Seismic Site Characterization in the Charlevoix Region, Québec

Abstract

Abstract Significant earthquake site effect potential exists in Charlevoix, Québec, 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 microtremor horizontal-to-vertical spectral ratio (MHVSR) datasets was performed to obtain shear-wave velocity (VS) depth (z) profiles at the six array sites. The 164 calculated MHVSRs are used to obtain a spatial distribution of the MHVSR fundamental frequency (f0HV) within, and to define the boundaries of, the buried river valleys. The Du Gouffre River valley is marked by low f0HV (0.44–1 Hz), with f0HV increasing progressively from the valley center to its edges and decreasing southward toward the river mouth. The Malbaie River valley is marked by higher f0HV (1–2 Hz), with several areas of higher f0HV within the valley, indicating a shallower sediment depth. The inverted VS(z) profiles demonstrate that sediments are thickest (∼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 VS(z) with other available VS(z) data in Charlevoix to derive a linear average VS–z relationship of Charlevoix soils. Soils in Charlevoix have an intermediate VS–z relationship between that of low VS soils in Ottawa–Gatineau and high VS soils in Montreal and Québec City. At greater depths, Charlevoix’s VS–z relationship is most similar to clays in nearby Saguenay, Québec.