P-wave seismic reflection survey of the Kakuda-Yahiko fault group in Niigata Prefecture, central Japan

Authors: Naomi Kano, Kazuo Yamaguchi and Yasuo Awata

Index Map

Abstarct:Kakuda-Yahiko fault group is a major buried fault along the western border of the Niigata Plain. We conducted a P-wave seismic reflection survey across the middle part of the fault group. The survey line is 7.5 km long, extending from Maki Town to Nishikawa Town in Niigata Prefecture. Inclined reflectors have been detected at the depth ranging from nearly 0 m to around 500 m in the western part of the line. Only the very shallow part could be imaged, however, in the eastern part of the line. In the middle of the reflection profile obtained, the shallow subsurface structure has been imaged. It makes possible to estimate the Quaternary activity of the fault group. The shallow reflectors at the depth less than about 150m on the east of the distance 500 m are inferred to be the Holocene deposits. A strong reflector correlative to the top of the lower Holocene gravel bed gently dips eastward with a relative depth of about 50 m between the distance 1500 m and 2500 m, while it is almost flat outside of this portion. Therefore, the eastward-inclined structure of the Holocene deposits is considered to be a flexure, which is ascribable to the recent activity of the Kakuda-Yahiko fault group.