Abstract
Investigations of earthquakes worldwide show that rock falls are the most abundant type of landslide that is triggered by earthquakes. In many locations throughout the world, the juxtaposition of steep rock slopes and population centers have resulted in major hazards to people and property. In the United States, Salt Lake City, Utah, is one of the most notable examples of a major population center adjacent to mountainous slopes.
Slope stability analyses for rock slopes under seismic shaking conditions are useful for specific sites but have little application to large areas. To evaluate the seismic susceptibility to rock falls for a large area, empirical methods are required because of the extreme variability of lithologic and fracture characteristics of rock throughout a large area.
Susceptibility to seismically-induced rock falls is governed by the characteristics and properties of fractures and joints in all but the weakest rocks. Because of this, we modified an engineering classification originally used in tunnel design known as the rock mass quality designation (Q) for use in rating the susceptibility of rock slopes to seismically-induced failure. This classification describes six fracture characteristics of rock bodies and gives them numerical values. Our modification involves major changes in the rating system for the stress reduction factor to account for aperture in rock slopes, the major factor in rock-fall susceptibility.
Analysis of rock-fall concentrations and Q-values for the 1980 earthquake sequence near Mammoth Lakes, California, defines a well-constrained upper bound that shows the number of rock falls per site decreases rapidly with increasing Q.
The decay of number of rock falls per site versus Q can be modelled as a probability density function of the form P = Nae-aQ. Using this model, four categories of rock-fall susceptibility can be discriminated based on the percentages of rock falls within different ranges of Q.
Because of the similarities of lithology and slope between the Eastern Sierra Nevada Range near Mammoth Lakes and the Wasatch Front near Salt Lake City, Utah, the probabilities derived from analysis of the Mammoth Lakes region were used to predict rock-fall probabilities for rock slopes near Salt Lake City in response to a magnitude 6.0 earthquake. These predicted probabilities were then used to generalize zones of rock-fall susceptibility.
