Seismic guidelines underestimate impact of 'The Big One' on metro Vancouver buildings

November 30, 2020

Scientists examining the effects of a megathrust earthquake in the Pacific Northwest say tall buildings across Metro Vancouver will experience greater shaking than currently accounted for by Canada's national seismic hazard model.

The region lies above the Georgia sedimentary basin, which is made up of layers of glacial and river sediments sitting on top of sedimentary rock. In the event of an earthquake, it would jiggle and amplify the seismic waves, causing more intense and longer-lasting tremors. However, the amplification caused by the sedimentary basin is not explicitly accounted for in the 2015 seismic hazard model, which informs Canada's national building code.

The latest U.S. national seismic hazard model now explicitly accounts for sedimentary basin amplification, but Canada's latest seismic hazard model, released this October, still doesn't, says lead researcher Carlos Molina Hutt, a structural and earthquake engineering professor at UBC.

"As a result, we're underestimating the seismic hazard of a magnitude-9 earthquake in Metro Vancouver, particularly at long periods. This means we're under-predicting the shaking that our tall buildings will experience," he warned. "Fortunately, Natural Resources Canada, responsible for the development of our national seismic hazard model, recognizes the potential importance of basin effects in certain parts of Vancouver and is actively reviewing and participating in research on the topic. They intend to address basin effects in the next seismic hazard model."

Using physics-based computer simulations, the researchers found that regions where the Georgia Basin is deepest will have the greatest seismic amplification. Delta and Richmond will experience the most amplification, followed by Surrey, New Westminster, Burnaby, Vancouver and North Vancouver. West Vancouver, which sits just outside the basin, will have the least.

Older, tall buildings at greater risk

The researchers also evaluated the impact of the magnitude-9 simulations on tall reinforced concrete shear wall buildings, of which there are more than 3,000 located in the Lower Mainland. They found that those built to building codes from the 1980s and earlier are at the greatest risk of severe damage or even collapse, with buildings in the 10- to 20-storey range experiencing the worst impacts.

"We have these pockets of tall buildings within the Georgia Basin--in Vancouver, Burnaby, Surrey and New Westminster. In general, based on a comparison of the code requirements in the past versus the code requirements now, many of our older buildings are vulnerable to these large earthquakes, particularly if we consider the amplification effect of the Georgia Basin," said Molina Hutt. The differences in expected performance between new buildings and older constructions reflects continuous improvements in seismic hazard estimates and engineering design provisions.

"When we build a structure, it only needs to meet the code of the time when it was built. If there is a future change in the code, you don't have to go back and upgrade your building. To address vulnerable existing buildings, jurisdictions must explore different seismic risk reduction policy options and adopt the most effective mitigation strategies," Molina Hutt added.

The study, published recently in Earthquake Engineering & Structural Dynamics, notes that concrete is the predominant construction material for buildings taller than eight storeys in the city of Vancouver, constituting 90 per cent of a total 752 buildings identified. Of these, more than 300 are reinforced concrete shear wall constructions that pre-date 1980.

"Typically, people think that, if we have a magnitude-9 Cascadia subduction zone earthquake, it will be worse in Victoria, because they're closer to the seismic source. But the reality is that, for tall buildings, we're going to be worse off in Vancouver, because this basin amplifies the shaking in taller structures," Molina Hutt noted. The probability of a magnitude 8 or 9 Cascadia earthquake is estimated to be 14 per cent in the next 50 years.

"We're collaborating closely with our neighbours to the south, who are taking active steps to account for these basin amplification effects," said Molina Hutt. "Our work attempts to assess the impacts of neglecting these effects so we can appreciate their significance and take action."
"Impacts of simulated M9 Cascadia Subduction Zone earthquakes considering amplifications due to the Georgia sedimentary basin on reinforced concrete shear wall buildings" was published recently in Earthquake Engineering & Structural Dynamics.

University of British Columbia

Related Earthquake Articles from Brightsurf:

Healthcare's earthquake: Lessons from COVID-19
Leaders and clinician researchers from Beth Israel Lahey Health propose using complexity science to identify strategies that healthcare organizations can use to respond better to the ongoing pandemic and to anticipate future challenges to healthcare delivery.

Earthquake lightning: Mysterious luminescence phenomena
Photoemission induced by rock fracturing can occur as a result of landslides associated with earthquakes.

How earthquake swarms arise
A new fault simulator maps out how interactions between pressure, friction and fluids rising through a fault zone can lead to slow-motion quakes and seismic swarms.

Typhoon changed earthquake patterns
Intensive erosion can temporarily change the earthquake activity (seismicity) of a region significantly.

Cause of abnormal groundwater rise after large earthquake
Abnormal rises in groundwater levels after large earthquakes has been observed all over the world, but the cause has remained unknown due to a lack of comparative data before & after earthquakes.

New clues to deep earthquake mystery
A new understanding of our planet's deepest earthquakes could help unravel one of the most mysterious geophysical processes on Earth.

Fracking and earthquake risk
Earthquakes caused by hydraulic fracturing can damage property and endanger lives.

Earthquake symmetry
A recent study investigated around 100,000 localized seismic events to search for patterns in the data.

Crowdsourcing speeds up earthquake monitoring
Data produced by Internet users can help to speed up the detection of earthquakes.

Geophysics: A surprising, cascading earthquake
The Kaikoura earthquake in New Zealand in 2016 caused widespread damage.

Read More: Earthquake News and Earthquake Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to