Seismic Microzonation in Saguenay: Site-Specific Ground Motion...

Saguenay sits inside a seismically active graben. The 1988 magnitude 5.9 earthquake reminded everyone that moderate quakes here produce unusually strong short-period shaking because of the deep, rigid rock basement. Local amplification changes street by street. A standard NBCC classification is not enough. Our MASW surveys capture Vs profiles across the marine clay and till that blanket much of the Jonquière and Chicoutimi sectors. We then build a microzonation map showing where spectral acceleration amplifies from site class C to E, letting structural designers adjust base shear without overbuilding the entire project. Over 145,000 people live in this metropolitan area, and development keeps pushing into zones where the clay thickness exceeds 30 meters. We have run microzonation grids for schools, hospital expansions, and industrial plants near the aluminum smelters, correlating each CPT and borehole log to a shear-wave velocity profile before drawing the class boundaries.

Site class changes from C to E in less than 100 meters in parts of Chicoutimi. A single borehole can misclassify the entire lot.

Scope of work

The boroughs of Saguenay grew fast after the 1976 merger. Old neighborhoods in La Baie sit on sensitive post-glacial clays that amplify long-period motion, while the bedrock outcrops on the north shore of the Saguenay River deliver totally different ground response. A single uniform hazard spectrum for the whole city makes no engineering sense. We start with the NBCC 2020 reference rock motion for the Charlevoix-Kamouraska source zone and propagate it upward through a column of soil. The analysis uses the nonlinear hyperbolic model from Darendeli (2001) for clay and the DCP-based modulus degradation curves for the sandy till. Inclinometer and piezometer data feed into the site response model where pore pressure buildup during the design earthquake could trigger cyclic softening. The final deliverable is a grid of surface response spectra at 50-meter spacing, overlaid on the cadastral base. Municipal planners use these maps to flag parcels where site class E requires deeper foundations or Improvement before rezoning.

Area-specific notes

We deploy a 24-channel geophone spread with a 5-kg sledgehammer source for the active MASW lines inside city limits. The low-frequency geophones (4.5 Hz) are critical here because the soft clay masks the fundamental-mode dispersion below 8 Hz. A single line shot without a borehole tie is risky. We always drill at least one calibration SPT hole per microzonation block, logging the stratigraphy and sampling the clay for index tests. The Saguenay Graben fault geometry means the bedrock surface can step 10 meters across a short distance. A map built purely on interpolation between widely spaced borings misses those steps and underestimates the impedance contrast. We run passive MAM arrays at night when truck traffic dies down, extending the dispersion curve to 2 Hz so the Vs profile reaches the seismic bedrock at 1,500 m/s. Without that deep constraint, the site period calculation drifts.

Reference parameters

Parameter	Typical value
Minimum Vs30 measured in Saguenay clay	120–150 m/s (Site Class E)
Typical grid spacing	50 m x 50 m
Seismic source model	Charlevoix-Kamouraska zone, Mmax 7.0
Design ground motion return period	2,475 years (NBCC 2020)
Site response software	DEEPSOIL v7.0 (nonlinear)
Depth to bedrock in Jonquière	5–45 m
Spectral period range	0.01–5.0 s

Linked services

MASW and MAM Acquisition

Active and passive surface-wave surveys to extract shear-wave velocity profiles down to 40 m depth. Low-frequency spreads capture the deep impedance contrast at the till-bedrock interface.

Site Response Analysis

One-dimensional nonlinear or equivalent-linear modeling using DEEPSOIL. Input motions are matched to the NBCC 2020 uniform hazard spectrum for the Saguenay region.

Microzonation Mapping

GIS deliverables with Vs30, site period, and surface spectral acceleration grids. Ready for municipal planning submissions and structural base-shear calculations.

Quick answers

How much does a seismic microzonation study cost for a Saguenay site?

Cost depends on the area to map and the number of calibration boreholes. For a typical commercial lot with one SPT calibration hole and a MASW grid, budgets range from CA$5,300 to CA$20,680. Larger industrial parcels requiring passive MAM arrays and deeper boreholes fall at the upper end.

Which Saguenay neighborhoods show the highest amplification?

The marine clay plains in Jonquière and La Baie typically produce the strongest site amplification. Vs30 values can drop below 150 m/s where the clay exceeds 20 meters in thickness, pushing the site into class E. The bedrock terraces in Chicoutimi-Nord tend to classify as site B or C.

How long does a microzonation study take from start to finish?

Fieldwork typically takes three to five days for a standard grid. Processing, site response modeling, and GIS mapping add another two to three weeks. We schedule field acquisition between May and October because frozen ground in winter alters the near-surface velocities.

Can you use existing borehole logs for the microzonation?

Yes, provided the logs describe the stratigraphy in enough detail and the coordinates are accurate. We still run at least one new MASW line to calibrate the legacy data. Boreholes without SPT N-values or lab index tests are of limited use for site response modeling.

Seismic Microzonation in Saguenay: Site-Specific Ground Motion Analysis

Scope of work

Area-specific notes

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