Journal of the Japan Society of Erosion Control Engineering, Vol.65,No.2, 2012

Preliminary study on landslide-triggering rainfall thresholds after an earthquake

Yasuhiro SHUIN, Norifumi HOTTA, Masakazu SUZUKI, Nobutomo OSANAI and Atsushi OKAMOTO


Abstract

This study used two conceptual models to examine the effects of rainstorms on landslides after an earthquake:1) a tank model for calculating the soil water index and2) a process]based model. The process]based model used a digital terrain model with10]m resolution to calculate the regional potential for shallow landslides, based on the distribution of shallow infiltration water, Darcy's law, and a safety factor estimated by infinite slope stability analysis. We used this process]based model as a conceptual model, rather than as a physically based model, and calculated safety factors for each cell area; values less than1were compiled as the potential landslide area index, after eliminating cells that were unstable under no]rainfall conditions. The two models were applied to the Funyu Experimental Forest of Utsunomiya University in Tochigi Prefecture, Japan. At the end of August1998, a heavy rainfall event caused many shallow landslides in the study area, whereas other heavy rainfall events from1994to2003did not cause severe landslides. The relationships between the two indices (soil water index and potential landslide area index)showed a close correlation, confirming the validity of both approaches for the estimation of rainfall properties. Response analysis of the effect of earthquakes on soil strength parameters indicated that landslide]triggering rainfall thresholds varied with decreasing values of effective soil cohesion. This implies that the relationships between effective soil cohesion and the soil water index triggering landslides derived from two conceptual models were valid for the re]estimation of the influence of rainfall properties on landslide occurrence after an earthquake.

Key wordsFearthquake, landslide, soil water index, rainfall thresholds, effective soil cohesion


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