At about 9:00 pm on January 26, 1700, the Cascadia thrust fault broke along a front perhaps 1000 kilometers long.
The fault lies about 100 km west of Vancouver Island, and runs south from B.C. to northern California. It’s the line where the North America plate is gradually overrunning the much smaller Juan de Fuca plate. Normally the plates are locked, but every 500 years or so the pressure overwhelms the resistance of the rock. With every such “subduction” quake, a little more of Juan de Fuca sinks beneath the larger plate.
The 1700 Cascadia quake, like the one off Sumatra on December 26, released enormous energy. Some of it went into raising or sinking shorelines on the coast of Vancouver Island and Washington; some of it went into a tsunami that crossed the Pacific in a few hours and inflicted damage on the Japanese coast. Detailed Japanese records of that event enabled geophysicists to pinpoint the time of the Cascadia quake.
That quake had an estimated “moment magnitude” of 9, like the Sumatra quake, and it was a bit smaller than two of the worst quakes of the 20th century: a 1960 quake in southern Chile measured 9.5, and the Good Friday Alaska quake of 1964 measured 9.2.
The impact of Cascadia is recorded in Japan and also in First Nations accounts and coastal geology. Peoples of Vancouver Island tell about a quake that occurred at night and waves that wiped out villages and their inhabitants, except for the fortunate few who could reach high ground. Tidal marshes on Vancouver Island’s west coast have layers of tsunami-driven sand overlying layers of peat.
A choice of disasters
Subduction quakes appear to hit B.C., on average, about every 500 years. But the actual gap may be much shorter or much longer. In the intervals, “crustal” quakes occur within the North America plate. “Subcrustal” quakes occur deep within the buried Juan de Fuca plate.
Such quakes shake the region less violently but more often. John J. Clague, a geologist at SFU, says that on average we get a magnitude 7 quake every 40 or 50 years, a 6 every 20 years, and a 5 every 5 years.
B.C.’s last sizable earthquake was the 1946 Courtenay quake, a 7.3. Two very large quakes, possibly subduction events, occurred around 300 AD and 1500 B.C.; the older one, says Clague, may have caused southern Vancouver Island to subside while raising the Vancouver area by “tens of centimeters.”
The 1700 Cascadia event and earlier earthquakes affected scattered populations of no more than a few thousand. Over 2.5 million people now live in southwestern B.C., many in especially hazardous areas like the Fraser delta. Clague warns that in B.C. today a similar subduction quake, or even a modest magnitude 7.5, would cause damage estimated at between $17 billion and $40 billion.
When it happens to us
We have a fair idea of what happened in 1700, and if we assume a close repetition of the Cascadia quake in the near future we can predict what the effects would be, and where.
The greatest hazard would come from ground shaking. A strong crustal quake close to Vancouver, Clague says, would inflict about 30 seconds of violent shaking; a magnitude 9 subduction quake would shake the region for one to three minutes. With an epicenter off Vancouver Island, that might mean less damage in the Lower Mainland, or it might mean more.
Earthquake waves go quickly through solid rock. But they slow down in sand and gravel—enough to liquefy the soil, destroying its bearing capacity. The Fraser delta consists of over 200 meters of water-saturated clays, sand and gravel. In a modern magnitude 9 quake, the Fraser delta and floodplain would lose their ability to support heavy structures. The delta alone is now home to 200,000 people. Other deltas, landfills and shorelines would also liquefy. High-rise buildings on such soils would probably collapse.
In 1946, the Courtenay quake triggered over 300 landslides. Caused partly by liquefaction and partly by ground shaking, slides from a magnitude 9 quake would cover, Clague says, 500,000 square kilometers. That would include all of southern Vancouver Island, northwest Washington State, the Gulf Islands, and southwest B.C..
Between ground shaking, liquefaction, and landslides, Vancouver and Victoria would be cut off from the rest of the world. Highways and railroads would be cut. Bridges would fall. Docks would be unusable, whether sunken into landfill or simply knocked down. B.C. Ferries terminals, the Roberts Bank coal terminal, and Vancouver harbour would all be crippled. Vancouver International Airport might be under water, or so broken up as to be usable only by helicopters. Even if it survived intact, the roads and bridges serving it might be wrecked.
Power lines would probably be down. Water supplies from Coquitlam and the North Shore might survive but would almost surely be contaminated by landslides in the watersheds. Fires breaking out in collapsed buildings would be hard to fight, and hospitals would be as overwhelmed as those in Thailand and Sri Lanka.
Government agencies could be paralyzed by disrupted transport and communications, not to mention the deaths of many personnel. The nearest source of help would be Seattle, probably as damaged as ourselves.
Spared from tsunamis
And what about tsunamis? Clague offers some consolation on that score: while tsunamis would certainly smash the west coast of Vancouver Island, the central coast, and the Queen Charlottes, they would lose energy as they came through Juan de Fuca Strait. The ruined shorelines of Vancouver might be flooded, but by subsidence rather than tsunamis.
In fact, Clague notes, when a crustal quake lifted parts of Seattle by 7 meters a thousand years ago, it caused a severe tsunami in Puget Sound—but not in the Strait of Georgia. The greatest damage from a Cascadia tsunami could again be inflicted on Japan.
Clague sees a crustal quake as more likely to hit us before a subduction quake does. For every magnitude 8 or 9 event, the Pacific Northwest gets about 50 quakes of 6 or 7. He cites the 1994 Northridge quake in California. A 6.7, it caused over $15 billion in damage.
So we could suffer enormous damage and loss of life from a crustal quake, only to recognize the continuing threat from a still larger subduction quake. Rebuilding our cities would involve a far higher level of earthquake resistance than we now have, and we could not risk spreading the cost over a decade or more, as we have been doing with school-building upgrades.
Private investment might be slow to arrive when the costs would be so high and the chance of further losses seemed so much greater. Governments would have to draw higher taxes from a weaker economy, just to get us back to today’s living standards.
The Sumatra quake did teach our governments one important lesson: The plight of the victims mattered enormously to people all over the world. Ottawa and Washington, after a feeble first response, realized that public concern has a magnitude of at least 9 on the political scale. Efforts to help the victims in South Asia will continue, but it should also now be politically possible to invest in the necessary infrastructure and emergency planning for B.C.’s vulnerable communities.
We responded to South Asia because we could imagine ourselves in the victims’ place. Some day, inevitably, we will be victims too.
Regular Tyee contributor Crawford Kilian recently re-read his 1983 novel Tsunami and found the descriptions depressingly accurate.