When the earth whispers …
... seismologists listen
By Joe McKendy
January 3, 2018
On the day that Simply Science caught up with seismologist Dr. John Cassidy in Victoria, there was a small earthquake in that city.
That was, of course, a coincidence, but not a big one: Canada has over 4,000 earthquakes a year, roughly 11 each day. All but about 50 are too small to be noticed by anyone but a seismologist.
From paper to pixels
The reason John and his colleagues with Natural Resources Canada can detect even a quiver of motion is increasingly sophisticated computers, data processing and tools, such as GPS and modern seismometers.
“Digital data has made a huge difference in the way we do our science,” he says. “Digital monitoring was just becoming mainstream in the 1990s. Before that, most shaking was captured on paper.”
GPS technology produced by the digital revolution was essential to one particular discovery by Natural Resources Canada scientists that has improved our understanding of where big earthquakes are likely to strike.
Beginning in 1991, Herb Dragert and Garry Rogers deployed GPS units on Vancouver Island — one of the first permanent GPS networks in North America — to relay data continuously on the slightest surface movements.
Over several years, the two scientists discovered repeated episodes of slow fault slip, movements of several millimetres. These episodes, each lasting about two weeks, are accompanied by seismic tremors that cause intermittent ground vibrations as slight as those produced by a windstorm.
This process as a whole — known as episodic tremor and slip — originates deep below the surface along faults that form the boundaries of tectonic plates, the irregularly shaped masses of solid rock that form the earth’s crust and upper mantle. The plates can be thousands of kilometres in length and range from 15 to 200 kilometres thick.
John refers to the phenomenon as “a little tectonic dance” that moves the entirety of Vancouver Island about one centimetre toward the British Columbia coast over roughly 14 months, then several millimetres back in the opposite direction in about two weeks.
Local research, global impact
Seismologists use the data on episodic tremor and slip to map out the buildup of long-term stress points in large faults with the potential to cause major earthquakes. For example, in the case of the Cascadia subduction fault, which runs from northern Vancouver Island to northern California, they have determined that the closest part of the “locked portion” — where stored energy is certain to cause earthquakes — is only about 75 km from Victoria. By determining the precise location where energy is being stored, seismologists can estimate the ground shaking from future subduction earthquakes — important information for engineers designing buildings and infrastructure.
Today, around the world, the connection between slip tremors and big earthquakes gives seismologists vital clues about what could happen in their own regions, especially in active subduction zones like Canada’s west coast.
Types of earthquakes
The surface of the earth is always changing due to the slow deformation of the outer, brittle portions of tectonic plates. These plates are constantly moving relative to one another at speeds of about 2–10 cm each year, about the same rate as fingernails grow.
The plates can move in different ways: slide past one another, collide or pull apart. All three of these types of movement occur off the British Columbia coast.
During the largest earthquakes — like the magnitude 8–9 earthquakes in Japan, Chile, Sumatra and Mexico — plates suddenly move by 10 or 20 metres in seconds or minutes, thrusting one plate beneath the other in the process known as subduction.
Some earthquakes occur far beneath the surface of the earth, as much as 50 or even 100 km. These deep earthquakes occur within the ocean plate being pushed beneath southwest British Columbia. They cause less severe shaking than a shallow earthquake, not usually exceeding magnitude 7, with few if any aftershocks. A magnitude 6.8 earthquake about 50 km below the surface near Seattle in 2001 caused about $2 billion in damage.
Crustal earthquakes take place near the surface of the earth. Although typically less than magnitude 7, they can cause more damage to nearby communities than a larger but more distant subduction earthquake.
Types of shaking
Earthquakes can move in vertical or horizontal directions. Different strengths of earthquakes cause different types of shaking: a magnitude 2 earthquake of just two or three seconds of shaking can feel “like a single jolt,” says John, while more powerful earthquakes produce a rolling force that can buckle the ground in waves. The duration of a quake usually relates to its force. While the shaking for a magnitude 2 earthquake is only a few seconds, a magnitude 9 earthquake can produce several minutes of shaking.
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