2012 Volume 52 Issue 6 Pages 222-230
The 2011 off the Pacific coast of Tohoku Earthquake induced 4 soilslide-avalanches of pyroclastic fall deposits in Shirakawa city and Nakagawa town, where 6+ and 6− seismic intensities of JMA scale were recorded, respectively. The causal factors of these landslides were slope-parallel bedding of pyroclastics that include very weak paleosol, in which a sliding surface was made, and that beds that slid had been undercut to have lost the support from lower slope. These landslides were highly mobile, which is suggested by apparent friction angles ranging from 10° to 16°. Trees on slopes have been transported on debris in a standing position. Soilslide-avalanches like those described above have been induced by many other earthquakes, which suggests that some pyroclastic fall deposits are very susceptible to earthquake and that we have to identify them to mitigate earthquake induced landslide hazards.
