Field observations of groundwater which may induce slope failures and landslides were conducted at mountain slopes from the1960s, and various physical models were developed at around the same time. After the 1980s, measurement techniques were developed that can acquire the data of pore water pressure fluctuations during not a snow cover period but an earthquake. With the evolution of investigation technologies, the flow processes of groundwater in slopes can now be visualized. In addition, large-scale analyses using a distributed model are possible to simulate the change in spatial distribution and flow of groundwater before and after the construction of landslide control works like drainage boring, well and tunnel. In future, observed data should be classified into types and made available as open data sets. It is necessary to improve investigation technologies to clarify the hydro-geological structure and three-dimensional groundwater flow analysis in fissured and weathered slopes. A fluidized moving body, whether generated by a slope failure, deep-seated landslide or reactivated landslide, flows down over long distances and spreads widely, causing severe damage in the lower catchment. Therefore, studies on the interaction between displacement and deformation of landslides and pore-water pressure should be conducted by indoor or outdoor experiments and numerical simulations. In addition, groundwater hydrology and hydraulics in landslide areas should be established as a new research field based on the knowledge gained from previous studies.

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Many landslides occurred during the 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0) . Among them, the most catastrophic one occurred in Hanokidaira Area, Shirakawa City. There are two permanent strong motion observation stations nearby, but they showed significant difference in their main shock observation records. To better understand the initiation mechanism of this landslide, it is necessary to have a better understanding on the seismic response of the landslide area during the main shock with high accuracy by taking into account site effects. Therefore, in this study, we estimated the seismic waveform at the Hanokidaira landslide basing on the site effects substitution method that has been identified to be a promising tool for inland earthquakes. We monitored the aftershocks on Harokidara landslide area, and then the site effects at Hanokidaira landslide area have been evaluated. Through this study, we also validated the applicability of the method to a huge subduction earthquake basing on strong motion records. We found that the difference of the site effects between Hanokidaira landslide and those two permanent observation stations is significant.

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It is well reported that landslides occur frequently during a period that a ground water level rises due to the heavy rain and snow melting. To perform a highly precise level of risk evaluation of these landslides the estimation method of ground water level fluctuation is necessary to develop including it for the snow melting period. Therefore, we examined the estimation method of water level fluctuation in consideration of the infiltration effect of snow melting water into under ground in snow melting period. The estimation method suggested the Ψ function method that calculates the maximum correlation in the observation values of ground water level and the estimated values analyzing effective snow melting amount by GA algorithm method. The presented method confirms the validity and precision through a comparison between the actual field observation data and estimation results.

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