| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 95 === The 21st September 1999 Chi-Chi earthquake (ML=7.3) triggered several large landslides in central Taiwan. Among them, the catastrophic Chiufenerhshan landslide resulted in 39 deaths and enormous capital loss. The landslide region is located in the Nantou area about 10 km west from the town of Puli. About 36 million m3 rock mass moved downward slope to the Sezikeng river valley and formed three barrier lakes. The sliding area distributed at the west limb of the Taanshan syncline is a typical dip-slope terrain, and its main rock formation consists of inter-bed sandstone and shale. The goal of this study is to characterize the geomorphic and structural features before and after this landslide. Subsequently, the influence of the observed structural and geomorphic features is discussed and possible sliding mechanisms are proposed for the catastrophic Chiufenerhshan landslide.
In order to better characterize the geomorphic and structural changes of this landslide, detailed digital morphological data before and after the event are necessary. However, only one DEM (40 m resolution, 1986) before the Chi-Chi earthquake is available, which cannot satisfy the intended geomorphic and structural analysis. Thus, a higher resolution DEM (2 m resolution, 1998) of the Chiufenerhshan area is reconstructed using the method of aerial photogrammetry. The reconstructed DEM and two other available DEMs (9 m and 1 m resolution, 1999 and 2002, respectively) are used to analyze the characteristics of the geomorphology and structure of the Chiufenerhshan area before and after the Chi-Chi earthquake.
The prominent structural discontinuities of the Chiufenerhshan area, including cracks, joint sets, and three fault zones are mapped and discovered through the analysis of the DEMs. The cracks and the stream gullies on the dip-slope apparently formed along the observed joint sets and may effectively reduce the lateral support of the sliding mass. After the Chi-Chi earthquake, the boundaries of sliding area are confined at the west limb of the Taanshan syncline, the major joint sets, and the cracks. The deposit area is restricted by the ridges around the Sezikeng valley. These phenomena imply that the eventual landslide is strongly influenced by the original slope morphology and structure. Several geomorphic observations indicate that the foot of the slope is still preserved and the initial failure location should be above it. It is possible, however, that the foot of the slope is partly destroyed and covered by sliding materials.
If the sliding bedding plane was detached and infiltrated by water during the earthquake, it may dramatically reduce the frictional force on the sliding plane. Consequently, most of the gravitational force was supported only by the strength of the slope strata leading to the slope failure either by shearing-off or buckling mechanisms. Based on the Coulomb-Mohr failure criteria, structural discontinuities such as faults, joints, or cracks must exist in preferable orientations in the sliding mass in order for the shearing-off mechanism to occur. Field investigations on structural discontinuities suggest that the buckling failure mechanism is most probable for the landslide rather than the shearing-off mechanism. The buckling hinge may initiate along the observed weak fault zones which are situated on the sliding plane, and a range of young’s modulus (16.89 ~ 22.68 GPa) for the slope body can thus calculated assuming such failure. It is inferred that the Chiufenerhshan landslide slid along the bedding plane, and the dip-slope strata initially failed along the fault concentrated zones instead of destruction at the lowest portion of the slope.
|
|---|
