Liquefaction evidence for the strength of ground motions resulting from late Holocene Cascadia subduction earthquakes, with emphasis on the event of 1700 AD

During the past decade, paleoseismic studies done by many researchers in th e coastal regions of the Pacific Northwest have shown that regional down-dr opping and subsequent tsunami inundation occurred in response to a major ea rthquake along the Cascadia subduction zone. This earthquake occurred almos t certainly in 1700 A.D., and is believed by many to have been of M 8.5-9 o r perhaps larger. In order to characterize the severity of ground motions f rom this earthquake, we report on a field search and analysis of seismicall y induced liquefaction features. The search was conducted chiefly along the banks of islands in the lowermost Columbia River of Oregon and Washington and in stream banks along smaller rivers throughout southwestern Washington . To a lesser extent, the investigation included rivers in central Oregon. Numerous small- to moderate-sized liquefaction features from the earthquake of 1700 A.D. were found in some regions, but there was a notable lack of l iquefaction features in others. The regional distribution of liquefaction features is evaluated as a functi on of geologic and geotechnical factors in different field settings near th e coast. Our use of widely different field settings, each in which we indep endently assess the strength of shaking and arrive at the same conclusion, enhances the credibility of our interpretations. Our regional inventory of liquefaction features and preliminary geotechnica l analysis of liquefaction potential provide substantial evidence for only moderate levels of ground shaking in coastal Washington and Oregon during t he subduction earthquake of 1700 A.D. Additionally, it appears that a simil ar conclusion can be reached for an earlier subduction earthquake that occu rred within the past 1100 years, which also has been characterized by other s as being M 8 or greater. On the basis of more limited data for older even ts collected in our regional study, it appears that seismic shaking has bee n no stronger throughout Holocene time. Our interpreted levels of shaking a re considerably lower than current estimates in the technical literature th at use theoretical and statistical models to predict ground motions of subd uction earthquakes in the Cascadia region. Because of the influence of esti mated ground motions from Cascadia subduction-zone earthquakes on seismic h azard evaluations, more paleoliquefaction and geotechnical field studies ar e needed to definitively bracket the strength of shaking. With further work , it should be possible to extend the record of seismic shaking through muc h of Holocene time in large portions of Washington and Oregon.