Characteristics of the recent landslides in the mid Niigata Region - Comparison between the landslides by the heavy rainfall on 13 July 2004, and by the intensive earthquakes on 23 October 2004

Niigata Region, Japan, is known as that designated landslides are more than 2000 sites which are the most abundant in Japan, and still now we have experienced with landslides in the mountainous areas in Niigata Region (Yamagishi and Ayalew 2004; Yamagishi et al. 2004). Most of the landslides are deep-seated and taking place on gentle slopes in the Neogene to Pliocene mudstone areas. These triggers are mostly by snow melting. However, on 13 July 2004, heavy rainfalls due to the intensive activities of rain front occurred in the Mid Niigata Region, Japan. They are as much as 400 mm in 24 hours, and brought about serious flooding by breaking the river banks, as well as landslides. The heavy rainfall-triggered landslides are recognized as 3600 sites. Most of the landslides are shallow seated, but some of them are more or less deep-seated and associated with long-run mudflows. Followed by such heavy rainfalls, the southern region of Mid Niigata Region was attacked by intensive earthquake of M 6.8 on Richter scale on 23 October 2004. The main earthquake was followed by intensive and small after-shocks until December 2004. By these earthquakes, many landslides also occurred in the hilly and mountainous areas. These landslides are classified into three types;one is deep-seated slides, the second shallow landslides, the third is flowing slides. Namely, in 2004, Niigata Region has been experienced with different induced landslides which are also different from the used landslides characteristic of Niigata, Japan. Therefore, in this paper, we are describing the distribution and characteristics of the heavy rainfall-induced and the intensive earthquake-induced landslides, and then comparing in features and scales with the different-trigger landslides.