Some fluidized landslides triggered by the 2011 Tohoku Earthquake (Mw 9.0), Japan

The 2011 Tohoku Earthquake off the Pacific coast of Japan generated a large tsunami and many landslides, resulting in a great number of casualties. Although almost all casualties resulted from the tsunami, some long-travel, fluidized small-scale landslides also killed 13 people. After the earthquake, we surveyed seven of these catastrophic landslides triggered by the earthquake. We found that most of them have nearly identical geological features, with slopes consisting of pyroclastic deposits formed at different times, and with a palaeosol layer that outcropped in most cases after the landslide. Above the palaeosol there are layers of pumice and scoria. The palaeosol had a natural moisture content of ~ 160%, and the pumice and scoria had a moisture content of ~ 145%. From field observations we concluded that the sliding surface originated in the very upper part of palaeosol, and liquefaction occurred in both layers, resulting in the fluidization of displaced landslides. To examine the trigger and movement mechanism of these landslides, we monitored the ground motion of one landslide area during the many aftershocks, and compared the results with records obtained by a national seismic station nearby. We inferred that strong seismic motion occurred in the landslide area during the main shock. We sampled the palaeosol and pyroclastic deposits, and performed undrained static/cyclic shear tests on the materials both in a saturated state and at natural moisture content. The results indicate that high pore-water pressure was generated, resulting in decreased shear strength even in samples with the natural moisture content. The shear strength of the palaeosol lowered to a very small value with continuous increase of shear rate, enabling the high mobility of the displaced landslide materials.