Earthquake Swarm Off the Coast of British Columbia, Canada
The sequence of earthquakes that has been occurring off the coast of British Columbia since August 25 2008, represents a seismic phenomenon known as a seismic "swarm"-- an episode of high earthquake activity in which the largest earthquake does not occur at the beginning of the episode and in which the largest earthquake is not substantially larger than other earthquakes of the episode. The swarm includes more than 100 earthquakes, over 25 of which have magnitudes of 4 or larger.
The swarm is occurring along the boundary that separates the Pacific plate from a small block of the earth's lithosphere that has been termed the Explorer plate and that is itself bordered on the east by the North American plate. The broad-scale tectonics of the region reflect the motion of the Pacific plate to the north-northwest with respect to the North American plate at a velocity of about 4.5 cm/yr. Seismic swarms are a typical mode of seismic activity along the boundary between the Pacific plate and the Explorer plate.
Additional information about this series and other Canadian earthquakes can be found on the Earthquakes Canada website of Natural Resources Canada.