REGIONAL MAPPING OF THE FUNDAMENTAL SITE FREQUENCY IN THE CHARLEVOIX REGION, QUEBEC, CANADA: EARLY RESULTS

One of the most significant factors affecting the level of earthquake ground shaking is the underlying geology or site conditions. The fundamental site frequency (f₀) is an important site-specific parameter to establish the primary resonance mode of the soil column. This earthquake site effect is particularly important in Eastern Canada where a strong impedance contrast exists between the soft Quaternary deposits and underlying hard metamorphic and igneous rocks. Our objective of is to determine the spatial distribution of f₀ inside the meteoritic impact crater of Charlevoix, Quebec, Canada, the most seismically active region in eastern North America. Here, most of the population lives within two buried valleys filled by up to 250 meters of glacial, glaciomarine and fluvial deposits with significant earthquake site effect potential. We use an innovative approach considering valley geometry and internal stratigraphy to model f₀ from the thicknesses and representative shear wave velocity (v_s) values determined for geological layers.

We collected microtremor-horizontal-to-vertical-spectral-ratio (MHVSR) and v_s profiles, mostly in the two buried valleys. The MHVSR’s are used to obtain a preliminary spatial distribution of f₀ in the main buried valleys. The f₀ values vary between 0.5 to 20 Hz with significant differences between the valleys. The first valley is marked by low f₀ between 0.5 to 1 Hz, with values increasing progressively from the valley center to its edges and decreasing towards its southern end. The second valley is marked by higher f₀ between 1.4 to 2.5 Hz, with several areas of high f₀ within the valley. The MHVSR f₀ results suggest the first valley is deep with a simple valley profile, deepening towards the south. In contrast, the second valley is much shallower and has a more complex valley profile. V_s profiles were interpreted from dispersion curve estimates collected from multichannel analysis of surface wave (MASW) and ambient-vibration-array recordings. These v_s profiles supplement available geologic and geophysical data to assign representative v_s values for the glacial and glaciomarine sediments. Our 3D geophysical models of Charlevoix, innovatively developed using both in situ seismic measurements and geologic reconstruction, will impact future prediction of earthquake site response.