ASSESSING THE NEAR-FIELD SUBDUCTION ZONE TSUNAMI HAZARD OF COASTAL BRITISH COLUMBIA, CANADA

Coastal British Columbia has the highest tsunami hazard in Canada due to its location relative to a multitude of near- and far-field subduction zones. Great megathrust earthquakes on the Cascadia subduction zone are the most significant source of large tsunamis likely to impact a large proportion of the Canadian Pacific coastline. The Explorer plate segment of the Cascadia subduction zone may rupture with the Juan de Fuca plate segment to the south, or may rupture independently. Megathrust earthquakes associated with subduction of the Pacific plate beneath Haida Gwaii are also a potentially dangerous near-field source, as demonstrated by the 2012 M_w7.8 earthquake that produced up to 13 m of tsunami run-up on the west coast of the islands. There was little damage due to the current lack of population and infrastructure in the affected area. The Winona microplate region off northern Vancouver Island may also have tsunamigenic potential. Earthquakes on submarine faults within the crust of the North America plate are subduction zone related sources that present a lower level of tsunami hazard (longer recurrence interval and smaller magnitudes), but they may occur in regions with significant population and infrastructure at risk. An example is the Devil’s Mountain fault in Juan de Fuca Strait, a region of the Cascadia forearc undergoing compressional crustal deformation. Waves caused by earthquake-triggered landslides may provide the largest hazard in some areas, but this is difficult to quantify.

We describe methods that have been used to assess and quantify tsunami hazard on the Pacific coast of Canada from near-field subduction zone related earthquake sources, and discuss the limitations of these preliminary assessments. We outline the types of data and modelling studies that are critically required to provide improved probabilistic tsunami hazard analyses in coastal British Columbia. These include: (1) probabilistic modelling of a wide range of scenario fault ruptures, which requires improved geodetic, structural, and thermal constraints, as well as collection of high-resolution bathymetry and onshore topography data for adequate tsunami propagation and inundation modelling; (2) paleoseismic data to provide constraints on the size and frequency of past events.