USING GIS IN THE PROBABILISTIC ASSESSMENT OF EARTHQUAKE TRIGGERED LANDSLIDE HAZARDS

Summit Ridge, located in the North Santa Cruz Mountains, California, was heavily affected by landsliding during the 1989 Loma Prieta earthquake (M w =7.0). Widespread shallow failures, along with 17 deep-seated landslides, caused extensive damage. In order to study possible methods of assessing landslide hazard a geographical information system (GIS) was utilised. Geological, geotechnical, geomorphological and seismological data were used in a deterministic analysis using two standard earthquake slope stability models (pseudo-static and Newmark displacement). The models were used to assess the landslide potential that existed during the Loma Prieta event. It was found that, of these two models, the Newmark displacement model proved most successful at predicting the location of shallow unstable slopes. However, because both models are deterministic in nature they did not take into account any of the error and uncertainty in the input parameters. Therefore, a probabilistic Newmark displacement method of analysis was developed. This was achieved by incorporating the principles behind a recently developed probabilistic technique which utilises the pseudo-static slope stability model. The method calculates the probability that a slope will exceed a certain critical value of Newmark displacement and thus fail. The use of such an analysis resulted in a more realistic distribution of hazard when compared with the distribution of actual landslides triggered by the 1989 earthquake. Subsequent to this, the probabilistic method was used to undertake a landslide hazard assessment for Summit Ridge given a postulated earthquake located on the Northern East Bay segment of the Hayward fault.