State of the art simulations reveal that Vancouver’s tall concrete buildings are especially vulnerable to some types of seismic waves.
By Shi En Kim (@goes_by_kim)
Citation: Kim, S. E., 2021, High-rises at risk: Building codes underestimate Vancouver’s seismic hazard, Temblor, http://doi.org/10.32858/temblor.161
If you’ve lived through a serious earthquake and the building you were in emerged relatively unscathed, you may have local building codes to thank. Building codes are construction standards that are guided by science and set by the government to uphold public health, safety and welfare. In an earthquake-prone region, these guidelines often require foundational supports beneath the soil or limit the allowed types of building materials, both of which determine the ability of the building to withstand a specified level of shaking.
Building codes are regularly revised as scientists develop new ground motion models to more accurately describe how future earthquakes might affect a region. In Metro Vancouver — Canada’s third largest metropolitan area and the region with the country’s highest seismic risk — the existing models used to develop present-day building codes have underestimated the region’s seismic hazard, according to a new study led by researchers from the University of British Columbia.
Metro Vancouver lies near the Cascadia Subduction Zone, an active tectonic plate boundary stretching 620 miles (1,000 kilometers) from Northern California to northern Vancouver Island. Should the fault act up, the impact would be exacerbated by the fact that Metro Vancouver sits on the Georgia sedimentary basin. Here, the deep sedimentary deposits of the basin are softer and less compact than the surrounding bedrock. This type of rock will amplify shaking caused by seismic waves.
“We’re surrounded by mountains, and we have most of our buildings and infrastructure in this sedimentary basin,” says Carlos Molina Hutt, referring to Metro Vancouver. Molina Hutt is a professor of structural and earthquake engineering at the University of British Columbia and an author of the study. He and his co-authors looked at how the Georgia sedimentary basin — a factor not explicitly considered in the current building code, he says — could affect shaking in the event of a large earthquake in the Cascadia subduction zone.
Simulating shaking in Metro Vancouver
Ground motion models, which serve as the foundation for Canada’s national seismic hazard model, typically rely on past observations of earthquakes from all around the world. The data are then extrapolated to other parts of the globe such as Metro Vancouver. But this area has unique geological features not captured by these models, such as its soft sedimentary basin. Additionally, the last large-magnitude Cascadia subduction zone earthquake here occurred more than 300 years ago, so there are few records of past earthquakes that ground motion models can consult.
To improve these models, Molina Hutt and his colleagues instead relied on a different technique altogether: observations of shaking from a simulated earthquake in the Cascadia Subduction Zone, dubbed the “M9 scenario.” Researchers from the U.S. Geological Survey (USGS) and the University of Washington, Seattle developed the M9 simulation by creating a detailed 3D computer model of the Cascadia landscape, within which they triggered an artificial magnitude-9.0 earthquake. The researchers generated 30 possible earthquake scenarios, varying parameters such as the earthquake location and the direction of the rupture — toss-up parameters in future earthquakes. Molina Hutt and his co-authors used these M9 simulations to determine how much the sedimentary basin could amplify shaking and what effect this shaking could have on buildings.
The M9 simulations work better than traditional ground motion models because they’re more detailed and location-specific, says Molina Hutt. The team’s calculations show that the M9 simulations highlight what traditional ground models had missed: The soft Georgia basin amplifies long period earthquake waves.
After extracting the relevant earthquake details from the M9 scenarios, Molina Hutt and his colleagues simulated how concrete shear wall structures — the most common type of high-rise in Metro Vancouver — would fare during an earthquake. They found that high-rise buildings like those found in Metro Vancouver are more vulnerable than their shorter neighbors. Just like how a longer pendulum traces out wider arcs in space, taller buildings will sway farther in an earthquake, with longer time intervals for each sway. This long period sway matches long period ground shaking that the researchers found to be amplified in the basin. That’s bad news, because similar periods of shaking translate to more efficient transfer of destructive energy from ground to building. The team found that older buildings adhering to Canada’s building codes predating 1990 are at higher risk of severe damage or even collapse than modern buildings.
Older buildings are at risk
“Some of these tall buildings that were constructed before 1990 are not going to fare too well [in an earthquake],” says Molina Hutt. “Obviously, there’s a lot of work that’s needed to better quantify the impact [of earthquakes] on these buildings and see what we can do to enhance their performance.”
The study is important for guiding new policy, says Tiegan Hobbs, a research scientist for the Geological Survey of Canada who was not involved in the study. She says the Vancouver area has many tall concrete buildings from the 60s and 70s. Molina Hutt and his co-authors were able to “use the most state-of-the-art research to figure out exactly how much of a problem [these buildings are].”
“To have studies that look at the seismic risk from a really rigorous engineering approach, like the simulations [this team] was doing on these concrete building typologies is a great contribution,” says Hobbs.
Old buildings erected under outdated building codes aren’t required by law to be retrofitted to comply with the most recent codes. To enact such a policy would be costly and complicated. Molina Hutt says he doesn’t recommend a sweeping mandate just yet. For now, researchers are still trying to accurately determine the seismic risk to different types of buildings in the area. “What we want to do is raise awareness,” says Molina Hutt. He hopes this work can push the government “to explore whether there’s a need for mandated assessment of certain types of buildings or a mandated retrofit.” He says, “Our work is helping to shape that discussion and inform that conversation.”
Kakoty P., Dyaga S. M., Molina Hutt C. (2021) Impacts of simulated M9 Cascadia Subduction Zone earthquakes considering amplifications due to the Georgia sedimentary basin on reinforced concrete shear wall buildings. Earthquake Engineering and Structural Dynamics, 50, 237–256.