Classification of Causes about Deep-seated Landslide and Those Characteristics of Geographical Predisposition

Abstract

Deep-seated landslides are classified into three different types in terms of their cause: deep-seated landslides associated with seismic motion, deep-seated landslides associated with heavy rainfall, and deep-seated landslides due to an unknown cause, and their occurrence process and geographical predisposition are characteristically different with respect to each cause. As for the characteristics of the occurrence process, deep-seated landslides associated with seismic motion tend to commonly occur at the same time as seismic motion with a magnitude and intensity that exceeds 5, spreading over a wide area. Typical deep-seated landslides associated with heavy rainfall occur as a result of long periods of heavy rain over a span of 2 to 3 days or more, during heavy rain and until several hours or several tens of hours after heavy rain has almost stopped, and in multiple locations in some cases. Small scale and large-scale deep-seated landslides due to an unknown cause occur, basically independently, through a tertiary creep process several days or several years after the discovery of deformation, respectively. As for the characteristics of the geographical predisposition, deep-seated landslides associated with seismic motion do not necessarily require landslide/bed rock creep deformation as an essential condition, and the deep-seated landslide occurrence area inclination angle ranges widely from 10° to 40°. On the other hand, typical deep-seated landslides associated with heavy rainfall and those due to an unknown cause form a deep-seated landslide occurrence area inclination angle of 30° to 35° and 35° to 50° at a bed rock creep deformation location, respectively, further forming a steep inclination with a most frequent inclination value of around 40° around the deep-seated landslide occurrence area.