MODELING CASCADIA TSUNAMIS INTO DISCOVERY BAY AND HOOD CANAL

In Washington State, new simulations of Cascadia tsunamis include areas far from the Pacific Coast: the Strait of Juan de Fuca and Puget Sound. In these simulations, areas at the heads of narrow waterways experience significantly higher tsunami run-ups than other coastal areas. Discovery Bay, on the Strait of Juan de Fuca, has high run-ups in simulations, and also has a geologic record of tsunami deposits. The tidal marsh at Discovery Bay contains at least nine tsunami deposits that span the last 2,500 years. New GeoClaw tsunami simulations of four different rupture styles of great Cascadia earthquake (M_w ≥ 9) show maximum run-ups of 3–11 meters at Discovery Bay. Another site that shows high run-ups in Cascadia tsunami simulations is Lynch Cove, at the head of Hood Canal in Puget Sound. Run-ups at Lynch Cove, while not as high as those at Discovery Bay, show inundation of the tidal marsh at the head of Hood Canal, with maximum run-ups between 2–7 meters. Two newly-identified tsunami deposits in the tidal marsh sediments at Lynch Cove are likely from Cascadia tsunamis. Other tsunami sources, including the Seattle fault and transoceanic tsunamis, were tested and rejected as sources of the deposits at Lynch Cove. Radiocarbon ages also suggest that the deposits are from Cascadia tsunamis rather than local sources. In the Hood Canal simulation, tsunami waves oscillate for at least 12 hours. Oscillating waves can increase in height as they converge. This may explain why some simulations had later waves that were bigger than earlier waves. Because of this, some areas in Puget Sound may continue to experience waves for many hours after the initial tsunami wave, and these waves may be larger than earlier waves.