Effect of Soil Moisture on Landslide Occurrences by the Earthquake : Tohoku Pacific Ocean Earthquake

2011年3月11日，東北太平洋沖地震が発生，地震に伴い発生した津波により，東北太平洋側地域に甚大な被害をもたらした。また，福島県白河市葉ノ木平地区等では，人的な被害を伴う土砂災害が発生した。被害は決して小さなものばかりではなかったが，全体的に見ると，地震の規模や記録された地震動の加速度の大きさにもかかわらず，山地部における斜面崩壊等の土砂災害は少数であり，規模も小さかった。また，人的被害を伴う土砂災害は，福島・栃木県に限定されていた。この原因について検討するために，地震発生時の土壌水分状態と地震の加速度を考慮した静的な斜面安定解析を行い，土壌水分状態が斜面安定性に与える影響を検討した。その結果，地震発生時期の先行降雨が少なかったことで，斜面安定性は大きく低下しなかったことが判明，仮に夏季の降雨イベント後に地震が発生していたら，実際よりも大きな被害がもたらされたと結論付けられた。
The pacific side of Tohoku region, Japan was suffered by a devastating earthquake on 11th March, 2011. Several hillside slopes were collapsed due to the earthquake in inland region, and a number of people were also killed by the sediment related disaster. Contrasting with the devastating earthquake which suffered wide area, the landslides were limited in its number, area, and size. To understand the reasons of this limited landslide occurrences, distribution of the maximum earthquake acceleration was investigated. The effects of the slope topographical properties such as slope gradient, direction, and bedrock geomorphology were also investigated by the field observation. Numerical experiments were also conducted to demonstrate the effects of the slope topographical properties and soil moisture conditions on the landslide occurrences. From the field investigations and numerical simulations, it is elucidated that the reasons of the limited landslide occurrences are 1) limited suffering area of strong acceleration by the earthquake and 2) relatively dry soil moisture condition of slopes at the earthquake occurrence.