Geospatial modelling of shear-wave velocity and fundamental site period of Quaternary marine and glacial sediments in the Ottawa and St. Lawrence Valleys, Canada

Abstract

The shear wave velocity of surficial sediments (VS) and the fundamental site period (TO) are important parameters for analysis of the free-field seismic response. Their spatial distribution in the Ottawa and St. Lawrence Valleys, Canada, was determined applying a standardized method consisting of (i) updating the Quaternary geology; (ii) classifying the surficial units with similar physical properties into three broad categories: upper sandy sediments, intermediate clayey sediments and basal glacial and non-glacial deposits; (iii) delineating the spatial thickness of each category by way of 3D geologic modelling; (iv) sorting of available geophysical data with respect to each individual category and assigning representative VS relationships: a power velocity-depth function for sand and clay units combined, VS=119+8.1 Depth0.5 (m/s), and a constant interval VS equal to the observed geometric mean velocity for glacial and non-glacial deposits, VS=385 (m/s). The respective TO values were computed as the ratio between the soil thickness and the average VS from ground surface to the bedrock. Validation of model results was conducted with VS and TO field data and available TO estimates from detailed urban-scale seismic zonation studies. The analyses of the uncertainty originating from the variation of the VS measurements showed that the standard deviations were roughly one-third of the modelled VS and TO values.