Chapter 4 - Coseismic landslides

Abstract

Coseismic landslides are important effects of strong earthquakes and one of the most poorly understood. Such landslides are driven by inertial forces acting on slopes and can vary in size from individual rock blocks of a few m3 in volume to catastrophic rock avalanches of many Mm3, which can claim many thousands of lives. The capability of an earthquake to trigger a landslide is a function of energy arriving at the site, which is itself related to earthquake magnitude and epicentral distance. However, as much as these two parameters can be shown to exert a limit on the ability of the earthquake to cause landslides, it is clear from the spatial distribution of landslides caused by earthquakes that these are not the controlling factors. A detailed consideration of ground accelerations recorded during earthquakes indicates that threshold or yield accelerations are required to initiate movement on slopes. These yield accelerations are strongly dependent on the degree of stability of the slope under ambient stress conditions, development of porewater pressures during shaking and degradation of strength and stiffness under cyclic loading. Traditional engineering analyses have tended to focus on horizontal accelerations; however, a growing body of evidence points to the importance of vertical accelerations in seismic slope stability. In addition to the polarity of ground motions, there is an important influence from topographic amplification of shaking. This can give rise to ground motions that are 5–10 times higher than free-field accelerations, meaning that ground shaking at slope crests and ridges can be considerably higher than the vibration at the foot, or behind the crest, of the slope. This variation in ground motions means that shaking recorded on free-field instruments is unlikely to be representative of ground motions driving slope instability. Therefore, several significant unknowns severely limit understandings of this group of dynamic slope processes.