Abstract
In the earthquake engineering practice, fragility curves are useful tools for predicting the extent of probable damage. They show the probability of damage to a particular-style structure as a function of strong motion parameters, and they allow estimation of a level of damage probability for a known ground motion index. The problem, however, is that strong ground motion records are often unavailable in seriously devastated areas, thus frustrating all attempts to rationally deduce the whole picture of the devastation at a great cost of many lives and properties. Other than that, we should recognize that not only intense shakes but also ground deformations can be equally or often more responsible for the devastation. Aftermaths of an earthquake are often more devastating than its immediate effect, especially in mountainous terrains. Large strains built up in soils and rocks along a dislocated fault can trigger post-earthquake disasters such as landslides and debris flows, which can last long causing serious problems for rehabilitations and land conservations. Therefore one of what required of us is to deduce as much hidden signs as possible from observable change of landforms. Earthquake-induced landform changes have a wide range of ground movement. Among them, tectonic deformations under the action of deep-seated forces may hint the presence of a zone of deformed rock along the exposed and/or hidden fault, namely the zones which became more susceptible to landslides than they had been in the past. Coherent mass movements are generally less catastrophic than chaotic mass movements. However, they can surely cause long-lasting problems for rehabilitations. Moreover even a chaotic mass movement can be preceded by a slow and coherent mass movement at its early stage. Recent development of remote sensing technologies has enabled us to detect precise landform changes in a scientific manner. However, the methods allow us to detect displacements only in the Eulerian description, in which the description of motion is made in terms of the spatial coordinates which does not follow the motion of soil particles. Discussions of earthquake-inflicted geotechnical issues require more direct description of soil particle movements because soils are typically history-dependent materials. The first half of this chapter presents a method to extract Lagrangian components of displacements from available set of elevation data. The method is then applied to an active folding zone affected by the Mid-Niigata Prefecture Earthquake of October 23rd 2004. The surface Lagrangian displacements and the cause-and-effect relationships for the reported damages are discussed in detail.
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