Seismic Site Characterization in the Charlevoix Québec

Significant earthquake site effect potential exists in Charlevoix, Quebec, the most seismically active region in the eastern Canada. Most of the population there lives on two buried river valleys (Du Gouffre in the west and Malbaie in the east) filled by <= 350 m of glacial, glaciomarine, and fluvial deposits atop hard rock. To constrain seismic site conditions and material properties in the two buried river valleys, we performed 164 single -station microtremor measurements and passive-and active-source seismic array testing at six sites. Joint inversion of fundamental-mode Rayleigh-wave dispersion and micro -tremor horizontal-to-vertical spectral ratio (MHVSR) datasets was performed to obtain shear-wave velocity (V-S) depth (z) profiles at the six array sites. The 164 calculated MHVSRs are used to obtain a spatial distribution of the MHVSR fundamental frequency (f(0HV)) within, and to define the boundaries of, the buried river valleys. The Du Gouffre River valley is marked by low f(0HV) (0.44-1 Hz), with f(0HV) increasing progressively from the valley center to its edges and decreasing southward toward the river mouth. The Malbaie River valley is marked by higher f(0HV) (1-2 Hz), with several areas of higher f0HV within the valley, indicating a shallower sediment depth. The inverted V-S(z) profiles demonstrate that sediments are thickest ( similar to 150 m) under Baie Saint-Paul at the mouth of the Du Gouffre River and thin to <50 m headward as well as in the neighboring Malbaie valley. We compile our inverted V-S(z) with other available V-S(z) data in Charlevoix to derive a linear average V-S-z relationship of Charlevoix soils. Soils in Charlevoix have an intermediate V-S-z relationship between that of low VS soils in Ottawa-Gatineau and high V-S soils in Montreal and Quebec City. At greater depths, Charlevoix's V-S-z relationship is most similar to clays in nearby Saguenay, Quebec.

Automated summary

Charlevoix, Quebec, the most seismically active region in eastern Canada, has significant earthquake site effect potential. Through single-station microtremor measurements and seismic array testing, researchers have studied the seismic site conditions and material properties in the Du Gouffre and Malbaie river valleys, and identified differences in their geological features.