Assessing the geometry of buried valleys from MHVSR data: Application to seismic microzonation mapping of Charlevoix, Quebec, Canada.

Abstract:

The development of a regional seismic zonation requires knowledge of the surface and subsurface geological and geophysical properties. Specifically, for the mapping of the fundamental site frequency (f₀), the depth to bedrock is one of the most important parameters, the other one being the shear wave velocity. However, at the regional level, f₀ measurements and/or geological data are typically sparse or clustered. As a result, the zonation output remains based on geological interpretation and on extrapolations to large areas of low data coverage. As part of a f₀ mapping project in Charlevoix, a reinterpretation of the stratigraphic architecture of Quaternary sediments is required, especially for two buried valleys where the bedrock topography was poorly constrained in previous geologic mapping. Here we use MHVSR data to add several new constraints in basin geometry for the interpretation of several cross-sections through the valleys.

We recorded ambient seismic noise at sites located along survey lines perpendicular and parallel to the main valley axes. Microtremor horizontal-to-vertical spectral ratio (MHVSR) data show that f₀ varies between 0.5 to 10 Hz in the first valley and between 1.5 to 20 Hz in the second valley. In the first valley, f₀ increases progressively from the valley center toward its edges suggesting a gradual slope in the bedrock topography. A clear trend in f₀ is also obtained along the main valley axis where deepening towards the south is observed. In contrast, f₀ data in the second valley suggests a narrower valley than that suggested by the surficial geology map, which is consistent with several new observations of shallow and outcropping bedrock not represented on the surficial map. Additionally, seismic array recordings of ambient vibrations and Rayleigh surface waves were collected to obtain an estimate of the shear wave velocity of the Quaternary deposits. Shear wave velocity data are used to determine bedrock depth from f₀. Our new MHVSR and shear wave velocity datasets have improved both the distribution of relevant data for seismic microzonation as well as its spatial distribution (less clustered). We use these new datasets to generate several geologic cross-sections and to develop a 3D subsurface model of the Charlevoix region.