The formation of causative factors and the prediction of outbreak for repeated landslides on the same slope

以1999年台湾集集地震引发的两次大型滑坡——秋芬二山滑坡和曹岭滑坡——为研究对象，研究了滑坡破坏和再稳定后边坡地理特征的变化。此外，本项目期间发生的新泻县中部地震和北汉头地震也是本研究的对象。对于有150年滑坡记录的草岭滑坡，估计的最大加速度与滑坡发生之间不存在明显的相关性。滑坡的发生不能仅仅用地震来解释，即提出了诱发因素的影响。通过简单模型的试计算表明，重力蠕变作用下存在屈曲褶皱的可能性，屈曲褶皱的位置在斜坡下部。两大滑坡的下坡屈曲褶皱和侵蚀作用逐年加重。在曹岭滑坡的GPS观测中，边坡中部水平位移约为5cm/年，边坡下部水平位移约为1cm/年，坡度起伏幅度为1 ~ 2cm/年。这个斜坡逐渐不稳定了。进行了简单地震勘探，研究了应力释放对地面强度的影响。发现弹性波速以每年400 ~ 600m/s的速度递减。利用GIS对新泻县中部地震引起的滑坡分布进行了分析。滑坡地形出现滑坡的面积率为5.26%，非滑坡地形出现滑坡的面积率为0.16%。这说明地震滑坡的发生受地面历史的强烈控制，在尚未发生滑坡的斜坡上不容易发生地震滑坡。这种现象被认为与上部荷载被移走较少的边坡的再失稳有关

Changes in geographical features of a slope after failure and re-instabilization of the slope by those were investigated for two large-scale landslides -- the Chiu-fen-erh-shan and the Tsaoling -- triggered by the Chi-Chi earthquake in 1999 in Taiwan. Furthermore, the mid Niigata prefecture earthquake and the Noto-Hantou earthquake which occurred during the term of this project were targeted for this study, too.1. For the Tsaoling landslide which has the record of landslides in the past 150 years, the clear correlation between estimated maximum acceleration and occurrence of landslide wasn't admitted. The occurrence of landslide can't be explained only by earthquake, that is, the influence of causative factors is suggested.2. Trial calculations by a simple model show that there is a possibility of buckling fold by gravitational creep and the position of buckling fold is on the lower part of slope.3. The buckling fold and erosion on the lower slope of both the landslides progress year a … More fter year. The GPS survey at the Tsaoling landslide showed the horizontal displacement of about 5cm/year on the central part of slope and 1cm/year on the lower part of slope and the upheaval of 1-2cm/year. The slope is instabilizing gradually.4. Simple seismic exploration was carried out to investigate decrease in the ground strength by stress release. The tendency that an elastic wave speed was decreasing at the rate of 400-600m/s a year was found.5. The distribution of landslides caused by the mid Niigata prefecture earthquake was analyzed by GIS. The area rate of landslides which appeared in the landslide topography was 5.26% and that on the non-landslide topography was 0.16%. This result shows that the occurrence of landslide by an earthquake is strongly controlled by the history of the ground and a landslide by an earthquake doesn't happen easily in a slope where a landslide hasn't occurred yet. This phenomenon is thought to concern the re-instabilization of a slope from which upper load is removed Less