Shake it off : Quakin’ on Vancouver Island

Maybe you thought we were being attacked, a gas leak exploded, a car ran into your house, or you were tripping out. Maybe you were shaken awake or maybe you were away for the break and missed out.

Either way, the magnitude 4.9 earthquake on December 29 sprouted 20 km north of Victoria rattled houses and resulted in a flooding of frantic status updates on Facebook feeds. It was felt throughout the southwest coast of Vancouver Island and well into the Vancouver area, inspiring a reminder to many that our homes are situated in an earthquake-prone zone.

Earthquakes are a common occurrence in the Pacific Northwest, where we have an active plate tectonic boundary, said VIU Earth Sciences professor Sandra Johnson. “The Juan de Fuca plate is sub-ducting below the North American plate along the west coast of North America from around northern California to the north end of Vancouver Island,” she said. “Friction between the plates causes stress to build up, and when all that pent up energy is released the ground shakes. This is not unusual here, since there is an active plate boundary located west of Vancouver Island.”

The last earthquake in the area of this size was in May when there were earthquakes of M4.3 and M4.9 offshore of Tofino, but such tremors, and many much larger, happen regularly around the world. Each year we get many small earthquakes, several of these which are large enough to be felt; larger ones capable of doing damage are rare, and may happen every few decades. It is common knowledge to those who live on Vancouver Island and in the Vancouver area that the “Big One” could theoretically happen at any time.

“Big ones (earthquakes larger than M8), tend to happen on a cycle of several hundred years or more; it’s not regular,” Johnson said. “The last really big one in our area was in the year 1700.”

It is difficult to determine whether these smaller quakes have a connection to the expected destructive one. While the plate boundary is the same, typically the truly large earthquakes occur offshore and not as deep as the December 29 earthquake, said Johnson. “Stress between the two plates is accumulating, releasing, and shifting all the time. A large earthquake will happen when a significant amount of stress accumulates and is released all at the same time. Smaller earthquakes can be precursors to large events, or may be isolated occurrences. It’s pretty tough to know.”

During large earthquakes, coastal areas tend to be flooded by tsunami waters, which bring lots of sediment with them, and so geologists have drilled into sediment records and identified these types of tsunami deposits from past large earthquake events, explained Johnson. “There’s no way to know when another ‘megathrust’ earthquake will occur, but it’s very likely to be in the next 500 years,” she said.

In the meantime, it’s best to know how to educate yourself in the case of a larger tremor so you can rest easy. The VIU website has several helpful resources on how to act in the case of an earthquake, as well as how to prepare an earthquake kit for your home or car.

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Left: Past earthquake events (Magnitude 5 or larger) are summarized on this map from the Geological Survey of Canada.

Right:  Since 1872, there have been 11 earthquakes with a magnitude higher than 5.7 in the region, and many more than that with smaller magnitudes. There are regular earthquakes that are too small to be detected by people, but can be detected on the sensitive instruments used by seismologists. This figure summarizes all the earthquakes greater than magnitude 2.0 since the year 2000.

Infographs via earthquakescanada.nrcan.gc.ca

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