ESTABLISHING PRECISE AGES FOR PAST CASCADIA SEISMICITY USING MULTIPROXY TREE-RING RECORDS (Invited Presentation)

Precise dating and characterization of past seismicity on the Cascadia Subduction Zone and in the surrounding region is integral to assessing geologic hazards in the Pacific Northwest. In general, our ability to accurately interpret the suite of paleoseismic evidence scattered across coastal Cascadia has been hindered by large age uncertainties. Dendrochronology is one of the most precise geochronologic tools utilized to date past earthquakes in the Cascadia region. Large earthquakes can cause substantial land-level changes, including coastal subsidence, and large fault scarps that dramatically change the local hydrology. These effects kill and preserve ancient forests through drowning and rapid burial, producing “ghost forests” and buried soils. Here, we use a multiproxy approach, including novel high-resolution ¹⁴C records from ghost forest tree rings, traditional dendrochronology, and stratigraphic relationships to generate precise geochronology of earthquake-induced forest deaths along the Cascadian coast. By providing precise dates of these large events, these data allow us to better understand the timing and potential interplay of crustal faults with megathrust subduction earthquakes and improve our understanding of earthquake recurrence beyond the limited time span of instrumental records. Additionally, we generate multi-centennial, tree-ring records along the Cascadian coast that allow us to assess the impacts of climate in conjunction with extreme events on ancient coastal landscapes. These records can be used by other disciplines to calibrate the geochronology of their studies.