The Lost Lake Landslide: Evidence for an Earthquake-Triggered Landslide Vashon Island, Washington

Abstract

I address the large-scale stability and potential past and future triggers of the Lost Lake Landslide on Vashon Island, one of the largest mapped landslides in the Puget Lowland. I focus on three landslide triggers; groundwater fluctuation, the Seattle Fault Zone and the Tacoma Fault Zone and identify the most likely trigger. No previous work has analyzed the trigger, age, or stability of the slope. Using an end member approach, I calculate the factor of safety and seismic critical acceleration using a two-dimensional limit equilibrium model for three different Lost Lake Landslide scenarios. Two scenarios are a reconstruction of potential past failure and one scenario is a future failure of the modern slope. Using USGS ShakeMaps I compare modeled Seattle Fault Zone and Tacoma Fault Zone peak ground accelerations with calculated critical accelerations from this study. I find that significant groundwater fluctuations have a surprisingly low influence on large-scale slope stability. Additionally, shaking from either a Seattle Fault Zone or Tacoma Fault Zone earthquake could have triggered the Lost Lake landslide. A Tacoma Fault Zone earthquake is a more likely trigger due to its greater exceedance of the required critical acceleration to cause a slope failure. My results indicate that large magnitude crustal earthquakes can potentially trigger extremely large landslides in the Puget Lowland. As a first order assessment, factor of safety and critical acceleration analysis can potentially identify other large co-seismic landslides in the Puget Lowland.