Assessment of tsunami coastal hazard along the Northwest Vancouver Island from coseismic sources in Cascadia and Alaska subduction zones

Abstract

The distant and near-field tsunamis from the Pacific Ocean have frequently impacted the western coast of Canada over time. Local seismic sources including the Cascadia Subduction Zone and distant sources including the Aleutian Islands may impose destructive tsunamis on the west coast communities of Canada. Among those, Northwest Vancouver Island has a rich cultural history and strong first nations communities. Unlike populated areas around the Strait of Georgia and Salish Sea, less research has been carried out on the vulnerability of the Northwest Vancouver Island communities to tsunami hazards. There are several First Nations Territories within this region including Kyuquot and Nuchatlaht in addition to several villages including Gold River, Port Alice, Tahsis, and Zeballos. Understanding tsunami hazards is key to protecting these communities from the destruction of the tsunami.In this study, the Strathcona Regional District, British Columbia (BC), Canada, in partnership with the Ka:’yu:’k’t’h’/Che:k:tles7et’h’ First Nations and Nuchatlaht First Nation, engaged Northwest Hydraulic Consultants Ltd. (NHC), Ocean Networks Canada Society (ONC), and Northwest Seismic Consultants Ltd. to undertake a tsunami risk assessment in the region. We at ONC developed integrated topographic and bathymetric digital elevation models (DEM) of Northwest Vancouver Island and performed tsunami modelling to support inundation, vulnerability, and hazard mapping. We used a numerical modelling approach to estimate the tsunami hazard on the coastal communities of Northwest Vancouver Island. Tsunami wave propagation was simulated using the well-known FUNWAVE-TVD model that has been extensively used for tsunamis around the world.The results demonstrated that the tsunami waves generated by both near-and far-field earthquakes impact Northwest Vancouver Island communities, however, the impact of the nearfield tsunami is more significant. The most impacted areas are semi-enclosed bays and inlets due to the energy focusing in the narrow regions and wave shoaling in the shallow areas enhanced by possible resonances. For the Cascadia scenario, the tsunami wave amplitudes reach up to 6 m in the proximity of Kyuquot located on the open ocean side of Vancouver Island coastline and up to 4.5 m in the vicinity of the communities within the inlets. For the near-field source, the tsunami arrival times were estimated 25 minutes along the outer west coastline of Vancouver Island and SO minutes inside the inlets, while for the far-field source, the maximum tsunami wave will reach these areas in about 3 to 5 hours. Results of this study suggest that sea level rise does not significantly influence the tsunami wave amplitude at offshore locations, but the propagation of these waves overland will lead to greater extents of flooding as a result of sea level rise. The outputs of this study were presented in several hazard, inundation, and assets-at-risk maps by NHC, as well as provided recommendations on emergency management and risk reduction.