TEMPORAL CLUSTERING OF LANDSLIDE-DAMMED LAKES IN WESTERN OREGON USING DENDROCHRONOLOGY

Large magnitude earthquakes and climatic events in mountainous settings commonly trigger thousands of landslides, and these slope failures constitute a significant fraction of event-driven damage. In the Oregon Coast Range (OCR), large, deep-seated landslides are common, yet established landslide dating techniques are not sufficiently accurate to pinpoint a specific triggering event, such as the 1700AD Cascadia subduction zone earthquake. Dendrochronology, or tree ring analysis, is the only known method capable of dating landslides with annual (or sub-annual) accuracy. We utilize standing, drowned Douglas-fir snags at landslide-dammed lakes in the OCR to establish robust landslide ages with annual accuracy, and we demonstrate that ¹⁴C dating of landslide detritus both corroborates the dendrochronology ages and demonstrates the limitations of using ¹⁴C dating alone. Our initial results show that several landslides in the OCR occurred in the absence of a known triggering event, such as Klickitat and Wasson Lakes, which formed in 1751 and 1819 AD, respectively. Some sites, however, date to the same calendar year, such as Burchard and Esmond Lakes, which formed in early 1890 AD, potentially corresponding with one of the more significant historic flood events on the Willamette River. Landslide ages at additional sites also appear to demonstrate temporal clustering, though the tree ring record at those sites is still being established. Contemporaneous triggering of deep-seated landslides encourages us to explore forcing mechanisms that favor triggering such landslides and why so few deep-seated landslides have historically failed catastrophically.