The earthquake of M6.8 occurred at 17:56 on Oct. 23, 2004 in mid Niigata prefecture, and seismic intensity 7 was recorded in Kawaguchi]cho. After that, the aftershock activity has continued so actively that several aftershocks of seismic intensity 6+ were observed, and they caused heavy damage in various places. The main shock appeared right near Yamakoshi]mura, and seismic intensity at this district was 6+. This area is the foremost area of frequent landslides in Japan, and by this earthquake many slope failures and landslides occured on the wide range of Higashiyama hill with center at Yamakoshi]mura. Slope failures and landslides appeared at 1,662 points and the total of sediment yield is estimated to reach 7~107 m3 as a result of the analysis of aerial photographs. There are ten large]scale landslides that have sediment yield of more than 106 m3 individually, and five of them concentrate along the River Imo. Moreover, landslide dams were formed at more than 30 points along the River Imo, and urgent measures have been taken.
Key wordsFthe mid Niigata prefecture earthquake, sediment]related disaster,
slope failure, landslide dam, Niigata prefecture, earthquake