Geomorphic Features on the Movement of Reactivated Landslides due to Earthquakes

Strong earthquakes with M6.8 (JMA at maximum) in the Chuetsu region, Niigata prefecture, Japan on October 23, 2004, generated a number of landslides around the epicenter. This region consists of Tertiary mudstone and sandstone, and was originally a landslide-prone area. Some of the earthquake-induced landslides formed natural dams and merged houses under water. These landslides derived from the reactivation of old landslide masses, according to the landslide maps (NIED). Topographic analysis focusing on the landslide movement was carried out with landslide maps before and after the earthquakes. Slope of source areas in the reactivated landslides ranged from 7 to 27 degrees. Equivalent coefficient of friction H/L, where H is runout height and L is horizontal runout distance, was proportional to the slope of the source area in the reactivated landslides. The regression line of the above relationship agreed well with that in original landslides. The width of landslide mass expanded 1.2-1.3 times of the width before slide on average. This means that both original and reactivated landslides had equality of motion. The results allow estimation of the runout distance and deposition area of future landslides in this region.