Sunday 28 October 2012

Magnitude 7.7 Earthquake hits coast of British Columbia; tsunami threatens Hawaii.

On Saturady 27 October, slightly before 8.05 pm, local time (slightly before 3.05 am on Sunday 28 October, GMT), The Uniteed States Geological Survey recorded a Magnitude 7.7 Earthquke at a depth of 17.5 km on Queen Charlotte Island, off the coast of British Columbia. This is a very large  quake with the potential to cause considerable damage, though the area is sparsely populated which helps to mitigate this; the USGS estimates there is a 32% chance of a quake this size in this area causing at least one fatality, though no damage or casualties have been reported at the time of writing, 4½ hours after the quake.

The location of the 27 October Earthquake, and the areas that recieved the worst shaking. Damage to buildings is likely within the green circle, and more likely within the yellow circles. USGS.

The quake has been followed by a number of aftershocks, including over 10 with magnitudes over 4.0 so far, adding to the danger of serious casualties. In addition the NOAA Pacific Tsunami Warning Center has issued a severe tsunami alert for Hawaii, where an evacuation of coastal areas is underway.

The Queen Charlotte Islands lie on the boundary between the Pacific and North American Plates. This is a subductive margin, with the Pacific Plate passing under the islands as it sinks into the Earth. This is not a smooth process, and the plates frequantly stick together, then break apart once the peressure has built up sufficiently, leading to Earthquakes.

Map showing the boundary between the Pacific and North American Plates alomg the coast of British Columbia. Irving et al. (2012).

Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up tp far and the overlying plate snaps back., causing an Earthquake and a tsunami.

Simplified graphic showing tsunami generation along a convergent margin. NASA/JPL/CalTech.