Comparison of Seismic Slope-Performance Models

Researchers, emergency response and lifeline managers, and municipal planners are beginning to recognize the utility of seismic landslide hazard zonation. With this recognition, the decisions made based on resulting maps could have widespread social and economic impact in the event of a large earthquake. This report compares several popular permanent displacement models for assessing seismic slope-performance. The approaches are implemented in a raster GIS to expose potential differences and assess the effects of using a particular approach within a decision-making context. It is observed that each approach forecasts notably different levels of slope-performance. Thus, considering the variety of spatial seismic landslide analysis approaches and the effect of basing a decision on a map created using a single one of them, it is suggested that less reliance be put on the traditional paper map format. Instead, multiple approaches can be used to investigate many scenario earthquakes under a variety of conditions in a computer-based spatial decision support system. INTRODUCTION Keefer (1984) observed that earthquakes of moderate to high magnitude can cause landslides over an area as large as 500,000 km2 . These landslides also have large damage potential as illustrated by the recent effects of the 1989 Loma Prieta and 1994 Northridge earthquakes (Harp and Jibson, 1995; Keefer, 1998). Accordingly, researchers, emergency response and lifeline managers, and municipal planners are beginning to recognize the utility of seismic landslide hazard and risk zonation. With this recognition, the decisions made based on resulting hazard or risk maps could have widespread social and economic impact in the event of a large earthquake. Therefore, investigating and comparing several popular techniques for seismic slopeperformance zonation is important. The state of the art in seismic landslide hazard zonation using geographic information systems (GIS) was summarized by Ho and Miles (1997), who suggested several potential approaches using dynamic permanent-displacement models. In the short time since then, considerable effort has been spent improving seismic landslide hazard zonation techniques using spatial technologies (Miles and Ho, 1999; Jibson and others, 1998; McCrink and Real, 1996). This report extends the study of Ho and Miles (1997) by implementing several seismic slope-performance models using raster GIS to expose any differences between the approaches and assess the potential effects of using a particular approach within a decision-making context. The report begins by summarizing the approaches that exist for determining seismic landslide hazard. The general procedure of a permanent-displacement analysis the class of approaches chosen in the report for investigation is then described. The report concludes by discussing the implementation of each individual approach and the differences among these approaches. PERMANENT-DISPLACEMENT ANALYSIS Three basic approaches exist for conducting seismic landslide hazard analysis. These consist of the statistical, pseudo-static, and permanent-displacement approaches. A statistical approach assesses hazard by assuming the past predicts the future. Hazard is assessed through correlation of past landslides with several influential factors. Results of a statistically based analysis can range from an estimated probability of failure to some index indicating degrees of hazard. Pseudo-static analysis employs a traditional static slope-stability analysis with the addition of a horizontal force component that models the effects of earthquake-induced ground-motions. A pseudo-static analysis yields a factor of safety against seismic slope failure. This effectively provides a simple binary index of whether a slope is expected to fail or not at a given level of seismic acceleration. Permanent-displacement techniques provide information regarding actual slope-performance through calculation of some index of relative or actual displacement based on commonly accepted characterizations of earthquake-shaking severity. Permanent-displacement analysis is chosen for investigation because of its higher information content, better modeling of ground-motion, and increasing acceptance in the earthquake engineering community. Newmark's Sliding Block Analogy In his landmark paper, Newmark (1965) noted that the transient effects of earthquake motions can cause permanent deformation of slopes prior to complete failure. Newmark proposed modeling a slope subjected to earthquake-induced accelerations as a friction block resting on an inclined plane subjected to the same accelerations as the modeled slope (Figure 1). Therefore, in each instance when the sum of the static and dynamic forces exceed the shear resistance of the sliding interface the block will displace. The interface shear resistance is commonly characterized by the critical acceleration (ac) of the modeled slope, which is the base acceleration needed to overcome the shear resistance. Newmark (1965) defined the following relationship to calculate critical acceleration in the case of planar slip: