Spatial Distribution of Earthquake-Induced Landslides in the 2004 Mid-Niigata Prefecture Earthquake

Abstract

Japan is a seismically active island nation and which has long been at risk of earthquakes and related disasters due to its geological location. At the Japanese Standard Time (JST) 17:56 on October 23, 2004 an earthquake with a moment magnitude of 6.8 struck MidNiigata prefecture in Japan and caused many landslides and slope failures in the Chuetsu region of Niigata prefecture. This catastrophic earthquake was followed by severe aftershocks and caused many types of landslides such as surficial slides, shallow slides, and deep slides. This study analyzed the correlations between landslide occurrence and the slope angle, slope aspect, rock type, bedding plane orientation and dip, and distance from the epicentre fault line by using landslide occurrence ratio (LOR), based on geographic information system (GIS) technology. The LOR is defined as percentage of area affected by seismic landslides. Surficial slides are correlated well with slope angle and distance from the epicenter fault line. Shallow slopes that face parallel to the fault direction are more prone to failure. In addition to that shallow slides occurrence ratio decrease with distance from epicenter fault line. Deep slides are strongly affected by the geological structure. In addition, to examine the spatial distribution of earthquake-induced landslide, it is essential to consider the seismic effects for designing slope stability, retaining walls, bridges, and other engineering structures. It is important to determine the shear strength parameters on a failure surface for stability evaluation and engineering analysis of a landslide, as these parameters are mainly determined by the test, back analysis method and engineering experience analogy method is used. Among these methods, the back calculation is a procedure, which offers the opportunity to estimate the shear strength properties along the failure surface by the mathematical method. Soil strength calculation by back analysis avoids many of the problems related to laboratory testing and is widely used, especially in association with landslide repair studies. This study presents the rational procedure to calculate the horizontal seismic coefficient according to the distance from the epicenter fault line and shear strength of the weathered soil. The seismic coefficient is found similar with past research works.