Many bridges, dams and other major infrastructure around the Lower Mainland have undergone seismic upgrades, but questions linger as to exactly how big an earthquake they could withstand and which structures might fail.
Public imagination focuses on “the big one” – a magnitude 8 to 9 super-quake centred far offshore and deep beneath the seabed in the Cascadia subduction zone.
That would be similar to the magnitude 9.0 earthquake that has devastated Japan and would rock the entire Pacific Northwest.
But experts in the field worry more about a much closer and shallower earthquake in the 6.5 to 7.5 range that could strike at any time, rather than the every 500 years typical of Cascadia quakes.
Such a quake close to Vancouver might last only 30 seconds rather than minutes but be more intense and cause more damage to buildings and structures in the region, says SFU earth sciences professor John Clague.
“The greatest hazard is really conveyed by these magnitude 7 local earthquakes,” he said. “You get stronger ground acceleration from those.”
The impact depends on how far away the epicentre is and how deep it is underground, as well as local soil conditions.
A 7.0 quake centred almost on top of Vancouver would be much more destructive than the identical quake 80 or 100 kilometres away.
Clague said he remains concerned various buildings, bridges and transportation links would fail.
“I would worry about critical infrastructure on the Fraser River delta,” he said, citing the airport, Tsawwassen ferry terminal and Deltaport as vulnerable to damage from soil liquefaction.
The phenomenon where shaken ground turns to mush could happen not just in the silty lowlands of Richmond and Delta, he said, but anywhere large amounts of fill underlies buildings or structures.
“We’ve got all this development all around False Creek built on a lot of fill,” Clague said, noting that vulnerable area extends east as far as Clark Drive.
Rock slides would almost certainly plug the Sea-to-Sky Highway and possibly the Fraser Canyon, Clague said.
“You’re limited then, particularly if the airport goes, in how you can get relief into the area,” he said.
“If bridges go, you can end up with parts of the Lower Mainland being isolated from one another without the ability to provide relief except internally with whatever resources you have.”
Transportation ministry officials say new bridges – like the 10-lane Port Mann under construction and new Pitt River bridge – and seismic upgrades on other key spans are designed to withstand a 6.5 to 7.0 local earthquake centred on Vancouver or an up to magnitude 8.0 Cascadia megathrust earthquake.
Engineers expect them to “dance” but not fall in those types of quakes.
The Alex Fraser Bridge, built in 1986, also meets stringent seismic standards, according to the ministry.
The more than $105 million spent on refits in recent years have focused on older “lifeline bridges” needed to connect communities and serve designated disaster response routes.
So far, the Lions Gate, Ironworkers Memorial, Oak Street, Queensborough bridges and the Massey Tunnel have been retrofitted to the seismic standard.
The Mission Bridge is being retrofitted and seismic upgrading is in the planning stage for one more lifeline bridge – the Agassiz-Rosedale Bridge.
Overpasses are also being examined and the Port Mann/Highway 1 project includes six retrofits, 15 replacements and 21 new structures all built to the current seismic standard.
TransLink spokesman Ken Hardie said the Golden Ears Bridge is also built to the latest standard.
Engineers estimate the new Langley-to-Maple Ridge span faces a one-in-10 chance in its 50-year life of being temporarily shut for inspections due to a quake, with more serious damage much less likely.
Other TransLink-controlled bridges – the Pattullo and Knight Street – would be more vulnerable to a major quake, particularly the 73-year-old Pattullo.
“We’ve done some upgrading on the Pattullo, but certainly nothing that is going to prevent it from being closed if we have any kind of major incident,” Hardie said.
“The choice there was not to invest the large amount of money in the Pattullo if we’re going to replace it.”
In a major quake, he said, TransLink would expect to see the Knight and Pattullo closed for inspections, with the Knight likely reopening sooner than a potentially more damaged Pattullo.
The Knight – and any other bridge or overpass touching down in Richmond – could be affected by soil liquefaction.
“The structure could be fine but the approach could be out of commission,” Hardie noted.
Delta Mayor Lois Jackson said she has strong confidence dikes that protect Delta would hold in a major quake.
Cleveland Dam, which was seismically upgraded to current standards in the 1990s, holds back Capilano Reservoir. FILE PHOTO
Most local dams up to latest earthquake standard
A burst dam during Japan’s earthquake that flooded a downstream town has also put the spotlight on the seismic readiness of dams around the Lower Mainland.
Metro Vancouver has upgraded both its Cleveland Dam and Seymour Falls Dam that hold back the two North Shore reservoirs to a similar seismic standard as the province’s bridges.
In a total failure, the Cleveland Dam could release a deluge of water from the Capilano Reservoir towards the Park Royal shopping centre and the north footings of the Lions Gate Bridge in West Vancouver.
The failure of the dam at the Seymour Reservoir might threaten downstream homes in the Riverside Drive area of North Vancouver.
And B.C. Hydro’s Coquitlam Dam holds back Coquitlam Lake above populated parts of Coquitlam and Port Coquitlam and low-lying areas around Derby Reach Regional Park.
But Metro Vancouver security and emergency planning coordinator Rod Tulett said a catastrophic failure where a dam instantly vanishes is “highly unlikely.”
A partial failure would mean a much slower water release and less damage downstream, he said.
Tulett said another area of concern is slope stability, citing the potential for mud slides or land slides on the North Shore and lower Sea-to-Sky corridor, particularly if a quake strikes during a late fall rain storm when soils are saturated.
B.C. Hydro finished a $65-million seismic upgrade of the Coquitlam Dam in 2008, while Metro finished a $50-million retrofit at Seymour Falls in 2009.
Both now meet Canadian Dam Association Guidelines, which require them to withstand a one-in-10,000 year probability quake.
“That will survive literally any amount of shaking at that point,” said Stephen Rigbey, B.C. Hydro’s manager of dam safety, adding they would certainly withstand a 7.0 local quake centred on Vancouver.
He noted seven to eight major dams were damaged last year in China when a magnitude 8.7 quake struck.
“None failed and all were able to withstand to a degree where they were able to lower all the reservoirs and fix the dams,” Rigbey said.
B.C. Hydro has also just announced an $800-million upgrade of its old Ruskin Dam, which holds back Hayward Lake in Mission, in part to make it safer in the event of an earthquake.
Metro Vancouver is also spending big money on ongoing upgrades.
A $230-million new water tunnel beneath the Fraser River to serve the Surrey area is being built in part because the existing pipe is vulnerable to being knocked out by even a minor quake.
A report released last year by the Institute for Catastrophic Loss Reduction also warned that earthquake-triggered fires have potential to cause more damage in Metro Vancouver than the shaking.
Downtown Vancouver is considered particularly vulnerable, because transformers on wooden poles may arc and explode, sparking fires in buildings that are often just inches away.
“Vancouver appears to be the only major city in North America that has not relocated its electric transmission underground in the city core,” it said.
Potential disruption of water and sewer lines are also a major concern.
Metro Vancouver’s Tulett said the experience in Japan – arguably the best prepared nation in the world – should be a wake-up call for residents here.
“Everybody should be prepared for at least 72 hours unaided,” he said. “As people have seen in Japan or New Zealand, there is nobody to help you for the first few days.”