PROPENSITY FOR DEEP-SEATED LANDSLIDES IN THE OREGON COASTAL RANGES DURING CASCADIA MEGATHRUST EARTHQUAKES THROUGH DENDROCHRONOLOGICAL DATING OF LANDSLIDE-DAMMED LAKES (Invited Presentation)

Seismic hazard analyses of the Cascadia subduction zone largely focus on primary threats posed by ground motion and tsunamis, and less attention is directed towards hazards posed by large, deep-seated landslides, which have the potential to overrun communities and dam rivers resulting in outburst floods. Despite an abundance of deep-seated landslides in the Oregon Coastal Ranges (OCR), no landslide has been conclusively linked to a Cascadia megathrust earthquake. While radiocarbon and surface roughness techniques provide compelling methods for dating landslides, they do not provide the precision to pinpoint definitively a landslide as coseismic. Landslide-dammed lakes with still-standing tree snags, or “ghost forests,” present the novel opportunity to use dendrochronology for determining the year (and potentially season) of tree death and, therefore, the age of the lake-forming landslide dams that killed them. We have dated several landslide-dammed lakes throughout the OCR to ascertain the effect of coseismic ground motion. Given that the last subduction zone earthquake in Cascadia has been constrained to January 26, 1700, we restrict our search to relatively recent landslides as implied by morphologic properties determined by lidar. Using dendrochronology, we constrained the age of Wasson Lake and its associated landslide dam to 1819 AD, consistent with radiocarbon analysis. At Klickitat Lake, we generated a preliminary date of 1751 AD, though more dendrochronological measurements of drowned snags are needed.

Utilization of tree ring chronologies from the OCR to correlate measured ring thicknesses from snags at landslide-dammed lakes provides the precision required to date a landslide with minimal uncertainty. Radiocarbon may be used to corroborate the dendrochronology-derived dates, and calculated ages can be used to improve existing surface roughness dating techniques. While we have not yet found a lake that dates to 1700 AD, we have demonstrated that our enhanced ability to date landslide-dammed lakes using tree rings provides a preferred methodology for investigating the role of earthquakes in the landscape in Cascadia.