6. Soil-Slide Avalanches of Pyroclastic Fall Deposits Induced by the 2011 Off the Pacific Coast of Tohoku Earthquake

The 2011 Off the Pacific Coast of Tohoku Earthquake set off four soil-slide avalanches of pyroclastic fall deposits within Shirakawa (Fukushima Prefecture) and Nakagawa (Tochigi Prefecture). At Shirakawa, a seismic intensity of 6+ on the JMA scale was recorded and at Nakagawa, 6−. Predominant factors behind these slides were (1) a bedding of pyroclastic fall deposits parallel to the original slope and (2) a sliding surface of weak paleosol that, having been undercut by erosion at the bottom of the slope, lost its ability to support the soil above it. These landslides were highly mobile, as suggested by apparent friction angles of 10–16°. Many trees on them, supported by root systems, remained standing within their root–soil plate; that is, they were transported and deposited upon the underlying debris in an upright position. Similar slides have been observed to occur in numerous other earthquakes, which suggests that some pyroclastic fall deposits are highly susceptible to seismic shocks. Here, to contribute to efforts toward mitigating the earthquake-induced hazards presented by such deposits, we identify some of the characteristics of soil-slide avalanches induced by the 2011 Off the Pacific Coast of Tohoku Earthquake.