VARIABLE COASTAL RESPONSE (SUBSIDENCE, TSUNAMI, BEACH RETREAT, AND BAY DEPOSITION) FOLLOWING EACH OF THE LAST FIVE CASCADIA EARTHQUAKE EVENTS, S.W. WASHINGTON COAST, USA

The Cascadia subduction zone (1000 km in length) has produced megathrust earthquakes with a mean recurrence interval of about 500 years during the late Holocene (0-5 ka). Modeling studies have focused on the last event at 1700 AD, estimated to be Mw 9. However, geologic deposits, in large bays and beach plains of the S.W. Washington coast, record information on coseismic subsidence, tsunami inundation, catastrophic beach retreat, and tidal flat deposition following the last five events (0.3, ~0.8, ~1.1, ~1.3, and ~1.7 ka). The origin of the 0.8 ka event, e.g., Southern Cascadia or Alaska source area, is unresolved at this time. Liquefaction, a measure of shaking intensity/duration, is also recorded in geologic deposits, but it is difficult to date prior to the last event at 0.3 ka. Subsided tidal marshes demonstrate abrupt sea-level rise of ~1.0, 0, ~1.0, <0.5, and ~1.0 m for the last five events, starting with the last event (0.3 ka). Tsunami inundation over barrier beach plains, corrected for the corresponding paleoshoreline, ranges from 0.7 km (0.3 ka event) to 1.3 km (0.8 ka event). In several different back-barrier localities of the central Cascadia margin the 0.3 ka event represents the least amount of tsunami inundation relative to the last 3-5 events. The vertical heights of catastrophic beach retreat scarps for the last 5 events range from 0 m (event 0.8 ka) to 14 m (event 1.7 ka). The larger retreat scarps probably correspond to several hundred meters of shoreward beach erosion. Muddy tidal flats in large tidal basins (Willapa Bay and Grays Harbor) demonstrate laminated sand-mud deposition (5-50 cm thick) after subsidence events. Unlike tsunami deposits, these laminated sequences continue well above the subsidence contacts, probably reflecting enhanced tidal flow. Future ‘probabilistic’ evaluations of coastal hazards will likely utilize the range of observed coastal responses to these great subduction zone earthquakes.