Seismic Zone Canada — NBCC 2020 · Sa(T) and kh by region
The National Building Code of Canada 2020, Division B Section 4.1.8, adopts the 5th generation seismic hazard model published by Natural Resources Canada (NRCan) in 2020. Ground motion is given as spectral acceleration Sa(T) at periods 0.2 s, 0.5 s, 1.0 s, 2.0 s, 5.0 s and 10.0 s for a 2% probability of exceedance in 50 years (2,475-year return period). Hazard is highest on the west coast (Cascadia Subduction Zone and Queen Charlotte Fault), moderate in the Saint Lawrence and Ottawa Valleys (stable continental interior seismicity), and low across the Prairies and much of the Canadian Shield. This tool returns regional Sa(0.2) values and the pseudo-static horizontal coefficient kh used for retaining walls, slopes and foundations.
What it is and when it applies
NBCC 2020 requires every new building to be designed for the mapped spectral accelerations at the project location, adjusted by Site Class (A through F per Clause 4.1.8.4) and Importance Category (Low, Normal, High, Post-Disaster). NRCan provides the 5th generation hazard values through the online Seismic Hazard Calculator at earthquakescanada.nrcan.gc.ca. For geotechnical design, PGA is derived from Sa(0.2)/2.5 or obtained directly from NRCan, then the pseudo-static horizontal coefficient is built: kh = 0.5·PGA/g for retaining walls and slopes with acceptable Newmark displacement (10-25 cm per Hynes-Griffin & Franklin 1984), and kh = PGA/g for rigid structures. Application is mandatory for buildings, bridges (CSA S6 Canadian Highway Bridge Design Code), industrial facilities, embankments and tailings dams (CDA 2013 guidelines).
Governing equations
Spectral acceleration and PGA (NBCC 2020 Clause 4.1.8.4):
S(T) = F(T,PGAref) · Sa(T), with F(T) the site coefficient for site-specific period
PGA ≈ Sa(0.2) / 2.5 for preliminary geotechnical assessment
Pseudo-static horizontal coefficient:
kh = 0.5·(PGA/g) → retaining walls and slopes with acceptable deformation
kh = 1.0·(PGA/g) → rigid structures with no tolerated deformation
kv = 0 (typical) or kv = ±0.5·kh (near-fault and critical projects)
Surface acceleration with site factor (NBCC 2020 Table 4.1.8.4-B):
amax = F(PGA) · PGA, with F(PGA): A=0.8, B=1.0, C=1.2, D=1.4, E=1.7
Mononobe-Okabe seismic earth pressure:
θ = arctan(kh/(1−kv)), then Kae follows the classic formulation
Worked example
| Parameter | Value |
|---|---|
| Location | Burnaby, BC — high Cascadia hazard |
| Sa(0.2) | 0.75 |
| PGA (from Sa(0.2)/2.5) | 0.30 g |
| Site Class D (stiff soil) | F(PGA) = 1.4 |
| Use | Retaining wall with acceptable deformation |
| Pseudo-static kh | 0.5 × 0.30 = 0.15 |
| Surface amax | 1.4 × 0.30 = 0.42 g |
With kh = 0.15 the Mononobe-Okabe seismic earth pressure gives θ = arctan(0.15/(1−0)) = 8.5°. That Kae multiplies the backfill weight above the wall and adds to the static Rankine or Coulomb thrust. Required checks per CSA S6 and CFEM (Canadian Foundation Engineering Manual, 5th ed.): FSoverturning ≥ 1.5 static / 1.1 seismic, FSsliding ≥ 1.5 static / 1.1 seismic. In Vancouver, liquefaction assessment is typically required in Fraser River delta deposits per BC Ministry of Transportation guidelines.
Result: Vancouver · Sa(0.2) = 0.75 · PGA = 0.30 g · kh = 0.15 · amax = 0.42 g · apply in Mononobe-Okabe with CFEM checks
Regional hazard table (NBCC 2020 / NRCan 2020)
| Level | Sa(0.2) | Representative regions |
|---|---|---|
| Very high | ≥ 1.00 | West coast Vancouver Island, Haida Gwaii (Queen Charlotte Fault), Tofino, offshore BC |
| High | 0.50-1.00 | Vancouver, Victoria, Lower Mainland, Whistler, west Fraser Valley |
| Moderate | 0.25-0.50 | Montreal, Ottawa, Quebec City, La Malbaie (Charlevoix seismic zone), Trois-Rivières |
| Low | < 0.25 | Toronto, Prairies (Calgary, Edmonton, Regina, Winnipeg), most of Ontario interior, Atlantic provinces interior |
Interpretation of results
Canadian seismic hazard is dominated by three regimes. The west coast is shaped by the Cascadia Subduction Zone (interplate megathrust capable of M9 events, last in 1700) and the Queen Charlotte transform fault (M7.8 in 2012). The Saint Lawrence and Ottawa Valleys host stable continental interior earthquakes, with the Charlevoix seismic zone producing M6-7 events (1663, 1925) and the Western Quebec zone generating M5+ events. The Prairies and Shield are seismically quiet with Sa(0.2) typically under 0.10. For geotechnical work, Sa(0.2) ≥ 0.50 triggers liquefaction screening in saturated sands per the CFEM and BC MOTI guidelines; tailings dams in BC, Yukon and NWT follow CDA 2013 with site-specific PSHA regardless of the NBCC value. Vancouver and Victoria also require consideration of basin amplification effects in the Georgia Strait sediments.
Reference standards
- NBCC 2020 — National Building Code of Canada, Division B Section 4.1.8
- NRCan 2020 — 5th generation seismic hazard model for Canada
- CSA S6:19 — Canadian Highway Bridge Design Code
- CFEM 5th ed. 2023 — Canadian Foundation Engineering Manual (CGS)
- CDA 2013 — Canadian Dam Association, Dam Safety Guidelines
- Hynes-Griffin & Franklin (1984) — Rationalizing the seismic coefficient method
- Boulanger & Idriss (2014) — CPT and SPT based liquefaction triggering procedures
Frequently asked questions
Why has Canadian hazard changed so much between NBCC editions?
Each NBCC cycle incorporates updated earthquake catalogs, new ground-motion prediction equations, and refined fault models. The 5th generation model released in 2020 uses the NGA-West2 GMMs for crustal events, the Abrahamson-Gregor-Addo 2016 model for Cascadia subduction, and updated recurrence rates for eastern stable-craton zones. Hazard values in Vancouver increased moderately, while values in eastern Canada were refined with finer spatial resolution.
How does the Cascadia M9 scenario influence design?
The last Cascadia megathrust rupture occurred in January 1700 (documented by Japanese tsunami records). Recurrence is estimated at 300-700 years. NBCC 2020 hazard integrates Cascadia into the 2% in 50 years probabilistic results, so west coast PGA values already include that scenario. For critical infrastructure (bridges, dams, schools) a deterministic M9 Cascadia scenario is often checked in addition to the code PSHA.
What about the Charlevoix seismic zone in Quebec?
Charlevoix has produced five M6+ earthquakes since 1663 (1663, 1791, 1860, 1870, 1925), making it the most active zone in eastern Canada. Sa(0.2) values around La Malbaie reach 0.60-0.80, comparable to Vancouver. The zone is thought to be related to a Cambrian rift system reactivated under current stress conditions. Projects in Quebec north of 47°N latitude should check NRCan values carefully rather than using generic eastern Canada assumptions.
Is NBCC 2020 enough for geotechnical design?
NBCC 2020 gives Sa(T) and the Site Class framework. Geotechnical design complements it with the CFEM for foundations, CSA S6 for bridges and abutments, CDA 2013 for dams, and project-specific analyses: Mononobe-Okabe for walls, Boulanger-Idriss for liquefaction, Newmark for slope displacement, and nonlinear FEM for dams and waterfront structures. For standard projects with acceptable deformation, kh = 0.5·PGA is adequate.