Ground motions estimates for a cascadia earthquake from liquefaction evidence

Paleoseismic studies conducted in the coastal regions of the Pacific Northwest in the past decade have revealed evidence of crustal downdropping and subsequent tsunami inundation, attributable to a large earthquake along the Cascadia subduction zone which occurred approximately 300 years ago, and most likely in 1700 AD. In order to characterize the severity of ground motions from this earthquake, we report on results of a field search for seismically induced liquefaction features. The search was made chiefly along the coastal portions of several river valleys in Washington, rivers along the central Oregon coast, as well as on islands in the Columbia River of Oregon and Washington. In this paper we focus only on the results of the Columbia River investigation. Numerous liquefaction features were found in some regions, but not in others. The regional distribution of liquefaction features is evaluated as a function of geologic and geotechnical factors at each site in order to estimate the intensity of ground shaking.