According to events and studies on landslides of rock slopes due to recent earthquakes mainly in Japan, the author remarks some differences of the characteristics and mechanisms among landslide phenomena. Rockfalls occur by the seismic inertia force on steeper reverse-dip slopes as topples and falls, which consist of loosen rock or jointed rock material in shallow portions. On the other hand, rockslides occur on gentler dip slopes as translational or rotational slide, which develop surface gravitational deformation, consist of mainly bed rock and intercalate sandy, silty or clayey strata of slip plane. Furthermore, rockslides are caused not by the seismic inertia force but by the cyclic shear load which results in both decrease of shear strength and liquefaction with excess pore water pressure of slip plane in earthquake duration.

1 Introduction

Many earthquake-induced landslides have occurred on rock slopes in the seismically active areas of the world. In Japan, earthquake-induced landslides have caused hundreds of deaths and billions of dollars in economic losses in recent years. Since Japan situates on the subduction of both the oceanic Pacific plate and the oceanic Philippines plate beneath both North American plate and Eurasian plate.

Characteristics and mechanisms of earthquake-induced landslide have been discussed based on their numerous historical records induced by seismic ground motions. For example, Voight & Pariseau (1978) introduced the distance at which earthquakes can trigger landslides is subject to the following: the stability of the potential slide mass, the orientation of the earthquake in relation to the slide mass, earthquake magnitude, focal depth, seismic attenuation, and aftershock distribution.

Wyllie & Mah (2004) introduced the performance of rock slopes during earthquakes by Keefer (1984, 1992) that the following five slope parameters have the greatest influence on stability during earthquakes:

- Slope angle Rock falls and slides rarely occur on slopes with angles less than about 25°;

- Weathering Highly weathered rock comprising core stones in a fine soil matrix, and residual soil are more likely to fail than fresh rock;

- Induration Poorly indurated rock in which the particles are weakly bounded is more likely to fail than stronger and well-indurated rock;

- Discontinuity characteristics Rock containing closely spaced or open discontinuities are more susceptible to failure than massive rock in which the discontinuities are closed and healed; and

- Water Slopes in which the water table is high, or where there has been recent rainfall, are susceptible to failure.

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