Case History of Landslide Movement during the Northridge Earthquake

We document a case history of Northridge earthquake induced movement of a coherent landslide mass comprised principally of weathered, previously sheared siltstone. Information on the residual shear strength of the material through which the sliding occurred is available both from backanalysis of a static (rainfall-induced) failure on the same slope and laboratory test data. Postearthquake field reconnaissance data establishes the seismic slope displacements near the slope crest. A yield coefficient is estimated for the slope based on residual shear strengths and measured groundwater levels at the time of the Northridge earthquake. This yield coefficient is used with four horizontal equivalent acceleration time histories (that approximate the seismic demand within the slide mass) in Newmark sliding block displacement analyses. The calculated displacements range from approximately 20 to 90 mm, which compares well with observed displacements at the slide scarp of approximately 50 mm.