The ground beneath Westwood Plateau tells a completely different story than the alluvial soils along the Fraser River near Maillardville. On the plateau, stiff glacial till and bedrock outcrops keep amplification factors modest. Down in the lowlands, however, we encounter deep sequences of soft silts and clays that can amplify long-period ground motion by a factor of two or more. Coquitlam sits in one of Canada’s highest seismic hazard zones, and the 2020 NBCC requires site-specific analysis for structures on Site Class D, E, or F soils. Our MASW surveys map shear-wave velocity profiles across these transitions, while deep SPT drilling provides the stratigraphic control needed to run one-dimensional ground response models that capture how each soil column will actually behave during a design earthquake.

In Coquitlam, two sites separated by 500 meters can show a 40% difference in spectral acceleration at 1 second period—purely due to soil column thickness.

Methodology and scope

The bedrock beneath Coquitlam is part of the Coast Plutonic Complex, but the overburden varies from zero to over 30 meters of glaciolacustrine silts and sandy outwash. This contrast creates sharp lateral changes in Vs30 that a regional hazard map cannot resolve. We run multi-channel surface wave lines at 50 to 100-meter spacing, then calibrate the results with CPT soundings where access permits, to obtain a continuous profile of cone resistance and pore pressure. Most commercial projects require characterization down to at least 30 meters per ASTM D4428, but for critical facilities we extend the bedrock model to 100 meters to capture basin-edge effects. The output is a grid of site amplification factors, fundamental periods, and response spectra that feed directly into structural analysis. For sites with marginal bearing capacity, we also integrate liquefaction triggering analysis using the NCEER semi-empirical procedure calibrated to local SPT blow count data.

Local considerations

One pattern we see repeatedly in Coquitlam is structural damage concentrated along the transition zone between bedrock and deep soil—a classic basin-edge effect. Buildings founded partly on till and partly on soft sediment undergo differential ground motion that linear-equivalent analysis often underestimates. The 2020 NBCC provides default amplification factors, but these are conservative envelopes that can drive foundation costs unnecessarily high. A site-specific microzonation study replaces generic factors with measured values, often reducing design spectral accelerations by 15 to 25% for short-period structures on stiff soil. The real danger is missing a thin liquefiable sand lens at 6 to 10 meters depth; these are common in the Coquitlam River floodplain and can trigger flow failure during a Cascadia event. Our field program is designed to catch those lenses before the structural engineer finalizes the foundation design.

Technical data

Parameter	Typical value
Vs30 range in Coquitlam	180 m/s (soft soil) to >760 m/s (bedrock)
Typical overburden thickness	3–35 meters across the study area
Site Classes (NBCC 2020)	C, D, E mapped; F for liquefiable soils
Maximum considered earthquake (MCE)	Mw 7.3 from Cascadia subduction zone
Peak ground acceleration (PGA) at rock	0.45g–0.55g for 2% in 50-year hazard
Key measurement standard	ASTM D4428/D4428M for cross-hole and surface wave
Amplification factor range	1.0 (bedrock) to 2.4 (deep soft soil)

Associated technical services

Surface Wave Geophysics (MASW/MAM)

We deploy 24-channel arrays with 2 Hz geophones to measure shear-wave velocity to 30–100 m depth. Results are inverted to 1D Vs profiles and interpolated into a 3D velocity model for the full parcel.

Deep Borehole Calibration

Mud-rotary or hollow-stem auger borings to 30–50 m, logged by a geotechnical engineer. SPT N-values every 1.5 m plus undisturbed Shelby tube samples for index and dynamic laboratory testing.

Site Response Analysis

One-dimensional equivalent-linear (SHAKE) or non-linear (DEEPSOIL) modeling using input motions scaled to the NBCC uniform hazard spectrum. Output includes surface acceleration time histories and design response spectra.

Liquefaction and Lateral Spread Mapping

Cone penetration testing paired with SPT data to evaluate factor of safety against liquefaction per NCEER methodology. We map post-liquefaction settlement and lateral spreading displacement for each design ground motion level.

Quick answers

Does the City of Coquitlam require a site-specific seismic study for a four-story wood-frame building?

Generally, no—most four-story wood-frame structures on Site Class C can use the NBCC 2020 static procedure. But if your geotechnical investigation classifies the site as Class D, E, or F, or if the building has vertical irregularities, the code triggers site-specific analysis. We always recommend checking the preliminary borehole data before making that call.

How long does a full microzonation study take in Coquitlam?

A typical program—MASW survey, two to four calibration boreholes, laboratory testing, and site response modeling—takes six to eight weeks from mobilization to final report. Sites with complex stratigraphy or deep liquefaction assessment may require an additional two weeks for non-linear analysis.

What ground motion records do you use for the site response analysis?

We select and scale real accelerograms from the PEER NGA-West2 database that match the NBCC 2020 uniform hazard spectrum for Coquitlam at the 2% in 50-year hazard level. Typically we use seven pairs of horizontal motions to meet the code's mean-spectrum matching requirement.

What is the typical cost range for a seismic microzonation study in the Coquitlam area?

Can you use existing geotechnical data from a previous investigation?

Yes, and we encourage it—it can reduce the number of new boreholes. We need the original field logs, SPT blow counts, and laboratory test results. We then fill gaps with targeted surface wave lines or a CPT sounding to build a defensible site model without duplicating work already paid for.

Seismic Microzonation in Coquitlam: Site-Specific Ground Response Analysis