Abstract
The 2004 Mid-Niigata Prefecture earthquake (M6.8) triggered a large number of landslides around the epicenter region. This region geologically consists of Neogene mudstone and sandstone, and is typical landslide-prone area. In this earthquake, many old landslides were reactivated and caused great damages. The initiation and the movement of these landslides were investigated by means of field survey and comparison of the landslide distribution maps, which were made before and after the earthquake. It was found that both old and new landslides were remarkably distributed on slopes facing to the west controlled by the geological structure in this region. Approximately half of the new landsides were occurred on the slopes of old landslides, but most of them were partial slides and only five percent of the new landslides was recognized as the reactivated slide of the entire old landslide masses. Topographic analysis on the entirely reactivated landslides showed that both the first slides (old landslides) and the second slides (reactivated ones) slid with less difference in their width and length. Equivalent coefficient of friction H/L (H: runout height; L: horizontal runout distance) for the second landslides was proportional to the slope of their source area as well as that of the first slides, even if the second landslides have higher mobility than the first ones.
