This one had a magnitude of 7 but its activity was unrelated to the sub ducting plate. Earthquakes are common in this area and fortunately this one was intra-plate and, while felt over a very large area, caused no casualties and did minor damage to buildings.
This earthquake of magnitude 7 occurred on Friday, December 6, 1918. Its epicenter was near the west coast of Vancouver Island at a depth of ten miles, and was felt very strongly at Estevan Point Lighthouse and at Nootka Lighthouse on the southern tip of Nootka Island. There was some damage done to the Estevan Point Lighthouse and to a wharf at Ucluelet. This earthquake awakened people all over Vancouver Island and in the greater Vancouver area.
It was felt in northern Washington State and at Kelowna, in the interior of British Columbia. Due to sparse population, no injuries and little damage resulted from this earthquake. Other than broken dishes and some instances of cracked plaster on central Vancouver Island, the only damage reported was to the Estevan Point lighthouse. Its steel reinforced concrete tower cracked for its full length in several places, and parts of the glass lens were smashed.
There is an average of one earthquake in southwest British Columbia every day. Nearly all are too small to be felt, but a damaging earthquake occurs somewhere in the region about once every twenty years. The largest earthquake in this century had a magnitude of 7.3. It hit in 1946 and was centered beneath Vancouver Island. Were this earthquake to occur today under Vancouver, damage would be in the billions of dollars.
Recently, scientists have recognized a history of infrequent, but great earthquakes on the fault separating the subducting Juan de Fuca and North America plates. The last great earthquake in 1700 AD affected the entire coast from northern California to southern British Columbia. The 1918 earthquake appears to be a crustal intraplate event occurring in the lithosphere of the America Plate, resulting from the complicated interaction of the Explorer, Juan de Fuca, and America plates.
The major tectonic boundary separating the Pacific and North American lithospheric plates lies near the west coast of North America. The San Andreas and Queen Charlotte transform faults dominate this boundary. However, in the region between northern Vancouver Island and northern California, the oceanic Juan de Fuca and Explorer plates lie between the major plates and are being subducted beneath North America.
These two smaller plates act independently of the two bigger ones, the Juan de Fuca moving under the North American Plate at approximately an inch and a half a year. While major concerns over earthquakes in this region tend to focus on subduction motions, it is important to note that this 1918 one was not of this type. Instead it was a crustal, intraplate earthquake. Rather than being directly associated with the subducting Nootka fault zone, it was linked to the stress in the continental crust as the North American Plate interacts with the Explorer and Juan de Fuca plates.