On the Initiation and Movement of Hanokidaira Landslide from the 2011 Tohoku Earthquake, Japan

A flow type landslide was triggered in Hanokidaira area by The 2011 off the Pacific coast of Tohoku Earthquake, killing more than ten people. The slope consists of pyroclastic flow deposits that were formed at different times, with a layer of paleosol that was outcropped. Above the paleosol there is a layer of pumice and scoria, which is very rich in natural moisture content (~145 %). From field observation, we inferred that the sliding surface originated on the boundary between the paleosol layer and the layer of pumice and scoria. To examine the possible trigger and movement mechanism of these landslides, we monitored the aftershocks on landslide area, and inferred the possible seismic response of the landslide area during the main shock. We took samples from both layers, and performed undrained static and cyclic shear tests on them in fully saturated or in natural water content states. The results showed that high pore-water pressure could be generated after failure with increase of shear displacement, resulting in great loss in the shear strength. We also performed seismic simulation tests on these samples by using the inferred seismic waveform, and the test results showed that the amplified vertical motion may have played a key role in the initiation of this landslide.