An Analysis of the Reactivation Potential of a Deep-seated Landslide in the Oregon Coast Range under varying Hydrologic conditions with Seismic Triggering

Abstract

Landslides can occur in many locations across the world and have the potential to be extremely destructive if failures occur near populated areas. Failures are most likely to occur on slopes that have already experienced numerous failures. This means they are a considerable hazard, and the risk involved with building in areas that have previously experienced landslides should be adequately understood. This study examines the reactivation potential of a deep-seated landslide located in the Oregon Coast Range. The analysis of this landslide included creating a map of the surface morphology and computing the factor of safety for the deposit using the Swedish method of slices. The surface morphology map determined that the landslide is relatively young and currently inactive. The factor of safety was computed for two conditions—the base conditions in the field and an estimated fully saturated condition. The factor of safety determined for both conditions was used to calculate the critical acceleration needed to induce failure. This calculation was completed using Newmark’s analysis of a sliding block. The slope is stable in base conditions with a factor of safety of 2.22 and it would take a critical acceleration of 22% of g to cause slope failure (Table 4). This ground acceleration can be related to a Modified Mercalli Intensity of seven or greater. For saturated conditions, the slope is also stable, but a smaller critical acceleration will produce failure, namely 7.7% of g, which is an earthquake of Modified Mercalli intensity of five or above (Table 4).