Catastrophic Landslides of Pyroclastics Induced by the 2011 off the Pacific Coast of Tohoku Earthquake

Abstract

The 2011 off the Pacific coast of Tohoku Earthquake induced 4 soilslide-avalanches of pyroclastic fall deposits with volumes from 500 m3 to 30000 m3 in Shirakawa and Nakagawa, where 6+ and 6− seismic intensities of JMA scale were recorded, respectively. These landslides occurred on rather gentle slopes with 13° to 23° with sliding surfaces in the depths of 3 m to 9 m but had a high mobility, which is shown by apparent friction angles ranging from 10° to 16°. Trees on slopes have been transported on debris in a standing position. 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 the beds that slid had been undercut to have lost the support from lower slope. The paleosols were rich in halloysite, which is a weathering product of volcanic ash and pumice. Soil slide-avalanches like those described above have been induced also by previous earthquakes, including 1949 Imaichi, 1968 Tokachioki, and 1978 Izu-Oshima-Kinkai earthquakes. Among these, the landslide numbers of the Tohoku Earthquake were much less than those of the other earthquakes, which could be related to less amounts of preceding rainfall before the Tohoku Earthquake than the others. The Tohoku Earthquake occurred during a dry season in Japan and had only 94 mm during 60 days before the earthquake, while the other earthquakes had 250–400 mm antecedent rainfalls. Pyroclastic fall deposits generally are distributed widely, so if much more rainfall had preceded the earthquake, more landslides could have been induced.