Earthquake of M6.4 Strikes off Vancouver Island

Location of the September 2011 earthquake (image from USGS)

Location of the September 2011 earthquake: Image courtesy USGS

Western Canada was struck by a large earthquake (recorded by the United States Geological Survey as measuring M6.4 on the Richter Scale) on Friday 9 September 2011. Earthquakes in this area, and of this scale, are unusual though by no means unheard of, and inevitably attract considerable interest in the news media.

The Vancouver Earthquake of 2011

The earthquake struck offshore on the Pacific coast of Canada, its epicentre just off Vancouver Island and some 180 miles up the coast from the capital, Victoria. It took place at a relatively shallow depth of just 23km. Despite the magnitude of the tremor and its submarine location, no tsunami was generated and no warnings issued by the NOAA’s Tsunami Warning Centre.

Although earthquakes of this magnitude can prove damaging, the Vancouver earthquake caused no reported injuries or damage, though it was felt widely within the region. One local resident living on the opposite side of Vancouver Island told Decoded Science that the earthquake felt like a dizzy spell ‘but when I noticed my desk swaying back and forth I knew it wasn’t just me’ (10 September 2011).

The Tectonic Setting of the Vancouver Earthquake

Vancouver Island’s location on the western seaboard of North America places it firmly in the seismically active belt known as the Pacific Ring of Fire, which broadly surrounds the Pacific Ocean. This zone of earthquake and volcanic activity is generated by the movement of the Pacific Plate, one of the tectonic plates which make up the earth’s surface, relative to the surrounding plates.

For much of the western coast of North America, the boundary is a transform boundary, with the Pacific and North American plates sliding laterally past one another (Natural Resources Canada). To the north the boundary is the Queen Charlotte Fault and to the south it is the notorious San Andreas Fault.

Vancouver Island (Image by GLetham)

Vancouver Island: Image by GLetham

Although the Vancouver earthquake occurred in this zone it took place not at the boundary between the Pacific and North American plates, but at the Cascadia Subduction Zone. Here, a smaller plate called the Juan de Fuca plate, which is thought to be the remnant of a much larger tectonic plate (known as the Farallon plate) is being forced beneath continental North America (Open University).

Historic Seismicity in the Region

As might be expected from such a seismically active region, major earthquakes are not uncommon in the Vancouver area. A look at the Natural Resources Canada earthquake listings for 2011 shows that British Columbia experienced six earthquakes of greater than M4.0  in 2011. A smaller earthquake (M3.8) occurred close to the epicentre of the September earthquake in March 2011.

The most significant earthquake to strike Canada occurred along the Cascadia Fault Zone in 1700, and has been assigned a magnitude of 9.0, making it one of the largest to occur in recorded history. This was by no means, however, a unique event locally: Natural Resources Canada notes that ‘geological evidence indicates that 13 great earthquakes have occurred in the last 6000 years’.

Sources

Bell, A. How Plate Tectonics Works. Open University. (1997).

Natural Resources Canada. Seismic Zones in Western Canada.  Accessed 10 September, 2011.

Natural Resources Canada. The M9 Cascadia Megathrust Earthquake of January 26, 1700. Accessed 10 September 2011.

United States Geological Survey. Magnitude 6.4 – Vancouver Island Canada Region. Accessed 10 September 2011.

© Copyright 2011 Jennifer Young, All rights Reserved. Written For: Decoded Science
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