根尾谷断層水鳥断層崖のトレンチ壁面の地質

抄録

An exploratory trench was excavated across the Midori fault scarp associated with the Nobiearthquake (M 8.0) of 1891, which was the largest earthquake recorded on inland Japan. The trench has been preserved for exhibition by covering it with a building and grouting it to prevent leaking of ground water. This preservation and exhibition of an exploratory trench across a seismic fault is the first attempted in the world.
The Midori Fault exposed on the trench walls has a strike of N 35°W and is almost vertical. The upthrown side of the fault consists of Mesozoic sedimentary rocks and alluvial gravel. The downthrown side consists, in ascending order, of wide fault gouge derived from Mesozoic rocks, alluvial gravel, paleo-soil, gravel due to collapse from the upthrown side and artificial fill. Clasts within alluvial gravel along the fault are rotated by the fault movement. The pre-1891 sediments have been displaced vertically by 5.5 to 6 meters and left-laterally by about 3 meters, indicating the Nobi earthquake of 1891 was the only seismic event on this fault over at least 1, 000 years that caused displacement.