Post Earthquake Evaluation of Higashi Takezawa Landslide Located at Niigata Prefecture of Japan

A 6.8M earthquake occurred in Niigata in October 2004 caused a huge damage that includes triggering of hundreds of landslides. Some of those landslides caused natural damming of the associated streams. Higashi Takezawa is one of the largest landslides that caused natural damming. Shear strengths of soil layers above and below the sliding surface of that landslide were remarkably different. Pseudo static analysis of the landslide showed that about 700 gal of peak ground acceleration (PGA) was enough to cause the slide moving at the time of earthquake. After the mobilization of residual shear strength, the stability was calculated and found to be reduced considerably, and about 350 gal of PGA was enough to keep the mass moving at that condition. However, if the earthquake has occurred in dry season or if the soil mass has not been softened by the excessive rainfall during the typhoon, the earthquake of that magnitude is not sufficient to cause failure. Dynamic safety factor of the landslide decreased exponentially with the increase in water pressure. This study is very useful to evaluate the stability of other potential landslides so that precaution can be taken against disaster during next earthquake events.