Seismic hazard could pose a severe threat to high-rises in Vancouver, Canada

Research using rigorous deterministic 3D physics-based simulations point out the vulnerability of tall concrete buildings in Vancouver, Canada.

Citation to original article: Kim, S. E., 2021, High-rises at risk: Building codes underestimate Vancouver’s seismic hazard, Temblor, http://doi.org/10.32858/temblor.161 

A research team from the University of British Columbia, with contributions by researchers from the U.S. Geological Survey (USGS) and the University of Washington, Seattle, produced a study published in the Journal of Earthquake Engineering and Structural Dynamics indicating that the existing models used to develop the regional building codes have underestimated Vancouver’s seismic hazard. In general, building codes are regularly revised as researchers develop new methods to better quantify how future earthquakes might affect a region.

The metropolitan area of Vancouver is situated in proximity of the Cascadia Subduction Zone, an active tectonic plate boundary stretching from Northern California to northern Vancouver Island, approximately 1,000 kilometers. It is believed that, in case of a large seismic event generated along the Cascadia Subduction Zone, the intensity of shaking at Metro Vancouver could be amplified by the existence of the Georgia sedimentary basin, which much of the buildings downtown Vancouver are founded on. As the seismic waves propagate through the deep sedimentary deposit of the Georgia basin, their amplitude becomes amplified by the response of the soil. Moreover, propagating seismic waves become trapped within the sedimentary basin due to multiple reflections on the basin’s bedrock boundary. Such amplifying mechanism is not explicitly considered in the current building codes.

Cascadia Subduction Zone (Source: Federal Emergency Management Agency, retrieved from the National Weather Service)

Carlos Molina Hutt, an assistant professor of Structural and Earthquake Engineering at the University of British Columbia and an author of the study characteristically mentioned: “We’re surrounded by mountains, and we have most of our buildings and infrastructure in this sedimentary basin.” Investigating how the Georgia sedimentary basin could affect the intensity of shaking in the event of a large earthquake along the Cascadia Subduction Zone was one of the primary goals of the study.

The research team adopted a different approach in developing the simulated earthquake scenarios for their study. Instead of relying on published ground motion models, which define Canada’s national seismic hazard, they developed a technique altogether, i.e., they defined a simulated earthquake in the Cascadia Subduction Zone, the “M9 scenario.” The differentiated approach was adopted due to the fact that typically ground motion models are based on observations from past earthquakes. Nonetheless, in the Metro Vancouver area, the last large-magnitude Cascadia Subduction Zone earthquake occurred more than 300 years ago, therefore there is a lack of local earthquake records within the database used to develop the existing ground motion models.

To circumvent this limitation, researchers from the U.S. Geological Survey (USGS) and the University of Washington, Seattle developed an M9 simulation by generating a detailed 3D physics-based computational model of the entire region. A suite of simulated M9 earthquake records were then propagated though the model and the response of the system elements were observed. Indeed, the results of the simulations confirmed the soft Georgia basin amplifies long period seismic waves.

Moreover, Carlos Molina Hutt and his team investigated how, commonly encountered within Metro Vancouver, concrete shear wall structures would behave during an earthquake scenario such as the simulated one. The authors concluded that high-rise buildings are more vulnerable than the shorter ones. The taller structures are associated with longer natural periods of vibration, which in this case match with the long period ground shaking amplified within the Georgia basin. This “double resonance” phenomenon can prove catastrophic for tall buildings, as seen during the Mexico City M8.0 1985 earthquake.

Additionally, the research team found that older buildings designed based on Canada’s building codes before 1990 are at higher risk of severe damage or even collapse than more recently constructed buildings. “Some of these tall buildings that were constructed before 1990 are not going to fare too well [in an earthquake],” says Carlos Molina Hutt, while he adds: “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. What we want to do is raise awareness.” He hopes this work can incentivize the canadian government “to explore whether there’s a need for mandated assessment of certain types of buildings or a mandated retrofit. Our work is helping to shape that discussion and inform that conversation,” Carlos Molina Hutt concluded.

References

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. https://doi.org/10.1002/eqe.3361