Western Study Provides Detailed Mapping of Seismic Activity in Metro Vancouver

While earthquake prediction is next to impossible, it is possible to design and build infrastructure to withstand seismic hazards like earthquake shaking if you have the right data. After seven years of investigation, a Western-led research team is revealing what they’ve learned about the variability in earthquake hazards for the metro Vancouver area, one of the most seismically active regions in Canada.

Earth sciences professor Sheri Molnar and her team, supported by Western’s Institute for Catastrophic Loss Reduction and British Columbia’s Ministry of Emergency Management and Climate Readiness, lead the Metropolitan Vancouver seismic microzonation mapping project. The 10-year (2017-2026) investigative research study assesses, analyzes and maps seismic hazards across Canada’s third-largest city centre with the aim of informing emergency preparedness, land-use planning, seismic risk and loss assessment and prioritizing seismic retrofit programs.

“We have known metro Vancouver’s susceptibility to seismic activity for some time, but for the first time ever, we now have a series of maps that systematically and comprehensively detail the subsurface ground conditions of the region, which can be used to predict the amplitude and frequency content (from rapid vibrations to rocking or swaying) of earthquake shaking,” said Molnar.

Using seismometers (instruments that record the Earth’s ground vibrations), Molnar and her team measured the amplitude and period (or frequency) of ground vibration across metro Vancouver from Sunset Beach to Surrey and Tsawwassen to Coquitlam. Their work was systematic – in some places tracking every 200 metres – and physically tough.

Since 2017, Molnar and the team have measured seismic vibrations at more than 2,300 locations and collected available geological data for another 15,000 unique sites for the analysis. The ground preferentially filters background vibrations based on the soil’s material properties and thickness, just as it would during low intensity earthquake shaking, making the exercise difficult but indispensable as it relates to emergency preparedness.

The current result is 29 highly detailed, comprehensive region-specific seismic hazard maps based on two different shaking intensity levels over 475 or 2,475 years. Some of these maps show where the ground is susceptible to changes in shaking intensity and occurrence of soil liquefaction and land sliding while others show the relative hazard potential of changes in shaking intensity and triggering of liquefaction and landslides given the region’s shaking intensity for the two chosen return periods. Soil liquefaction occurs when pressures build in water-logged sandy soils during earthquake shaking resulting in solid ground behaving temporarily like a liquid.

Design engineer codes, based on these seismic vibrations, are specific to sites or structures. Larger-scale seismic hazard assessments require regional seismic hazard mapping, where the relative differences in seismic shaking across an area are identified and mapped. This process is known as microzonation.

“Regional-scale assessments, like the one we are doing here in metro Vancouver, push the national-scale building code predictions of ground motions towards more local site-specific assessments that are typically only accomplished for high-consequence sites of interest,” said Molnar.

All 29 maps from the first phase of the project, which covers western metro Vancouver, will be available to government officials, city planners, insurance agencies, private developers and the public in late July. The next phase of mapping, which begins this summer, will cover the communities east of the Pitt River (Pitt Meadows, Maple Ridge) and east of Surrey (Langley).

These new regional seismic hazard maps and associated databases, technically reviewed by the members of Engineers and Geoscientists British Columbia, will be used by the government and the insurance industry for risk analysis and damage estimation to calculate potential losses resulting from earthquake shaking, landslides and soil liquefactions while the seismic data collected, generally, will lead to a better understanding of the local variability in subsurface ground conditions.

“While technical experts, like earthquake engineering professionals, catastrophe modelers and risk analysts, are the primary audience for these maps, we anticipate these will also be invaluable to emergency managers, land use planners, consultants, architects and other stakeholders,” said Molnar. “It is important to note that the maps show hazard, not risk, so the general public, including developers, real estate agents, insurance agents and homeowners, should consider expert opinion before making personal decisions.”

Engineers and Geoscientists British Columbia has developed professional guidelines for development and use of seismic microzonation mapping in the province, which are available online.

“These maps will make Vancouver a safer place to live, work and play,” said Molnar, who completed her PhD studies at University of Victoria and worked as a postdoctoral fellow in earthquake engineering at University of British Columbia.

Molnar also served as president and director of British Columbia Regional Chapter of the Earthquake Engineering Research Institute from 2013-2017.