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Cluster of moderate earthquakes shakes up the ocean floor

VANCOUVER (NEWS1130) – An unusual cluster of moderate earthquakes has been shaking up the ocean floor off the coast of Vancouver Island.

Since late last month, there have been at least eight undersea quakes, most in the magnitude 5.0 to 6.0 range in an area about 120 kilometres west of Port Hardy.

“It’s interesting. That particular area is a patch of real estate that is very earthquake-prone,” says John Clague, a professor in the Department of Earth Sciences at Simon Fraser University.

“We do get frequent earthquakes off northern Vancouver Island but I can’t remember when we’ve had about eight in quick succession that are in that range,” he tells News1130.

But Clague says the cluster of quakes is probably not a precursor to a massive, more damaging shaker.

“Probably the largest earthquake that could occur in that area is about a magnitude 7.0 or maybe a little larger. That is offshore, beneath the Pacific Ocean floor, and the nearest substantial community is well over 100 kilometres away. Even with an earthquake of magnitude 7.0 there is not likely to be damage on northern Vancouver Island.”

And because of the geography, Clague says seismic activity in the area is not connected to activity further south.

“I know people worry about these very large earthquakes that we get closer to the south coast. They would be disconnected; there wouldn’t be any relationship between this earthquake cluster and a larger quake that might occur off, say, southern Vancouver Island.”

There is also little risk of a tsunami spawning from quakes clustered around the north coast of the Island, with Clague pointing out they typically produce a horizontal movement, rather than the vertical motion needed to produce the big waves.

“The ones we have to worry about are, again, farther south — these very rare but extremely large earthquakes that occur off the southern coast of Vancouver Island, Washington and Oregon — and they are tsunami-producing earthquakes.”

The good news, as Clague mentions, is those quakes are rare, happening every 500 years on average.

“Yet we know they occur and we wish, as scientists, that we could tell people when the next one is. But when we look at the geologic record of these events, they just are so non-uniform,” he says.

“We get events that are only spaced 100 years apart and we get events that are spaced 1,000 years apart. Using that 500-year average is a little bit misleading because we just don’t know whether we are in a long gap between earthquakes or in a short gap.”

Clague tells us the last massive quake in southern BC was a little over 300 years ago.

“We’re definitely getting out there closer to the average for the intervals between these earthquakes. I know people get a little frustrated with that and, scientifically, we are looking at the lead-up to recent, comparable huge earthquakes in the Indian Ocean, Sumatra, and Japan to see if there’s anything unusual occurring in the earth’s crust. Those earthquakes, particularly the Japanese earthquake, were very well-monitored.”

Clague says hopefully, they can begin to identify precursor signals that can tell us when we are getting closer to the next big earthquake off the BC coast.