Published: June 18, 2014
VANCOUVER — The “slow” quakes emanate from the deep like clockwork every 14 months on Canada’s west coast.
They typically release the energy of a Magnitude 7 earthquake, but the powerful tectonic events are almost imperceptible because they occur slowly over two weeks, instead of in sudden jolts that last just seconds.
“They would represent a pretty big earthquake if they happened like a regular quake, but because they are so slow you are never able to feel anything,” says Pascal Audet, a seismologist at the University of Ottawa, and co-author of a report released Wednesday on the subtle, silent quakes.
The “slow” or “slow-slip” quakes are an intriguing but poorly understood feature of subduction zones like the one along the west coast where a giant oceanic plate dives under the continent.
While more famous for massive and often deadly earthquakes, many of the worlds’ subduction zones also frequently experience the slow subtle variety, says Audet.
There is a slow quake every 14 months under Vancouver Island, he says, while New Zealand sees one every two years and southern Japan has one every six months.
“The frequency varies from place to place,” says Audet, who has been poring over seismic data from quake zones around the world with Roland Burgmann at the University of California Berkeley.
They report in the journal Nature that the slow quakes appear to be the result of geological forces that “lubricate” deep sections of the subduction zones.
As oceanic plates are shoved beneath the continents pressure and temperatures rise, releasing fluids stored in the descending slabs of rock, Audet said in an interview.
The fluids, rich in silica, that are released allow the giant plates to slowly slip past each other — a process that typically takes 10 days to two weeks — before the movement stops. Then the cycle begins again, building towards the next slow quake.
He says the telltale evidence of the process is evident in veins of silica-rich quartz seen the rocks above the subduction zones. The more quartz, the more frequent the slow earthquakes, Audet says.
He says the slow quakes, which occur more frequently when the descending crust is younger and contains more fluid, occur 10 to 20 kilometres deeper along subduction zones than the sections that rupture unleashing damaging quakes.
The plates that dive past each other along the west coast along the Cascadia subduction zone have historically generated a massive “megathrust” quake about every 500 years. The last one struck in 1700, and geologists say pressure is slowly building towards the next “big one” that could rock the west coast from Vancouver Island down to northern California.
Slow quakes may be much smaller players but Audet says each one adds a little more pressure and strain to the section of the fault associated with bigger quakes. “You actually bring it a little closer to failure every time there is a slow earthquake,” he says.
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