Risk of Vancouver Island earthquake increases next month
Dustin Walker, Canwest News Service
Published: Tuesday, April 01, 2008
The chance of a major earthquake striking Vancouver Island will be slightly higher next month as tiny tremors rattle the earth 40 kilometres beneath our feet.

Seismologists forecast the southern half of the Island, including Nanaimo, to enter a period of what they call "episodic tremor and slip" activity in mid-April, as the Juan de Fuca plate grinds underneath the North American plate along what's known as the Cascadia subduction zone.

The fault that slopes down beneath Vancouver Island comes under increased pressure at these times causing small sub-surface tremors for about a two-week period, and that's also when seismologists think it's more likely there could be a major movement of the plates, causing a potential magnitude 9 earthquake.

Such a quake would also trigger a tsumani that would level the west coast and could topple tall or unstable buildings in cities like Nanaimo.

This type of earthquake last struck in 1700, wiping out aboriginal villages and sending a tsunami across the Pacific.

But there's no need to run to the mainland for cover, says one earthquake scientist, people who live along the coast from northern California to the northern tip of Vancouver Island probably face this risk about one-third of the time anyway.

ETS activity occurs beneath the southern half of Vancouver island about every 14 months, and people probably won't feel the shaking going on 40 kilometres beneath Nanaimo's surface, said Garry Rogers, an earthquake scientist with the Geological Survey of Canada.

Each time ETS activity takes place, more stress is added on the two plates, he explained.

Eventually, the pressure will be too much for the plates to take and the Cascadia subduction zone, which spans about 1,000 kilometres north from California to the Island, will rupture causing a major earthquake.

While studying ETS activity can provide some insight into earthquakes, it doesn't help scientists predict when they will happen.

These tremors take place at other locations along the fault at different times.

An 11-month cycle has been observed in California, for instance.

"It happens everywhere along the Cascadia subjuction zone, but it goes off at different times," said Rogers, adding that ETS activity is taking place somewhere about one-third of the time.

"One of these, it seems likely, will be the one that breaks the camel's back (and causes an earthquake). We have no idea which one it is."

And just because the ETS activity isn't happening near Nanaimo at the time, doesn't mean we're in the clear. A quake could originate in California but travel north to B.C.'s southern coast, for instance.

"When our subduction zone ruptures, the past history has been the entire thing or half the thing goes at one time," said Rogers.

And ETS activity doesn't run like clockwork. The northern half of Vancouver Island usually experienced these tremors about six months out of sync with the southern half of the Island.

But recently that schedule has changed all of a sudden, leaving scientists perplexed.

Rogers points out that the more probable source of major damage on the Island is not an extremely rare and deep underground subduction quake, but rather more common earthquakes that are less powerful but felt more intensely because they happen closer to the surface.

ETS activity was first discovered by a team of scientists, that included Rogers, on Vancouver Island in 2003.