REFINING CASCADIA SUBDUCTION ZONE EARTHQUAKE TIMING IN NETARTS BAY, OREGON, WITH NEW AGE DATA FROM A GHOST FOREST TREE AND MARSH STRATIGRAPHY

Netarts Bay on the Oregon Coast, USA, is located along the Cascadia Subduction Zone (CSZ), a 600- mile plate boundary extending from northern California to southern Canada. Stratigraphy in central Netarts Bay reveals a sequence of tsunami sands that stratigraphically overlie buried land surfaces visible in salt marsh bank exposures. Past coring studies have indicated that 4 of the last 5 CSZ events are preserved in the bay. A ghost forest is identifiable in places along the bay front, and we correlate those trees with trees buried more than 1 m below the salt marsh surface. We collected a slab from a tree buried 1.43 m below the modern marsh surface, and wiggle matched six C14 ages from growth rings and found the death of the tree to have occurred between 1555 – 1578 yrs. B.P. (95% confidence interval), which we interpret to be the timing of the 5th paleo earthquake on the CSZ. We are further refining the timing of death of this tree with new ages from additional growth rings selected along steep, unique portions of the C14 calibration curve.

Prehistoric earthquakes are identifiable within bank exposures as four pairs of buried marsh and sand deposits. We have collected charcoal samples and macroscopic organics (e.g. needles, twigs, rhizomes) that bracket earthquake horizons stratigraphically above the ghost forest horizon, to improve age estimates for this younger section. We generated an earthquake age model for central Netarts Bay using radiometric ages from earlier coring studies, our new AMS C14 dates collected from marsh stratigraphy, and the timing of death of the tree. We used OxCal v. 4.4 statistical software to combine these data and refine the timing of earthquakes preserved in this section. This research will improve the resolution of CSZ earthquake timing for several seismic cycles that can then be compared with earthquake ages from other high-resolution sites, and used to better evaluate evidence of complex paleo-earthquake rupture scenarios along strike. This will allow us to explore patterns of CSZ rupture length (full vs. partial rupture), which will improve our understanding of the frequency of 1700 CE M 9 earthquake scenarios in the last ~ 1,800 years before present, and could also be used to inform coastal hazard planning.