You can’t feel it, but the ground far beneath our feet on south-central Vancouver Island has been moving.
Seismologists have been recording thousands of “episodic tremor and slip events” occurring at a rapid rate since March 20 moving in a southeasterly direction down the Island.
More than 4,200 tiny tremors have been recorded so far between 25 and 40 kilometres below the surface, said John Cassidy, an earthquake seismologist with Natural Resources Canada and a professor at the University of Victoria.
“They’re not earthquakes. … On the instruments they look like series of a trains rolling by,” Cassidy said.
He said these bursts of small tremors and slips occur about every 15 months along faults that form the boundaries of tectonic plates — in our case, the Cascadia subduction zone from Brooks Peninsula on the Island to Northern California, where the Juan de Fuca plate system curves west beneath the North American plate.
The repeated episodes of slow fault slips — measuring about the size of five stacked dimes — usually occur over a period of several weeks, accompanied by tremors that appear on seismic records as prolonged, intermittent ground vibrations, Cassidy said.
The ETS activity has been going on for centuries, but has only recently been recorded using sophisticated GPS technology at more than 40 sites up and down the Island. They differ from earthquakes, which generate large, sharp shock waves that subside very quickly, but ETS can help in improving estimates on where and when the next earthquake might occur, Cassidy said.
By mapping out the areas on the subduction fault where stress is not accumulating over the long term, tremor events define the landward limit of the zone that could rupture during the next great earthquake.
That provides a more accurate estimate of how close the rupture could be to major West Coast cities such as Victoria, Vancouver and Seattle, and what shaking could occur, he said.
Each ETS episode adds a small amount of stress to the locked portion of the subduction zone. As the stress increases and approaches a critical level, an ETS event may provide the additional stress needed to overcome the friction on a fault, triggering a great earthquake.
According to the maps below, it seems that the slow slip event takes place along the entire Cascadia right now:
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