A shallow magnitude 6.6 earthquake hit offshore of Vancouver Island, British Columbia on Wednesday night. That’s larger than the Christchurch earthquake and was felt hundreds of kilometers away in Vancouver, yet no damage or injuries were reported. Why? Location, location, location.

Rocks, sand, trees, waves, and fog define the northwest coast of Vancouver Island. Image credit: Mika McKinnon

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Tectonic Setting

The Pacific Northwest has a triple-plate subduction zone, a mirror-image of the zone in Japan that spawned the megaquake and tsunami in 2011. A small plate, the Juan de Fuca plate, is diving under the North American plate. Off-shore, the Juan de Fuca and Pacific plates are spreading apart, a diverge boundary forming a mid-ocean ridge.

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Cascadia earthquake source locations. Image source: USGS

The plates move erratically, getting stuck and building up energy which is released during earthquakes. Subduction zones have the capacity for small shallow earthquakes, but also deeper megathrust earthquakes. Several hundred tiny earthquakes occur every year, but the average return period for a major event is approximately every 300 to 500 years. That average is deceptive — the spacing on megaquakes is highly irregular, historically happening as close together as a century and as far apart as a millennium.

History of Rattling

The last major megaquake in British Columbia was a massive magnitude 9 on the morning of January 26th, 1700. How do we know the time that precisely? Geologic evidence nearby of drowned forests and clays help pinpoint the local tsunami, but the exact timing comes from Japanese tsunami run-up records.

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Saw-toothed seismic patterns on Vancouver Island, British Columbia. Image credit: Natural Resources Canada (NRCan)

In between, the Cascadia Fault Zone does something weird: it saws back and forth. No, really! The same network of global position system stations that track the long-term stress accumulation along the subduction margin tracks a slow landward motion with seaward reverses. This creep-and-reverse produces non-earthquake tremor-like seismic signals. This suggests that at least the northern section of the fault (Vancouver Island) has repeated slow-slip events, instead of being fully locked.

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Thirty days of earthquakes in western Canada, including the orange-star M6.6 offshore of Vancouver Island. Image credit: NRCan

The most recent significant earthquake off British Columbia’s coastline was a M7.7 in October 2012, farther north offshore of the Haida Gwaii Islands. Earthquakes of at least magnitude 5.0+ are a regular occurrence in the region.

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M6.6 on April 23, 2014

Last night, British Columbia experienced a moderate magnitude 6.6 earthquake off the north coast of Vancouver Island, just under 100 kilometers south of Port Hardy. The US Geologic Survey automatically generated a shakemap for the event, where warmer colours reflect more intense shaking.

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Shakemap of the M6.6 offshort of Vancouver Island. Image credit: USGS

The earthquake epicenter was barely off-shore, but on the far side of the island from the nearest major settlement. The northwest coast of Vancouver Island is a mix of logging areas and parkland, with very few people even in summer when tourist and fieldwork populations peak.

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Seismograph at Corvallis, Oregon.

A pair of smaller aftershocks hit ten and thirty minutes after the main event — a M5.0 at 8:20 pm, and an M4.2 at 8:41 pm. Another M4.2 struck at 10:16 pm, and a smaller M4.1 ruptured in the early hours this morning at 5:07 am. None of the aftershocks are substantial enough to be felt at any significant distance from the epicenter, although if they keep up it may be a bit overly-exciting on the logging blocks today.

No significant damage or injuries were immediately reported.

The nearest community is Port Alice, a village of fewer than 1,000 residents 30 kilometers to the north of the earthquake. Port Hardy, 100 kilometers north of the earthquake on the northeast tip of the island, is as far north as you can drive on Vancouver Island on paved roads. It’s a small town of under 5,000 people, with a regular influx of visitors from either tourism or the resource industry. Oprah Winfrey once visited the region, hanging out on Jim Patterson’s yacht and checking out the amazing masks in Alert Bay. It’s nice jumping-off point for treks farther into the wilderness and a welcome collection of coffee shops, restaurants, and hotels for field-workers, but it’s entirely understandable if you’ve never heard of it before today.

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Residents in Port Hardy reported feeling between 30 and 45 seconds of strong shaking (or was it 10 seconds?), but no injuries.

Logging blocks on Vancouver Island. Map credit: Ancient Forest Alliance

San Josef is on the northwestern tip of the island closer to the earthquake, but it’s in a park, not a town. Getting out to Cape Scott Provincial Park is an adventure on washboard gravel roads, ducking out of the way of fully-loaded logging trucks. Last time I made it out, the park had a few unofficial-residents camping out year-round, but no one who shows up on census reports. Other than that, the west coast of north island is mostly logging, logging, and more logging blocks with an occasional creek. Aside from loggers, tree-planters, and geoscientists, few people are around to be rattled.

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Second-growth forest on Vancouver Island. Image credit: scumandvillainy

A magnitude 6.6 is a genuinely sizeable earthquake — not a megaquake catastrophe, but larger than the M6.3 that devastated Christchurch, New Zealand, in 2011. The Vancouver Island earthquake was nearly twice as big, and released over twice as much energy, as the one that flattened parts of Christchurch, so why no reported damage? It’s not better preparedness. Instead, as in real-estate, it comes down to location. Christchurch is on saturated sediments of an infilled valley that amplify shaking, while the majority of Vancouver Island is young bedrock that attenuates shaking. While the Christchurch earthquake was directly below the city, this earthquake hit offshore of the largely-uninhabited northwest coast of Vancouver Island. The low population density on the north end of Vancouver Island reduced the number of people directly impacted by this earthquake, but the shaking was strong enough to be felt as far south as Vancouver and as far inland as the Okanagan Valley.

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Did You Feel It? map and correlation between felt-magnitude and distance from the earthquake. Image credit: USGS

Overall, felt-shaking attenuates with distance, so some people more than 300 kilometers away in Victoria and Vancouver felt the earthquake, but not all of them. Shaking is amplified by loose sediments, so residents on the young delta of the Fraser River (Richmond and Delt)a felt it more intensely than those on bedrock (North Vancouver). Similarly, people in high rises were more likely to notice the earthquake (particularly with external hints like rattling blinds) as their buildings swayed, and those at ground-level were less likely to feel the shaking.

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A liquefaction hazard map for Vancouver traces surficial geology of bedrock, compacted sediments, and saturated sediments. Image credit: NRCan

If you were within the earthquake region, please file a Did You Feel It? report. These reports help seismologists better model how an earthquake’s shaking is amplified (or reduced) by surficial geology and built structures, helping advance our understanding. Even null reports (“Nope, I didn’t feel it.”) are important, keeping positive reports from swamping the system in areas that experienced weak shaking.

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Because this was a shallow event with no major vertical displacement, no tsunami was expected. Although the Winter Harbour buoy closest to the epicenter hasn’t been reporting data for the past few days, the NOAA tsunami monitoring DART buoy detected the non-event too-small-to-care tsunami with mere inches of displacement.

So... what now?

Was this the big one, and the Pacific Northwest can stop worrying about an impending megaquake? Not a chance. Earthquake magnitudes are a logarithmic scale: it would take 1,000 magnitude 7 earthquakes to release the same energy as one magnitude 9 megaquake. A moderate 6.6 earthquake can be devastating, but it does not release nearly enough energy let go of all the stress built up along the Cascadia Fault Zone over the past three centuries. The Big One is yet to come.

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What this does do is redistribute stress around the region. This earthquake ruptured some smaller fault landward of the massive Cascadia fault, releasing stress along the length that slipped and building up stress at the locked ends.

I wish I could include a beautiful map of fault lines on Vancouver Island, but I can’t. This is partly due to the limitations of science — fewer, larger earthquakes in a subduction zone make it harder to map faults compared to the frequent, smaller earthquakes in California — but also due to political limitations. With a smaller population than the United States, the Canadian budget for scientific research is also smaller. Canada has only have a couple hundred permanent seismograph stations, compared to the United States where the entire country is blanketed in seismic stations. Even worse, between policies that muzzle government scientists, massive funding cuts to science departments, and emptying public science libraries into dumpsters, even if the resources existed to produce glorious fault maps, we might not even seen them.

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Image credit: Southern California Earthquake Center

If you live in earthquake-country, make sure to prepare your home before you’re hit by a major earthquake. If you do feel an earthquake, remember the steps to safety are drop, cover, and hold on. Furniture has a nasty habit of walking away when the ground shakes, leaving puny humans exposed to whatever in their room wants to fall on their heads. We can’t predict when an earthquake will happen, but we can mitigate it some of the damage.

Parts of this article are modified from previous articles on British Columbia seismology on GeoMika. Check out more earthquake data on the USGS or the NRCan websites, or learn about the geology of Vancouver or Victoria. Learn more about earthquakes here, or here, or here. Practice earthquake preparedness in game-format, or join group earthquake-drill Shakeouts in BC or the United States.

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