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Topic

Subsurface Indicators of Active Faulting in the Central Strait of Georgia, British Columbia and Implications for Hazard and Risk

School of Earth and Ocean Sciences

Date & location

• Wednesday, May 21, 2025
• 10:00 A.M.
• Clearihue Building, Room C112

Examining Committee

Supervisory Committee

• Dr. Lucinda Leonard, School of Earth and Ocean Sciences, Ó£»¨Ó°ÊÓ (Co-Supervisor)
• Dr. Andrew Schaeffer, School of Earth and Ocean Sciences, UVic (Co-Supervisor)
• Dr. Tiegan Hobbs, School of Earth and Ocean Sciences, UVic (Member)

External Examiner

• Dr. David Mosher, Commission on the Limits of the Continental Shelf, United Nations

Chair of Oral Examination

• Dr. Jeremy Wulff, Department of Chemistry, UVic

Abstract

This thesis focusses on active faulting beneath the central Strait of Georgia, located between Metro Vancouver, Nanaimo, and British Columbia’s Sunshine Coast. Using ∼2200 km of seismic reflection data, a systematic approach is applied to interpret and catalogue subsurface evidence of active faulting across the study area. Fault interpretation is based on eight objective criteria: reflector offsets, reflector discontinuities, abrupt lateral changes in seismic unit, reflector truncations, associated folding and deformation, abrupt changes in dip, fault shadow, and fault plane reflectors. Some criteria appear highly localized and unlikely to delineate structures, whereas others exhibit consistency and lateral continuity across multiple seismic reflection profiles and along linear trends. This analysis enables the delineation of the Central Salish Sea fault zone (CSSFZ; referred to previously as the Fraser Delta fault) beyond its surface expression to a length of 12 km, with a possible extension up to 25 km. Subsurface evidence of active faulting is also present beneath a seafloor scarp offshore Bowen Island and a seafloor lineament near Gabriola Island.

Given CSSFZ’s proximity to densely populated areas of British Columbia, deterministic seismic hazard and risk modelling is conducted herein for various rupture scenarios. The CSSFZ’s main strand is modelled with an average dip of ∼75° southwest and strike of ∼123°. Based on fault orientation and the orientation of local maximum horizontal compressive stress (SHmax), the fault is presumed to accommodate oblique right-lateral slip with a reverse component, although scenarios ranging from pure right-lateral to pure reverse slip are considered. The most impactful modelled scenario – a magnitude 6.7 daytime oblique or reverse rupture – is projected to result in 1,300 deaths, 5,600 uninhabitable buildings, and $18.5 billion CAD (2019) in economic losses across southwest British Columbia due to ground shaking and building damage alone. These hazard and risk results establish a baseline assessment for a fault rupture offshore of Metro Vancouver; secondary hazards (e.g., aftershocks, liquefaction, fires, slope failures) and damage to critical infrastructure could result in further damage and casualties. In addition, the proximity of the CSSFZ to the Fraser River delta, an area prone to submarine slope failure, along with its potential for rightlateral oblique slip, suggests that a rupture may be tsunamigenic. Empirical relations indicate that seafloor displacement during a magnitude 6.7 rupture could generate damaging tsunami runups, with potential impacts for several coastal communities around the central Strait of Georgia. The results in this thesis have demonstrated hazard and risk implications for southwest British Columbia. It is therefore recommended that the CSSFZ be incorporated into future seismic and tsunami hazard and risk assessments.