Tsunamigenic potential of crustal faults in the southern Strait of Georgia and Boundary Bay

Abstract

In this thesis, I constrain rupture scenarios of active crustal faults in the southern Strait of Georgia and Boundary Bay in order to assess their tsunamigenic potential. The NW-SE-trending Drayton Harbor, Birch Bay, and Sandy Point faults had been previously identified on the southern side of Boundary Bay from aeromagnetic, LiDAR, and paleoseismic data; all show evidence of abrupt vertical Holocene displacements. South of Boundary Bay, the E-W-trending Skipjack Island fault zone was recently mapped on the basis of multibeam sonar imagery and seismic reflection data, with evidence for Holocene offsets of the seafloor and subsurface sediments. In addition, the Fraser River Delta fault had been hypothesized on the basis of a line of pockmarks and fluid seeps. Since these faults have only been recently mapped and identified as active, there is little information available on their structure, rupture style, and past large earthquakes. This makes it difficult to constrain rupture models to predict how fault slip could displace the seafloor during a large earthquake, for input to tsunami models.

I analyzed relocated earthquake hypocentres, earthquake mechanisms, bathymetry, topography, and aeromagnetic, seismic reflection, and magnetotelluric data, to constrain the location, strike, dip, and rupture width of each fault. Correlations between datasets enabled mapping of northwestward extensions of the Sandy Point and Birch Bay faults, as well as delineating the previously unmapped Fraser River Delta fault. These offshore faults appear to be associated with infilled basement valleys in the subsurface, perhaps due to differential glacial erosion of weakened fault zone material. The Drayton Harbor fault could not be definitively mapped across Boundary Bay, so was excluded from the rupture modelling. Rupture styles were constrained using a combination of earthquake mechanisms, stress orientations, other evidence of regional compression, and vertical paleoseismic offsets. Where possible, paleoseismic displacements in past earthquakes were used to constrain the amount of fault slip for scenario earthquakes; empirical relations between fault slip and fault length or area were used to estimate displacements for the Skipjack Island and Fraser River Delta faults.

The Birch Bay, Sandy Point, Skipjack Island, and Fraser River Delta faults all pose a significant tsunami risk to communities surrounding the southern Strait of Georgia and Boundary Bay. Considering both the originally mapped and extended lengths, the Birch Bay and Sandy Point faults could rupture in reverse-faulting earthquakes up to Mw 6.7-7.4 and 6.8-7.5, respectively, with seafloor uplift up to 2-2.5 m triggering damaging tsunami waves (up to at least 2.5 m) that could arrive onshore with little to no warning after the shaking begins. Similarly, the Fraser River Delta fault could host reverse or dextral-reverse slip earthquakes up to Mw 7.0-7.6, with seafloor uplift of 0.6-3.5 m. Ruptures on the Skipjack Island fault would likely have a larger strike-slip component; earthquakes of Mw 6.9-7.3 produce modelled seafloor uplift of 0.5-1.9 m. These results suggest that large tsunamigenic earthquakes on crustal faults in the southern Strait of Georgia should be included in future seismic and tsunami hazard assessments on both sides of the international border.