Dendrochronological dating of landslides in western Oregon: Searching for signals of the Cascadia AD 1700 earthquake

Large-magnitude earthquakes and hydrologic events in mountainous settings commonly trigger thousands of landslides, and slope failures typically constitute a significant proportion of the damage associated with these events. Large, dormant deep-seated landslides are ubiquitous in the Oregon Coast Range, western United States, yet a method for calculating landslide ages with the precision required to diagnose a specific triggering event, including the A.D. 1700 Cascadia earthquake, has remained elusive. Establishing a compelling connection between prehistoric slope instability and specific triggers requires landslide ages with precision greater than that provided by C-14 dating of detrital materials. Tree-ring analysis is the only known method capable of determining landslide age with this precision. Dozens of landslide-dammed lakes in western Oregon present an opportunity to use tree rings from drowned snags, or ghost forests, to establish the year of death, and thus landsliding. We cross-dated tree-ring indices from drowned Douglas fir trees with live tree-ring records from the Oregon Coast Range that exhibit synchronous, time-specific patterns due to regional climate variations. Our analyses determined that the landslides responsible for creating Wasson and Klickitat Lakes occurred in A.D. 1819 and 1751, respectively. The C-14 dates from selected tree rings and landslide deposit detritus are consistent with our tree-ring analysis, although the ages exhibit high variability, revealing the limitations of using C-14 dating alone. Because dendrochronology provides annual precision for landsliding, sampling of tree rings at additional landslide-dammed lakes throughout the Oregon Coast Range can be used to constrain the potential effects of ground motion and major storms on Cascadia landscapes.