Seismic Microzonation Studies in Ottawa: NBCC-Compliant Ground Motion Characterization

A standard NBCC site classification assigns a single letter class (A through E) based on Vs30 or equivalent SPT/CPT proxies, and applies a pair of site coefficients Fa and Fv. Seismic microzonation goes further: it maps how ground motion amplification varies spatially across the site, computes site-specific response spectra using local modulus degradation curves, and accounts for basin-edge effects and impedance contrasts that a single site class cannot capture. For critical structures in Ottawa—especially those on Leda clay with a strong bedrock contrast—the difference in design spectral acceleration can exceed 20%.

We use a combination of active MASW (multi-channel analysis of surface waves), passive microtremor arrays for deeper velocity information, and downhole seismic in boreholes. MASW works well in Ottawa because the stiff clay and till provide good coupling, and the method captures lateral variability efficiently. For sites near the Rideau River or in areas with known buried valleys, we add refraction microtremor lines to resolve the bedrock depth and shape. All methods follow ASTM D4428 procedures, and we cross-validate with CPTu logs wherever possible.

For a standard site footprint in the Ottawa area—roughly 1 to 5 hectares—a complete seismic microzonation including geophysical surveys, borehole downhole testing, dynamic laboratory testing on selected samples, and 1D site response analysis typically ranges from CA$6,160 to CA$23,080. The range depends on the number of geophysical lines, the depth to bedrock, and whether we need to collect undisturbed Shelby tube samples for resonant column or cyclic triaxial testing. Larger or geologically complex sites with significant lateral variability fall toward the upper end.

Not for every building. NBCC 2020 requires site classification for all structures, but site-specific response analysis—the core of microzonation—is triggered for higher Importance Category structures (post-disaster, schools, major assembly) or when the site conditions are unusual enough that the default site coefficients may be unconservative. In Ottawa, the combination of sensitive Leda clay and a shallow, irregular bedrock surface often puts sites into that 'unusual' category. Several municipalities in the National Capital Region also encourage microzonation for large subdivisions as part of the planning approval process, particularly after the lessons learned from the 2010 Val-des-Bois earthquake.