Minutes-to-hours: arrival times for tsunami on BC coast
New hazard assessment combines ocean science & Indigenous knowledge
May 8, 2023

Ocean Networks Canada (ONC) is developing a coastal hazard assessment framework that utilises a two-eyed seeing approach, interweaving Indigenous knowledge with its tsunami and flood hazard modelling services. Results from these assessments–which include the estimated arrival times and flooding extent of earthquake-induced tsunamis on the British Columbia (BC) coast–are being used to support emergency planning for coastal communities.

BC communities are particularly vulnerable to tsunamis because of their proximity to the seismically active Alaska-Aleutian and the Cascadia subduction zones, which are capable of producing megathrust earthquakes.

In two articles (accessible here, and here) published in Oceanography Magazine this year, ONC researchers discussed results from a series of collaborative tsunami hazard assessments (see IEEE publication) undertaken along the west coast of Canada.

FIGURE 1. Study areas along BC coastline where tsunami hazard assessment was performed by ONC. Earthquake data are shown as seafloor vertical displacement (m) for the Alaska-Aleutian and Cascadia subduction zones. Blue indicates subsidence and red indicates uplift of the topography.

Tsunami hazard results

In the most recent study, ONC was engaged by the Strathcona Regional District (working in partnership with Ka:’yu:’k’t’h’/Che:k:tles7et’h’ First Nations and Nuchatlaht First Nation), to undertake the Northwest Vancouver Island Tsunami Risk Assessment Project, alongside Northwest Hydraulic Consultants, and Northwest Seismic Consultants.

Through a combination of modelling, mapping, community engagement, and Indigenous knowledge, the researchers identified the expected height (amplitudes), current speed and direction (velocities), inundation extents, and arrival times of tsunami waves on the BC coast and the areas that would be hardest hit; findings that are backed up by historical records of past major events.

Tsunami waves generated by a magnitude 9.0 (M9) earthquake in the Cascadia subduction zone would reach the outer coast of northwest Vancouver Island in about 20 minutes. In this scenario, the tsunami would significantly impact local communities such as Kyuquot, with an estimated wave height of 5.9 metres reaching the shore in less than 30 minutes, followed by runup on the land that may exceed 12 metres of elevation.

A separate ONC study found that waves from a M9 Cascadia earthquake would travel into the Strait of Georgia and reach Boundary Bay (south of Vancouver) in three hours, with offshore waves of 1-2 metres tall.

The Alaska-Aleutian subduction zone is of interest due to the 1964 Alaska earthquake resulting in record tsunami waves hitting the BC coast. A tsunami triggered by a megathrust earthquake from this source would have significant impact in Prince Rupert and Haida Gwaii, with respective arrival times of 2.5 and 3.5 hours.

The tsunami current speeds will exceed 1.5 metres per second in several regions, which will be hazardous for mariners and boaters, in particular in shallow water regions and narrow passages in the case of both Cascadia and Alaska tsunami.

Impact of sea level rise

Current-day sea level and projected sea-level rise were modelled in the study to increase the accuracy of projected tsunami impacts. Results suggest that sea level rise would not significantly influence the amplitude of the waves but would intensify wave runup over land, leading to greater extents of flooding and damage.

Several overwater hazard graphics (Figure 2) and inundation maps (Figure 3) were created to represent the tsunami hazard information in each study area, including the tsunami wave amplitudes, current velocities, tsunami arrival times, and extent of flooding.

FIGURE 2. Maximum wave height for a Cascadia-generated tsunami in Esperanza Inlet and Nootka Sound as determined by the Northwest Vancouver Island Tsunami Risk Assessment Study. Colours show wave height from 0 metres (white) to above 7 metres (blue).

FIGURE 3. (Left) Inundation extent map for a Cascadia tsunami in Tahsis, BC, produced by the Northwest Hydraulic Consultant Ltd. in collaboration with ONC. (Right) Satellite image of Tahsis, BC, showing existing shoreline without inundation extent.

The science and ocean knowledge behind hazard assessments

The researchers used layers of Digital Elevation Models (DEMs) - which are 3-D representations of Earth’s surfaces - to identify areas that could be impacted by flooding under different scenarios. DEMs require large volumes of water depth and elevation data that are collected, checked for potential errors, merged, and converted for processing. As data can be sparse for remote communities, several additional Light Detection And Ranging (LiDAR) surveys were done to fill in the gaps.

The hazard zones identified by the modelling were then authenticated by those with lived experience, as well as intergenerational knowledge, of past tsunamis. The study site encompassed several First Nations territories including the Ka:’yu:’k’t’h’/Che:k:tles7et’h’ First Nations, Nuchatlaht First Nation, Ehattesaht/Chinehkint First Nations, Quatsino First Nations, and Mowachaht/Muchalaht First Nations.

Elders and community members from each nation shared stories, teachings, and oral histories of past tsunami events such as the 1964 and 1700 tsunamis in a series of interviews conducted by ONC Indigenous Engagement Research Associate, Jaquelynne LaFlamme, and Indigenous Community Liaison, Pieter Romer.

“These ‘boots on the ground’ tsunami experiences provide insights into the location, extent of damage, and evacuation routes for past tsunamis, and the adaptation measures that exist today as a result,” says Romer.

Soroush Kouhi, ONC Applied Science Specialist and co-author of the reports, says interweaving knowledge from coastal communities with scientific modelling offers significant insights for coastal safety and survival in the event of a megathrust earthquake.

“Knowing where coastal vulnerabilities lie and the predicted areas at risk strengthens hazard awareness, allowing decision-makers and communities to mitigate, prepare, respond, and recover from emergencies.”

FIGURE 4. ONC applied science specialist Soroush Kouhi on a visit to Tahsis, BC, Canada in September 2022 to learn about the study site geography and existing tsunami signs.

FIGURE 5. The study sites of the project includes territories of the Ka:’yu:’k’t’h’/Che:k:tles7et’h’, Nuchatlaht, Ehattesaht/Chinehkint, Quatsino, and Mowachaht/Muchalaht First Nations. Graphic courtesy of Northwest Hydraulic Consultants Ltd.

Building resiliency

ONC’s Learning and Engagement team has developed public education tools to raise tsunami risk awareness. To date, students in Kyuquot, Goldriver, Coal Harbour, Port Alice, Tahsis, and Zeballos have received tailored tsunami awareness programs delivered by the team.

This spring also saw the release of “Tsunami 11th Relative”, a documentary film produced and directed by Pieter Romer. The tour runs until June 2023.

Several tsunami signage illustrating evacuation routes, assembly locations, and inundation maps are under development, with installations planned for summer 2023. This collaborative research has also culminated into a report that authenticated and tailored emergency planning recommendations to fit the context of each affected community in Northwest Vancouver Island.

Access the Oceanography articles here:

Full model results can be found here: Assessment of tsunami coastal hazard along the Northwest Vancouver Island from coseismic sources in Cascadia and Alaska subduction zones

Full results from Phase 1 of the Northwest Vancouver Island Tsunami Risk Assessment Project can be accessed on Strathcona Regional District’s website and Phase 2 results will be released later this summer.

ONC’s coastal hazard assessment and mapping services are part of a suite of services for improving community safety, that include earthquake early warning and a pioneering geodesy project to monitor tectonic movement offshore along the boundary between the Juan de Fuca and North American plates.

Read more about ONC's earthquake early warning system.

For more information contact: Soroush Kouhi; or ONC communications:

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