Abstract
The Mid Niigata Prefecture Earthquake on 23 Oct. in 2004 caused the extensive damage to houses and the ground. The results of the extra survey on the relationship between the earthquake damage and geologic setting in the Yukyuzan area, Nagaoka city, are as follows: (1) Why did house damages concentrate within a zone on a terrace surface which is generally assumed to be stable?; Valley-shaped low grounds are confirmed on the Lower terraces where residential areas are in the studied area. The damage was concentrated along the embankment areas of bottom and slope in the valley-shaped low grounds. Geologic factors of the damage were inferred that alluvial muddy deposits and embankment displaced to the lower slope. (2) Why did the damage of the new town occurred sporadically on the ground of embankment in alluvial lowland? The damage distributed in the zone controlled by the ditch beside houses. The ditch was the old irrigation channel. The ground of residential areas inclines toward the northwest by the down slope of 0.7/100. The damage was concentrated in the downstream area of thick embankment and deep ditches, around one meter. The deformed or broken ditches beside damaged houses did not have a cover. The over earth pressure by the seismic vibration in the inclined residential grounds caused that the embankment was displaced with houses and pushed the wall of open ditches. Thereby the houses were seriously damaged by the seismic shock. Housing-land development will increasingly go toward the alluvial lowland in the future. It is important to take precautions against the ground displacement due to the seismic vibration, for example, enhancement of the breast wall of embankment and the sidewall of ditches, and installation of a steel or concrete cover on ditches, to resist against a seismic shock.
