B.C. earthquakes and the risk of a 'big one' — a scientific explanation

The magnitude 4.8 Victoria earthquake on Tuesday night woke many people from their sleep, and triggered many questions about earthquake risks on the South Coast.

Magnitude 4.8 earthquake on Tuesday not likely a precursor to something bigger

The Cascadia subduction zone is one of the most seismically active regions of the world. The earthquake on Dec. 29, 2015 did not happen in the 'locked' section but rather in the Juan de Fuca plate as it moved underneath the South Coast. (USGS)

Thousands of people were shaken awake last night across the B.C. South Coast during a magnitude 4.8 earthquake that struck at 11:39 pm PT just 19 kilometres north-northeast of Victoria.

For many people, it was the strongest quake they had ever felt — one of the largest in the area in 14 years. And following it there are lots of questions about what this means for the "big one."

This earthquake struck at a depth of 52 kilometres. Even though this earthquake is not classified as shallow or big, it was close enough to large populations, which is why so many people felt it.

Reports of the quake stretched as far north as the top of Vancouver Island, as far south as Seattle and as far east as the Fraser Valley. Fortunately, there were no reports of any major damage or injuries.

Weak to light shaking was reported across much of the Pacific Northwest. (USGS)

So far, no aftershocks have been reported. There is a slight chance small aftershocks may occur, but the likelihood will diminish over time.

Risk of the 'big one'

The South Coast of B.C. is situated in what's known as the Cascadia subduction zone, where ocean crust is trying to slide under the crust that North America sits on. It is likely this quake was located within the ocean plate that has moved underneath North America (and Victoria).

This is a different kind of earthquake from the type we would see during the "big one."

Along the boundary of these two plates, a section has become locked and no movement is happening. As a result, stress has been building up for centuries, and at some point, that stress will be relieved with a megathrust earthquake of magnitude 9.0 or higher.

A Washington station's seismogram shows the magnitude 4.8 earthquake Tuesday night. (Pacific Northwest Seismic Network )

Getting prepared

There is about a one in 100,000 chance of a megathrust earthquake hitting on any given day, and this latest quake is not likely to have affected those odds.

However, earthquakes like the one experienced Tuesday night can occur more frequently than that. Even though the magnitudes will be smaller than that of a megathrust earthquake, they may happen closer to big cities.

Now is the perfect time to get an earthquake kit together and make an earthquake plan with your family. 

This latest quake is a good reminder to reassess your earthquake kit. (Johanna Wagstaffe/CBC)

About the Author

Johanna Wagstaffe

Senior Meteorologist

Johanna Wagstaffe is a senior meteorologist for CBC, covering weather and science stories, with a background in seismology and earth science. Her weekly segment, Science Smart, answers viewers' science-related questions.

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