Vancouver Island ‘slow earthquakes’ due to fluid in rock
TORONTO – For the people of Vancouver, the earth is slowly sliding below them – and they don’t even know it.
Slow earthquakes – earthquakes that take place from anywhere between 10 to 14 days – were only just discovered about 12 years ago, here at home in Vancouver. But since then, scientists like Pascal Audet from the University of Ottawa have sought to better understand them.
Audet, a geophysics professor at the University of Ottawa, as well as researchers from the University of California, Berkeley, released a study in the scientific journal Nature on Wednesday with their recent findings about these peculiar earthquakes.
Regular earthquakes happen when two of Earth’s tectonic plates slide against one another. They can also occur when one plate slips beneath another. These earthquakes occur in a subduction zone, where an oceanic plate dives beneath a continent. Vancouver Island rests on the Cascadian subduction zone that runs from Vancouver to northern California.
The type of earthquakes that we feel may move several centimetres over a few minutes, whereas a slow earthquake may move just one centimetre a day, Audet told Global News. If the same fault moved as much as a regular earthquake, it would be equivalent to a magnitude seven earthquake.
Using sensitive seismographs as well as GPS, researchers monitored these regular slow earthquakes. Their study found that, in these slow earthquakes, when the plate slips down, it heats up, and releases a fluid. It’s the behaviour of that fluid that may be responsible for the slow motion and regularity of the slow earthquakes.
The water accumulates and is released when earth’s crusts are heated (it gets hotter the further down you go).
“Essentially, the water is accumulating because it’s constantly being created by this cooking of the oceanic plate. But this water also controls the stability of the fault. So once there’s a certain amount of water and pressure from the water, there will be a slow earthquake,” said Audet.
The water moves up towards the crust. There, the minerals that were dissolved in the water separate and settle.
The water contains silica, and when that dissolves it migrates up and moves into quartz veins. The more silica there is in the fluid, the more frequent these slow earthquakes are.
The researchers believe that these earthquakes depend on the amount of fluid and how the fault heals.
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What makes these slow earthquakes even more interesting is that they occur with regularity: in Vancouver once every 14 months.
These slow earthquakes happen around the world with their own type of regularity: in Costa Rica, southern Japan, and New Zealand. Each one has its own regularity. In Costa Rica it happens once every 15 months; in Japan once every six months; and New Zealand once every two years.
“We’re not making any new predictions on earthquakes here. We’re just trying to understand how faults work in a subduction zone,” Audet said. “We’re providing evidence that it has to be due to the presence and the role of the fluids in regulating these slow earthquakes.”
The last slow earthquake in Vancouver occurred between Sept. 7 to Oct. 8, 2013.
“It’s a very interesting time to be measuring these things and understanding what they mean,” Audet said.
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