At about 10:15 on the morning of Sunday, June 23, 1946, the chimneys of Cumberland began to come down. By the time the shaking stopped, three out of every four chimneys in Cumberland, Union Bay and Courtenay were on the ground, according to the Geological Survey of Canada. The earthquake, centred in the Forbidden Plateau area just west of Courtenay and Campbell River, measured magnitude 7.3. It remains the largest recorded onshore earthquake in Canada, and the reason most accounts of earthquakes on Vancouver Island start with that date.
It was not the largest earthquake this coast has known. A Vancouver Island earthquake map, read properly, shows three quite different sources of shaking: faults far offshore, a plate sliding under the island, and the crust of the island itself. Each has produced a major earthquake within the historical or oral record. This guide explains the Vancouver Island fault lines behind them, what the history shows, how to read the map, and what the risk means for Nanaimo and the rest of the mid island.
Why Vancouver Island shakes
Natural Resources Canada's seismologists put it simply: the largest and most frequent earthquakes in Canada occur along the west coast, and most are tied to plate motions and active faults. Across the country, the Geological Survey of Canada detects about 4,000 earthquakes a year, and most of them happen along the active plate boundaries off the west coast. About 50 earthquakes a year are strong enough to be felt somewhere in Canada, and the busiest stretch of that record runs along this coast.
The reason is plate tectonics. The earth's outer layer is broken into eight major plates and dozens of smaller ones, and they move up to fifteen centimetres a year, according to the Geological Survey of Canada. Plates can slide past each other, collide, or pull apart. Off the west coast of British Columbia there are examples of all three: a subduction zone where one plate sinks beneath another, a spreading ridge where new ocean floor is made, and a sliding fault. That mix is why the earthquake record here is so busy.
The three fault lines on a Vancouver Island earthquake map
The Vancouver Island fault lines are not one system but several. The Geological Survey of Canada describes three types of earthquake in southwestern British Columbia, and the distinction matters more than any single date. Subduction produces earthquakes within the sinking plate, typically at 30 to 60 km depth; earthquakes within the North American Plate, down to 30 km depth; and giant subduction earthquakes along the interface between the two plates, sometimes called megathrust or great earthquakes.
Each type has an example in the record. The megathrust type is the 1700 earthquake. The type within the sinking Juan de Fuca plate is the magnitude 6.8 Nisqually earthquake near Seattle in 2001, at a depth of about 50 km. The type within the continental plate is the island's own 1946 earthquake. North of the island, the Queen Charlotte Fault adds a fourth setting, with its magnitude 8.1 earthquake off Haida Gwaii in 1949.
When you look at a map of recorded earthquakes, these show up differently. The Survey notes that many of the larger earthquakes in the region occur offshore. Earthquakes within the subducting plate form a band stretching from mid Georgia Strait to southern Puget Sound at depths of 40 to 60 km, and they usually have few or no aftershocks. Earthquakes within the North American Plate are the shallowest, which is why the Survey says they can cause the most damage. Across Canada as a whole, the fewest earthquakes occur within the stable interior, including the plains of Saskatchewan and Manitoba.
The Queen Charlotte fault and Canada's largest earthquake
North of Vancouver Island, the Pacific plate slides northwest past North America. The boundary between them is the Queen Charlotte fault, which Geological Survey of Canada scientists call Canada's "San Andreas".
On August 22, 1949, a magnitude 8.1 earthquake broke a segment of that fault about 500 km long. It remains Canada's largest recorded earthquake. According to the Geological Survey of Canada, it was felt as far north as the Yukon and as far south as Oregon. Damage was limited because so few people lived nearby, but on Haida Gwaii chimneys toppled and cows were knocked off their feet, and on the mainland windows shattered and buildings swayed.
The same fault has produced three of the largest earthquakes in Canada's history, the survey notes, including magnitude 7.4 in 1970 and magnitude 7.0 in 1929. For people on the mid island, the Queen Charlotte fault is distant. It belongs on the map because it shows how much energy this coast's plate boundaries can release.
Cascadia: the plate under Vancouver Island
West of Vancouver Island, from the island's northern tip to northern California, the smaller Juan de Fuca plate is moving toward North America at roughly 2 to 4 cm a year, about the rate fingernails grow, and sliding beneath it. This is the Cascadia subduction zone, about 1,000 km long. The Geological Survey of Canada notes that the world's largest earthquakes occur in subduction zones, like the magnitude 9.0 earthquake in Japan in March 2011.
The important word in the scientific description is "locked". The Pacific Northwest Seismic Network explains that at depths shallower than about 30 km the two plates are locked together by friction, so strain slowly builds while the subduction forces keep acting. Meanwhile the smaller earthquakes carry on: about 400 are located each year between northern Vancouver Island and Seattle, a dozen or so are felt, and earthquakes capable of structural damage happen every decade or so, according to the Survey's fact sheet.
The 1700 earthquake was likely similar in many ways, the Geological Survey of Canada says, to the 2004 Sumatra subduction earthquake of magnitude 9.2. The 1964 Alaska earthquake of the same magnitude, which came from a subduction zone farther north, sent a tsunami into Alberni Inlet on Vancouver Island. Geological evidence from Vancouver Island to California shows that magnitude 9 earthquakes occur on the Cascadia fault about every 500 years on average, and that the interval between them has varied from 250 to 850 years.
The 1700 earthquake
The last one came at about 9 p.m. on January 26, 1700. The undersea Cascadia fault ruptured from northern Vancouver Island to northern California, a zone about 1,000 km long, causing very strong ground shaking for several minutes and spawning a tsunami that travelled across the Pacific, according to the Geological Survey of Canada. Its magnitude is estimated at 9.
The record of that night comes from two very different sources. On Vancouver Island, First Nations oral traditions describe it. According to the Geological Survey of Canada, oral reports describe houses collapsing in the Cowichan area because of landslides, and shaking so severe that people could not stand and so prolonged that it made them sick. On the west coast of the island, the tsunami destroyed the winter village of the Pachena Bay people, leaving no survivors.
Across the Pacific, the tsunami struck the coast of Japan, and Japanese records of its arrival are what allow scientists to state the earthquake's size and time so confidently. The geological record adds a third witness. Earthquakes of this size can cause sudden coastal subsidence, liquefaction and large tsunamis, and evidence of them from Vancouver Island to California shows that 1700 was not a one-off. The Pacific Northwest Seismic Network counts at least 19 great earthquakes along this margin in the past 10,000 years.
The 1946 earthquake on central Vancouver Island
The 1946 earthquake was the crust's turn. The Geological Survey of Canada describes it as Canada's largest recorded onshore earthquake. It was felt as far away as Portland, Oregon, and Prince Rupert.
The damage was heaviest in the Comox Valley, around Cumberland. Besides the chimneys in Cumberland, Union Bay and Courtenay, there was considerable damage in Comox, Port Alberni and Powell River across the strait. Chimneys came down in Victoria, and people in Victoria and Vancouver ran into the streets. Two deaths resulted: one person drowned when a small boat capsized in a wave generated by the earthquake, and another died of a heart attack in Seattle. More than 300 landslides were triggered, and liquefaction was widespread along the east coast of central Vancouver Island.
If you have driven the old roads north of Campbell River, the Geological Survey of Canada's photograph of road damage from 1946 is worth looking up: a reminder that the shaking reached far beyond the towns that recorded it.
Fault lines and the history of risk for Nanaimo
For Nanaimo and the rest of the east coast, the question is not only whether a great Cascadia earthquake will happen, but which kind of earthquake poses the biggest danger. The Geological Survey of Canada's fact sheet answers in terms of depth and frequency. Earthquakes within the overriding North American Plate can cause the most damage because they are closest to the surface, and the closer an earthquake is to the surface, the stronger the shaking. Earthquakes within the subducting Juan de Fuca plate have been the most frequent type of damaging earthquake in southwestern British Columbia. Megathrust earthquakes can reach magnitude 9 and bring large tsunamis, and on exposed coasts there would be only 15 to 30 minutes between the earthquake and the first waves.
The record shows what the first two types can do. The 2001 Nisqually earthquake, magnitude 6.8 and about 50 km down within the Juan de Fuca plate, caused more than $2 billion (U.S.) in damage around Seattle and was widely felt in Victoria and Vancouver. In 1918 a magnitude 6.9 earthquake within the North American Plate, near the west coast of Vancouver Island, awakened people all over the island and damaged the Estevan Point lighthouse so badly that it could not operate.
Local engineering reports put that risk in concrete terms. A 2002 assessment found that ground motion from a 1-in-2,500-year earthquake could compromise the South Fork Dam, the concrete arch where Nanaimo's drinking water supply is drawn off, and a 2016 report rated the two-storey classroom block at Rutherford Elementary School high risk in an earthquake.
The history of earthquakes on Vancouver Island therefore points in two directions at once. The 1700 earthquake shows what the subduction zone can do once in several centuries. The 1946 earthquake shows what the island's own crust can do within living memory.
How the coast compares with the rest of Canada
It helps to see the island against the rest of the country. The Geological Survey of Canada's review notes that some of the world's largest earthquakes have occurred in or next to Canada, all along the active plate boundaries off the west coast. Near the Yukon and British Columbia border, the Yakutat Bay region of Alaska was shaken in September 1899 by a series of major earthquakes, beginning with a magnitude 8.1 event on September 4 and followed by magnitude 7.4 and 8.2 events on September 10. They were felt strongly in northwestern British Columbia and southern Yukon.
Inland, the picture quiets down. The fewest earthquakes occur within the stable interior, including the plains of Saskatchewan and Manitoba, though a magnitude 5.4 earthquake in east-central British Columbia in 1986, felt strongly in Prince George, was a reminder that the Cordillera is not immune. Set against that, the mid island sits in the busiest part of the country's seismic map, close to the locked Cascadia boundary offshore and to the kind of crustal faulting that produced the 1946 earthquake.
Reading a Vancouver Island earthquake map
A Vancouver Island earthquake map usually shows dots for recorded earthquakes, sized by magnitude, and lines for the main plate boundaries. Reading it well means keeping three things in mind.
First, the dots are dominated by small earthquakes. Seismologists locate more than 4,000 earthquakes a year in and around Canada, the Geological Survey of Canada notes, and most are smaller than magnitude 3 and not felt. A crowded map is not a sign of imminent danger; it is a sign of good instruments.
Second, depth matters as much as position. An earthquake deep within the subducting plate beneath the southern island sits tens of kilometres down, while a crustal earthquake like 1946 happens within the continental crust above it, much nearer to towns and roads. Two dots side by side on a flat map can be very different events.
Third, the largest past earthquakes are not always where the dots cluster today. The 1700 rupture ran along the offshore Cascadia fault, which is locked, so strain builds there without being released. An empty stretch of map can be a stretch under strain. That is why a Vancouver Island earthquake map is best read alongside the history above.
Earthquake early warning in British Columbia
Earthquakes cannot be predicted, PreparedBC states. What is now possible is a few moments of warning. The Canadian Earthquake Early Warning system detects an earthquake immediately after it begins and can give seconds to tens of seconds of warning, by phone, TV or radio, before strong shaking arrives.
The system is designed to alert areas that will receive strong, potentially harmful shaking; in its coverage areas, public alerts are sent for earthquakes estimated at magnitude 5 or more. There is a limit built into the physics. Places very close to the epicentre may be in a "late alert zone", where the alert arrives after the shaking has started. PreparedBC's advice follows from that: if you feel the ground shaking, act at once and do not wait for an alert. The alerts are not an app and need no sign-up; they go out automatically through the National Public Alerting System to compatible phones, TV and radio. A phone set to Wi-Fi only, or one in a place with poor reception such as an underground garage, may not receive them. On the water, the standing safety net is the marine radio: Coast Guard radio centres around Vancouver Island keep watch on VHF channel 16, reserved for distress, urgency and safety traffic.
What to do when the ground shakes
PreparedBC's instructions are short. When you feel shaking or receive an alert, drop to your hands and knees; cover your head and neck and take shelter under a sturdy piece of furniture, or crawl to an interior wall; and hold on until the shaking stops. If you are inside, stay inside. Most injuries, the province notes, happen when people try to run for cover.
The details change with where you are. In bed, stay there and protect your head with a pillow or blanket. In a wheelchair, lock the wheels and stay in place. Driving, pull over away from overpasses, bridges and power lines, set the handbrake and stay in the vehicle until the shaking ends. Outdoors, move to a clear area only if you can do so safely.
On the coast, the most important instruction comes after the shaking stops: go to high ground immediately, without waiting for an official tsunami warning, and walk rather than drive to avoid traffic and debris. Aftershocks can continue for hours or days, and each one calls for the same response.
After the shaking stops
PreparedBC's advice for the minutes after an earthquake is just as specific. Count to 60 before getting up, so that displaced objects have a chance to settle. Move carefully and check for unstable objects above and around you. Expect aftershocks, and drop, cover and hold on each time.
Do not call 911 to report the earthquake itself; call only for serious injuries. Follow directions from local authorities and first responders. In Nanaimo, the City's emergency coordination centre sits in the downtown Fire Station #1, built to post-disaster standards and opened in 2023. If your home is badly damaged and unsafe, take your grab-and-go bags and leave for somewhere safer. If it is damaged but safe, stay and use your emergency kit. The province's EmergencyInfoBC site carries the most up-to-date official information after an earthquake.
Preparing a home for earthquakes on Vancouver Island
Most of what reduces harm happens long before the ground moves. PreparedBC recommends securing tall, free-standing furniture such as bookcases, cabinets and shelving to wall studs with L-brackets, corner brackets or aluminium moulding; fitting cabinet doors with latches; and strapping refrigerators, freezers, washers and dryers. Water heaters should be anchored snugly to the wall with straps, and the province suggests having a licensed gas fitter install a flexible gas line where needed.
The second step is a household plan, which PreparedBC offers as a printable fill-in form and an interactive planner, with attention to young children, older adults and others who need more help. The third is supplies. The province advises keeping enough non-perishable food, water and emergency supplies for the whole household for at least 3 days, and preferably one to two weeks, stored together where they are easy to reach, plus a smaller grab-and-go bag for each person and each pet. If your home is undamaged after an earthquake, staying put and using your kit leaves emergency reception centres free for people who need them.
Insurance is the step people tend to skip. PreparedBC advises discussing earthquake coverage with an insurance representative, and notes that provincial Disaster Financial Assistance is not available for losses caused by an earthquake.
Living with the map
None of this makes the mid island unusual. It makes it part of a coast where, as the Geological Survey of Canada's plate tectonics fact sheet puts it, boundaries of all three kinds are found off the coast, and where the earthquake record is correspondingly busy.
The one number that tends to stay with people is the one with no date attached to it. The Pacific Northwest Seismic Network counts at least 19 great earthquakes along this margin in 10,000 years, at intervals of hundreds of years. The last was in 1700. When the next one comes is the question no map can answer.
Questions
Are earthquakes common on Vancouver Island?
Yes. The largest and most frequent earthquakes in Canada occur along the west coast, according to Geological Survey of Canada scientists, who detect about 4,000 earthquakes a year across the country. About 400 earthquakes a year are located between northern Vancouver Island and Seattle, and about a dozen of them are felt.
What was the largest earthquake on Vancouver Island?
The largest historic earthquake on the island was the magnitude 7.3 earthquake of Sunday, June 23, 1946, centred in the Forbidden Plateau area west of Courtenay and Campbell River. The Geological Survey of Canada describes it as Canada's largest recorded onshore earthquake.
What damage did the 1946 earthquake cause?
It knocked down 75 per cent of the chimneys in Cumberland, Union Bay and Courtenay, did considerable damage in Comox, Port Alberni and Powell River, and shook down chimneys in Victoria. Two deaths resulted: one person drowned when a small boat capsized in an earthquake-generated wave, and another died of a heart attack in Seattle.
What is the Cascadia subduction zone?
It is the boundary west of Vancouver Island, about 1,000 km long, where the oceanic Juan de Fuca plate slides beneath North America at roughly 2 to 4 cm a year. Where the two plates meet at shallow depth they are locked together by friction, and strain slowly builds.
When was the last great Cascadia earthquake?
At about 9 p.m. on January 26, 1700. The Geological Survey of Canada puts its magnitude at about 9. The rupture ran roughly 1,000 km, from northern Vancouver Island to northern California, and the tsunami crossed the Pacific to Japan.
How is the date of the 1700 earthquake known?
Japanese records of the tsunami that reached Japan's Pacific coast gave the precise date and time. First Nations oral histories on Vancouver Island describe the shaking and the tsunami that destroyed the winter village at Pachena Bay.
How often do great earthquakes happen off Vancouver Island?
Geological evidence from Vancouver Island to California shows that magnitude 9 earthquakes strike the Cascadia fault about every 500 years on average, with the gap between them ranging from 250 to 850 years. The Pacific Northwest Seismic Network counts at least 19 great earthquakes along the margin in the past 10,000 years.
What is the Queen Charlotte fault?
It is the boundary between the Pacific and North American plates from northern Vancouver Island to Haida Gwaii, where the Pacific plate slides northwest relative to North America. Geological Survey of Canada scientists call it Canada's "San Andreas".
What was Canada's largest recorded earthquake?
The magnitude 8.1 earthquake of August 22, 1949, off Haida Gwaii on the Queen Charlotte fault. It broke a segment of the fault nearly 500 km long and was felt from the Yukon to Oregon.
Is the offshore megathrust the biggest risk for Nanaimo?
Not on its own. The Geological Survey of Canada notes that earthquakes within the overriding North American Plate can cause the most damage because they are closest to the surface, and that earthquakes within the subducting Juan de Fuca plate have been the most frequent damaging type in southwestern British Columbia. Megathrust earthquakes are the rarest and the largest.
How often do damaging earthquakes strike southwestern British Columbia?
The Geological Survey of Canada says earthquakes capable of causing structural damage happen every decade or so in the region from northern Vancouver Island to Seattle. The magnitude 6.8 Nisqually earthquake of 2001, within the Juan de Fuca plate near Seattle, caused more than $2 billion (U.S.) in damage and was felt across southwestern B.C.
What should I do when the ground starts shaking?
Drop to your hands and knees, cover your head and neck and take shelter under sturdy furniture, and hold on until the shaking stops, PreparedBC advises. If you are inside, stay inside; most injuries occur when people try to run.
What should I do on the coast after an earthquake?
PreparedBC advises that, once the shaking stops, people on the coast should go to high ground immediately without waiting for an official tsunami warning, and walk rather than drive.
What is earthquake early warning in B.C.?
The Canadian Earthquake Early Warning system detects an earthquake right after it starts and can give seconds to tens of seconds of warning by phone, TV or radio. In coverage areas, public alerts are sent for earthquakes estimated at magnitude 5 or more.
Should I wait for an alert before taking cover?
No. Places very close to the epicentre may be in a 'late alert zone' where the alert arrives after the shaking, so PreparedBC advises taking protective action as soon as you feel the ground shake.
Can earthquakes be predicted?
No. PreparedBC states plainly that earthquakes cannot be predicted. What science can say is where they are likely and how often, which is what an earthquake map shows.
Sources
- Canada's Earthquakes: 'The Good, the Bad, and the Ugly', Geoscience Canada, Geological Survey of Canada (Cassidy, Rogers, Lamontagne, Halchuk and Adams), 2010-03-01
- Earthquakes in Southwestern British Columbia (GeoFact), Geological Survey of Canada, Natural Resources Canada, 2011-01-01
- Plate Tectonics Shape (and Shake) British Columbia (GeoFact), Geological Survey of Canada, Natural Resources Canada, 2011-01-01
- Cascadia Subduction Zone, Pacific Northwest Seismic Network
- Canada's Ten Largest Earthquakes (GeoFact), Geological Survey of Canada
- Get prepared for an earthquake, PreparedBC, Government of British Columbia
