Abstract
The 2004 Mid Niigata prefecture earthquake (M6.8) occurred on 23rd in October 2004, severely damaging infrastructures in this area by strong shaking, liquefaction, and landslides; particularly a large number of landslides occurred in the mountainous areas in and around Yamakoshi Village, which has been now merged with Nagaoka city.
The Japan Society of Engineering Geology and the Japanese Landslide Society made a joint investigation team and made a ground survey in the affected mountainous areas from 5th to 7th in December, and reported the results at the special symposium organized by both the societies. After the earthquake, the affected area experienced heavy snow from December 2004 to March 2005; the snowfall was a record within recent 19 years. Almost all snow melted before the middle of May 2005, and the joint investigation team made a ground survey again from 21st to 25rd in May 2005 to check the effect of snow on landslides or destabilized slopes. The effects of the snow and its melting on the slopes were not large, but the reconnaissance team has found precursory features of future landslides or sediment yielding : new cracks and widening of existing cracks which were made by the earthquake. Long-term monitoring is essential for these features to mitigate future disaster. Here we report the results of our investigation on the change of the landslides or destabilized slopes between just after the earthquake and after the snow melt as well as newly visited landslide sites.
