fault starts moving in the depositional period of the Kuragari Formation, and its average displacement rate is of B class on the order of 0.1m per 1000 years. The Ichinohara Fault is a left-lateral fault and its vertical displacement is about 480m at the level of the upper Kono Formation in the survey line, and the average displacement rate is estimated as 0.48m per 1000 years (B class).
