People living on the west coast of Vancouver Island would have about twenty minutes before a tsunami hit, if there were a magnitude 9.0 quake in the Cascadia Subduction zone, researchers found.
Waves would be between one and seven metres in Esperanza Inlet and Nootka Sound after such an event, with the maximum wave amplitude in Tahsis, B.C. reaching three to four metres. Much of the low-lying areas of the village would be affected. Zeballos would also see increased wave amplitude in the case of a Cascadia quake, though that would be between two and three metres.
The research was conducted by Ocean Networks Canada, which has undertaken several tsunami hazard assessment projects on the west coast. Through modelling and drawing on Indigenous knowledge as well as accounting for various potential sea level rise scenarios, the researchers were able to determine the warning time and height of the waves for the western portion of the Island.
Using a network of nested grids, researchers laid out how a tsunami would travel through deep water. As the model approaches the coast, the resolution increased, giving the researchers more specific detail. That combined with existing digital elevation data for the seabed gave them a good description of the complex physics of tsunami propagation. From there, modelled sea level rise from previous research was included to account for changes over the next few decades.
“The impact of the locally generated tsunami is more significant in Boundary Bay, Port Alberni, and Northwest Vancouver Island, while a tsunami originating in Alaska would have greater impacts in Prince Rupert and Haida Gwaii,” the paper says.
Tsunami arrival times for the outer coast of Vancouver Island and Boundary Bay would be 20 minutes and three hours respectively for a Cascadia event. An Alaskan tsunami would take 2.5 and 3.5 hours to reach Haida Gwaii and Prince Rupert, respectively.
Indigenous Peoples have been living in tsunami risk areas for millennia, and have valuable knowledge and experience. Researchers highlighted Indigenous experiences as “critical for understanding current and future risks and implementing mitigation plans.”
Elders’ stories were gathered and used to authenticate modelling results. Stories, teachings and oral histories of tsunamis including that of 1700, and 1964 were collected, and used in a “two-eyed seeing” framework. The framework looks at how”different ways of knowing are used simultaneously for the benefit of all, helps to improve the science by learning from community stories,” the researchers write.
The modelling looked at a tsunami caused by an earthquake out at sea. However, Tahsis and other communities are also vulnerable to localized causes. These can be landslides into inlets and smaller quakes nearby. In February, the Strathcona Regional District board learned that a planned Natural Resource Canada Earthquake Early Warning network would not predict localized tsunamis. The SRD has been undergoing a public education campaign about tsunami risk and response, as well as setting up tsunami warning sirens for communities like Tahsis, Zeballos, Fair Harbour, Gold River and Walter’s Island.