Modeling of Tsunami Waves in the Southern Strait of Georgia Generated by a Potential Subaerial Landslide

Significant tsunami waves can be generated in coastal region of the World Ocean by submarine landslides, slumps, rockfalls, and avalanches. On the Cascadia subduction zone, which lies along the Pacific coast of southern British Columbia and northern Washington, tsunami hazards are dominated by megathrust events. Much of the southern Strait of Georgia is partially sheltered from such events but remains highly vulnerable to local landslide-triggered waves, which may cause significant damage with little or no warning. Despite their potential impact, landslide tsunamis in the Strait of Georgia have received little attention. In this study, we focus on modeling tsunami waves from a potential subaerial rigid landslide on the steep northeastern slope of Orcas Island, Washington State. Landslides here may be triggered by strong shaking during earthquakes on the Cascadia megathrust or the active Skipjack Island fault zone, near the southern boundary of the Canadian Fraser River delta. Using new high-resolution bathymetric and topographic data, we constrain a worst-case scenario “rigid” subaerial failure along NE Orcas Island. We calculate the initial computation of the landslide volume using the GIS software ESRI ArcGIS 10.7 (0.17 cubic kilometers). A three-dimensional non-hydrostatic model (NHWAVE) is used in conjunction with a two-dimensional completely nonlinear and dispersive model (FUNWAVE-TVD) to simulate the subaerial rigid landslide and tsunami generation and propagation. NHWAVE solves the Navier-Stokes equations associated with the complex behavior of the landslide source, and FUNWAVE-TVD solves high-order Boussinesq-type wave propagation equations. The simulated failure travels downslope at a peak velocity of 13.64 m/s before coming to a stop after about 85 seconds due to the friction between the slide and the underlying seabed. The strongest waves and currents would be located on the coastlines adjacent to and opposite the failure zone. Wave amplitudes rapidly diminish with distance from the source location. Near the tsunami source, the wave amplitude is around 17 to 19 m, with a runup of up to 28 m. Inundation maps reveal particularly vulnerable locations on the northern and eastern sides of Orcas Island. The southern end of Boundary Bay would experience high waves and strong currents.

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