Observations and Seismic Response Analysis of Urban Landslide Induced by the 2011 off the Pacific Coast of Tohoku Earthquake

Field observations of landslide movements in residential area induced by the 2011 off the Pacific coast of Tohoku Earthquake (M9.0) were conducted. The Midorigaoka 4-chome (Midorigaoka #4) subdivision in the southern part of Sendai City sprawled across the flatland stretching between the terrace and valley floor. The large landslides, the Midorigaoka #4 landslide, occurred in the widening fills of the lower part of this subdivision. The fills consist of soft and loose mixed bedrock material, sand, clay, and sandy silt with gravel. The ground water level was very shallow—0.5 to 1.1 m below the ground surface. Measurements on ground inclination using borehole inclinometers, and pore water pressure changes were made from June 2011 with high precision time interval of 100 Hz. The inclination measurement results indicate the self-dumping at the weak layers in ground structure. The effects of self-dumping varied depend on the thickness of weak layers and distance from epicenters. Excess pore water pressure in the landslide increased in direct proportion to horizontal peak ground velocity (PGV) during earthquakes. The failure process at the slip layer of landslide developed from small local failures to landslide movement was found by chance. The performance of landslide during strong motion has not been discussed based on observed facts. Precise field observation on landslide movement during earthquake should be significant to mitigate the landslide risk in urban region.