COASTAL MORPHOLOGICAL RESPONSE TO CO-SEISMIC SUBSIDENCE AND TSUNAMIS IN THE PACIFIC NORTHWEST

Evidence of co-seismic subsidence and tsunami inundation from nearby subduction zone earthquakes are found in the coastal sediments of the Pacific Northwest. Buried soils and drowned coastal forests suggest rapid subsidence of 0.5-2 m followed by decades of slow rebound. Thin layers of sand overlying marsh soils suggest inundation of tsunami waves. In addition, buried scarps, mapped with ground penetrating radar, are found parallel to the shoreline along the coast. The youngest scarp is correlated to the 1700 Cascadia earthquake and older scarps correlate to earlier subduction zone earthquakes with a recurrence interval of 500±300 years. The alongshore linear scarps suggest landward erosion of the shoreface and calculations estimate 100s of meters of shoreline retreat along the open coast. The morphological response of the coastal estuaries to these catastrophic events is less well known. Within Willapa Bay, a large estuary on the Washington coast, low frequency large-scale cycles of entrance channel realignment, as well as spit growth inside the bay are hypothesized to be responses to these events.

To explore the response of Willapa Bay to co-seismic subsidence and tsunami inundation we employ a process-based morphological model, Delft3D, that couples currents, waves, sediment transport and morphology change. The flow and wave components of the model have been calibrated with field measurements of water levels, currents, and waves collected in the bay during typical winter storm conditions. Model simulations explore the sediment transport and morphological response of the tsunami waves inundating the estuary and the response of the estuary entrance following the rapid change in relative sea level due to subsidence.

Model results show that under present conditions, lateral tidal-current asymmetries and the dissipation of wave energy on the shallow entrance shoal dominate morphological changes in the bay. Additional results suggest that residual circulation and sediment transport patterns are sensitive to water level and can be expected to change significantly when sea level is higher, such as immediately after the subsidence event. Further model simulations will explore the relative contributions of tsunami-related morphological change versus subsidence-related morphological change within the bay entrance.