Landslides in urban residential slopes induced by strong earthquakes in Japan

Abstract

Recent destructive earthquakes in urban regions, such as the 1978 Miyagiken-oki earthquake, the 1995 Kobe earthquake and the 2011 Tohoku earthquake have destabilized many gentle slopes in residential areas of large cities in Japan. Beyond the serious danger to residents of the earthquake affected areas, such landslides revealed the weaknesses of urban development in large cities of Japan. One of the typical large landslides, the Midorigaoka #4 landslide, occurred in residential fills in Sendai during the 2011 Tohoku earthquake. Inclination measurements in the landslide indicate self-dumping at weak layers in ground structure. Excess pore water pressure in the landslide increased in direct proportion to horizontal peak ground velocity during after-shocks suggesting that the landslide was initiated by the complete loss of shear strength along the slip layer during the main shock. A simple analog model, the roller slider model, can discriminate stable or unstable valley fills during strong motion of past earthquakes. The results of stability analysis by the simplified 3D method based on this model explain the degree of damage in each valley fill during the 2011 Tohoku earthquake. Considering risk mitigation against such landslides in residential lots, urban development should minimize artificial changes in landforms, especially avoiding valley fills. A new concept of a Counter line city is proposed that includes both minimization of risk and creating a favorable natural environment.