Consideration on the influence of seismic shaking on forest slope stability and landslide warning rainfall in the aftermath of the strong earthquake

The research concerning about the warning rainfall criteria of landslides after strong earthquakes is conducted associated with the great earthquake in Eastern Japan in 2011 (M=9.0). After this kind of strong earthquake, soil strength of the slopes in the region that were exposed to the strong seismic forces is generally reduced by seismic shaking (vibration). In this situation, the revised rainfall criteria for landslides are required. On this point of view, the response of landslide to rainfall under this weaken soil condition was studied. Hence, the impact of rainfall events on the specific landslide slopes that experienced the strong seismic shaking is analyzed using numerical simulation method i.e. finite element method FEM in order to evaluate the critical rainfall threshold for landslide occurrence. Consequently, we obtained following results. In the result of FEM analysis, the cracks induced by the earthquake Eire effective to increase the seepage and render the slopes instable. The slope deterioration mentioned above due to the impact of earthquake causes temporarily 40-50% reduction of critical rainfall for landslide occurrence, corresponding to decrease in safety factor "Fs" of the slopes from Fs= 1.02-1.04 to Fs=0.83-0.95. This reduction rate of critical rainfall is as same level as in the earlier researches based on field observations.