Thunder Bay sits on the Canadian Shield's edge, where Precambrian bedrock meets deep glaciolacustrine clays from Lake Superior's postglacial history. That geological contrast creates a seismic puzzle most standard code maps ignore. Ground shaking here isn't just a function of distance to a fault—it's amplified or dampened by the layered sediments beneath each neighborhood, from Port Arthur's thick varved clays to Fort William's till-over-shale sequences. We run microzonation campaigns that map local site response using direct downhole measurements, MASW profiles, and lab-based dynamic testing on undisturbed samples. The goal is practical: help structural designers, planners, and municipal engineers understand exactly how the ground will behave during a seismic event, so they can place critical infrastructure where the hazard is lowest and design foundations tuned to real spectral accelerations, not generic assumptions. For sites near the Kaministiquia River floodplain or the Mission Marsh area, where young alluvium overlies stiff glaciolacustrine deposits, a CPT test often provides the continuous stratigraphy needed to feed the seismic response model without losing the soft-layer signatures that borehole logs alone can miss.
Two boreholes 200 metres apart in Thunder Bay can sit on different NBCC site classes—and that changes the design seismic force by 30 % or more.
Process and scope
Thunder Bay's development pattern—stretching along the Lake Superior shoreline and climbing the Nor'Wester Mountains—means a single seismic microzonation map must span everything from shallow bedrock on ridge tops to 30-metre clay basins in the lowlands. The 1960s and 70s expansion onto reclaimed marshland introduced fill zones where seismic amplification factors can spike unexpectedly. We've built our microzonation workflow around the National Building Code of Canada (NBCC 2020) site classification framework, but we go further by calibrating Vs30 maps with measured shear wave velocities rather than proxy correlations. Each study starts with a desktop seismotectonic review—identifying active source zones, recurrence intervals, and attenuation relationships—then moves into field acquisition. We deploy surface wave arrays (MASW) and downhole seismic in boreholes to capture velocity profiles, and we extract thin-walled Shelby tube samples from key horizons for resonant column and cyclic triaxial testing. The output is a grid of ground motion parameters (PGA, spectral accelerations at 0.2s, 1.0s, 2.0s) mapped across the study area, often revealing that sites separated by a few hundred metres can fall into different NBCC site classes—a difference that shifts the design spectrum significantly for a mid-rise building.
Site-specific factors
We reviewed a proposed four-storey medical clinic near the Thunder Bay Regional Health Sciences Centre where the initial geotechnical report assumed NBCC Site Class C based on a single borehole. Our microzonation mapping across the parcel—just over two hectares—showed the southern half actually fell into Site Class D because of a buried soft clay lens that the single borehole missed entirely. The design team had to upgrade lateral force-resisting elements on that wing, adding about six percent to the structural steel weight but avoiding a critical under-design. This is the hidden cost risk with microzonation work: skipping it doesn't save money, it just moves the risk downstream to when steel is already ordered. In Thunder Bay's glaciolacustrine terrain, where soft varved silts can pinch out abruptly against till ridges, even a small site can straddle two seismic site classes. Municipal planners specifying zoning envelopes for new subdivisions near the Neebing River or Current River corridors face the same challenge at a larger scale—without sub-metre resolution Vs30 data, they risk permitting construction in amplification hotspots that a generic hazard map won't flag.
Applicable standards
NBCC 2020 (National Building Code of Canada, seismic provisions), CSA A23.3 (Design of concrete structures, seismic), ASTM D7400 (Downhole seismic testing), ASTM D4015 (Resonant column testing), ASTM D3999 (Cyclic triaxial testing)
Common questions
What does a seismic microzonation study cost for a typical project in Thunder Bay?
Budget ranges from CA$4,890 for a single-structure site with existing borehole data up to about CA$21,730 for a neighbourhood-scale campaign involving field surveys, multiple boreholes, and full dynamic lab testing. The spread depends on area size, depth to bedrock, and the number of Vs measurement points required to achieve the mapping resolution your structural engineer needs.
How long does a microzonation study take from fieldwork to final report?
Field acquisition typically runs one to two weeks for a moderate-sized site, depending on access and weather. Lab dynamic testing adds another three to four weeks because resonant column and cyclic triaxial tests on fine-grained glaciolacustrine soils require careful saturation and consolidation stages. The integrated report, including GIS deliverables and design spectra, is usually ready six to eight weeks after fieldwork wraps up.
Is microzonation required by the Ontario Building Code for Thunder Bay?
The Ontario Building Code references NBCC 2020 for seismic design, which mandates site classification based on Vs30 or other accepted methods. For critical facilities—hospitals, emergency response centres, schools—and for large subdivisions on variable ground, many Thunder Bay planning approvals now request a site-specific microzonation rather than relying on the default Site Class C assumption, because the cost of getting it wrong can be substantial.
