深層崩壊と誘因または地質の関係

抄録

In the Shimanto accretionary complex in the southwest Japan, most of the recent deep-seated landslides (DSLs) were induced by heavy rainfall over 1000 mm in a few days. In contrast, in the areas of volcanic rocks or Neogene sediments in the northeast Japan, recent DSLs were induced by strong seismic motion larger than six on the Japanese earthquake damage scale. These facts suggest that the regional characteristics exist in the trigger factors that induces DSLs (i.e., rainfall or earthquake), and that these factors are related strongly to the geological features of DSL sites. We therefore studied the regional characteristics of rainfall and earthquake on the basis of the distribution maps of trigger factors and the processes of DSL. The processes of DSL induced by two factors and the geological influences on them are summarized as follows. In the Shimanto accretionary complex in the southwest Japan, DSL induced by heavy rainfall is more likely compared with that by earthquake. This is because of the higher precipitation in the relevant area and the convergence of seepage water into the cracks of potential slip surfaces formed by rock creep. Earthquake-induced DSL is rare because active faults in this region are few and the mountainous basement is relatively hard. On the other hands, in the areas of volcanic rocks or Neogene sediments in the northeast Japan, earthquake-induced DSL is likely because earthquake stimulates greater ground motion in these areas and the mountainous basement is relatively soft.