Along the Pacific Coast, from southern British Columbia to northern California, three small tectonic plates lie between the Pacific and North American Plates. The Juan de Fuca Plate is the largest, stretching from the center of Vancouver Island to the southern border of Oregon, while the Explorer Plate sits north of it and the Gorda Plate lies off northern California; all three, like the Cocos Plate off Central America and the Nazca Plate along the west coast of South America, are remnants of the oceanic Farallon Plate, most of which has since subducted beneath the North and South American Plates.
On the west side of the three plates, they are spreading from the Pacific Plate along a broken oceanic ridge while, on their east side, they are subducting beneath the North American Plate. Between the spreading zones, each plate also has a transform-fault margin with the Pacific Plate, where the plates are scraping against one another; in addition, horizontal (east-west) faults form the boundaries between the Juan de Fuca Plate and its smaller neighbors. In this complex tectonic region, earthquakes may thus be generated at the subduction zones, along the transform faults or at the faults that separate the three plates.
Yesterday's 6.4 magnitude quake occured off the west coast of Vancouver Island, along the subduction zone and very near the margin between the Explorer and Juan de Fuca Plates. Rather shallow (14 miles deep), it was felt as far away as northern Washington but, fortunately, did not produce significant damage. Of course, there is always the potential for strong quakes in this region and those that occur along the subduction zone could produce a catastrophic tsunami. Hopefully, regularly occuring weak quakes might diminish the risk for such an event.
