A Tectonic Grinding Machine
Off the coast of British Columbia's Haida Gwaii archipelago lies a colossal fracture in the Earth's crust: the Queen Charlotte Fault. This 900-kilometer-long system is the Canadian counterpart to California's San Andreas Fault, the active boundary where the Pacific Plate grinds north-northwest past the North American Plate. The motion is relentless and rapid. The plates slide past each other at a rate of about 50 to 57 millimeters per year, making it one of the fastest-moving continent-ocean strike-slip faults on the planet.
This structure is a right-lateral strike-slip fault, meaning if you could stand on the North American Plate and look west, the Pacific Plate would be moving to your right. The fault extends from a complex junction with the Cascadia subduction zone off northern Vancouver Island, northward along the coast until it becomes the Fairweather Fault in Alaska. Together, they form the massive Queen Charlotte-Fairweather Fault system. While it is primarily a transform boundary, the plate motion is not perfectly parallel. A component of convergence—the plates pushing into each other—exists, particularly in the southern section near Haida Gwaii. This "transpressional" force causes crustal thickening and has recently been confirmed to be creating a new zone of subduction, where the Pacific Plate is beginning to dive under North America.
A History of Violent Shakes
The immense strain built up by this rapid plate motion is released in large, and sometimes great, earthquakes. The Queen Charlotte Fault is the most active seismic zone in Canada. Its history includes seven earthquakes of magnitude 7 or greater in the last century alone.
The most powerful of these was a magnitude 8.1 earthquake on August 22, 1949. This remains Canada's largest instrumentally recorded earthquake. The rupture extended for nearly 500 kilometers along the fault, and the shaking was so intense on Haida Gwaii that it knocked people and cattle off their feet. In the mainland town of Terrace, cars were seen bouncing, and the motion was described as being "on the heaving deck of a ship at sea".
More recently, on October 27, 2012, a magnitude 7.8 earthquake struck the region. This event was not a classic strike-slip rupture on the main fault. Instead, it was a thrust earthquake on a nearby, related fault, caused by the compressional stress between the plates. This quake generated a significant tsunami, with waves reaching up to 13 meters in some inlets on Haida Gwaii. It also caused the temporary disappearance of the hot springs on Hotspring Island in Gwaii Haanas National Park. The event showed the complex and varied seismic hazards posed by this dynamic plate boundary.