New quake theory proposed by Calvert
April 29, 2004,
Vol. 30, no. 1
By Carol Thorbes
Andrew Calvert, an associate professor of earth sciences at Simon Fraser University and an expert on seismic activity, has published a study in the scientific journal Nature. (See March 26 www.nature.com).
Calvert has connected the occurrence of hundreds of earthquakes to the grinding of overlapping rocks trapped between two of the tectonic (structural) plates that form the surface of the Earth.
A number of quakes, known as in-slab earthquakes, have measured as high as seven on the open-ended Richter scale, a scale for representing an earthquake's strength.
They are not as powerful as the much feared megathrust earthquakes, which can measure up to 9. But in-slab earthquakes worry engineers, city planners and emergency officials because they can do structural damage and are typically much closer to populated areas than megathrust earthquakes.
The earthquakes that Calvert studies have occurred beneath Vancouver Island's and Puget Sound's populated coastlines. Megathrusts usually occur far out to sea along a shifting faultline between two plates of the earth's surface.
“The closer you are to an earthquake, the more damage it is likely to cause,” says Calvert. “So if you're sitting directly above an in-slab earthquake, it's going to be more significant in terms of damage done than if you're 200 kilometres away.”
In-slab earthquakes occur deep below the earth's surface, approximately every 20 years.
Calvert's proposed cause of these quakes could make it easier for scientists to predict their maximum magnitude and potential damage in a given area.
Calvert has discovered that a layer of rock, trapped between two tectonic plates 20 to 40 kilometres below southern Vancouver Island, is responsible for the quakes.
The slippage of one of the plates (Juan de Fuca) beneath the trapped layer of rock is causing the plate to bend. In-slab earthquakes occur where the plate bends the most.
“Up until now scientists thought that geo-chemical reactions that released water in the rock forming Juan de Fuca plate caused these earthquakes in the plate,” explains Calvert.
“This new theory encourages us to focus on mapping faults and bends in the plates forming the Earth's subsurface to assess the magnitude and potential damage from future earthquakes.”
Scientists rely on controlled seismic surveys to image the Earth at great depths, and three such surveys formed the basis of Calvert's research.
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