5.6.7: Southwest British Columbia
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Northern Vancouver Island just doesn’t seem like the Earthquake Country. The highway north of Victoria runs past small towns along the east coast of the island; it is lined with firs, with breathtaking views of the Georgia Strait, the Gulf Islands, and on a clear day, the snowy peaks of the Coast Mountains. The road passes through Courtenay to Campbell River, past fishing villages and logging camps. One branch of the road crosses the Forbidden Plateau and Strathcona Provincial Park on its way to a lonely, storm-swept fjord below Gold River, on the Pacific Ocean side of the island.
This thinly populated region was the location of the largest crustal earthquakes in the short recorded history of the Cascadia region, an event of M 7.0 on December 6, 1918 and a larger earthquake of M 7.3 on June 23, 1946 (located in Figure 6-19).
The 1946 earthquake produced extensive chimney damage in Campbell River, Courtenay, and Comox, and there were many landslides in the mountains and liquefaction and slumping of coastal sediment. Despite extensive areas of intensity VIII from Campbell River to Courtenay, only one person was killed when his boat at Deep Bay was swamped by a wave, possibly generated by slumping of sediment into the water.
The 1918 earthquake struck along the primitive west coast of Vancouver Island, damaging the lighthouse at Estevan Point, south of Nootka Sound. The area of highest intensity was thinly populated, with widely scattered fishing villages accessible only by boat, and the damage was slight. The focus of the earthquake was about ten miles deep, and intensities up to VI were recorded. It was felt as far away as Seattle and the town of Kelowna in the Okanagan Valley east of the Cascades.
The seismograms of both earthquakes, as recorded at distant stations, show that the motion was consistent with left-lateral strike-slip on a crustal fault (or faults) striking northeasterly. This is the same strike as the Nootka Fault, a major left-lateral strike-slip transform fault on the deep ocean floor west of the continental slope, a fault that forms the boundary between the Juan de Fuca Plate and the Explorer Plate (Figure 6-19). However, the earthquakes are not located directly on the landward projection of the Nootka Fault but are offset about forty miles to the east.
The more heavily populated regions of Vancouver and Victoria experience quite a few small earthquakes, indicating that the region is a northern continuation of the seismically active crust beneath Puget Sound. This poses a dilemma for seismologists such as Garry Rogers of the Pacific Geoscience Centre in Sidney, B.C., concerned about estimates of seismic hazards in these areas. Should Rogers and his colleagues consider that earthquakes as large as the 1946 event, M 7.3, are possible in Vancouver or Victoria, or anywhere else in the shallow continental crust of southwestern British Columbia? Or should they conclude that the large crustal earthquakes in central Vancouver Island are part of a zone that has an unusually high seismic hazard because of its proximity to the offshore Nootka Fault, thereby reducing the perception of hazard to Vancouver and Victoria? The answers to those questions are not yet at hand.