Vs30 Derived from Geology: An Attempt in the Province of Quebec, Canada

The influence of local site conditions is important when assessing the distribution of building damage and seismic risk. The average shear-wave velocity of the top 30 m of soil, Vs30, is one of the most commonly used parameters to characterize site conditions. Topographic slope is one of the proxies used to estimate Vs30 and is often used as a preliminary estimate of site conditions since a dataset is available worldwide at a resolution of 30 arc-seconds. This paper first proposes to compare the accuracy of Vs30 derived from topographic slope against detailed Vs30 zonation in five regions of the province of Quebec, Canada. A general underestimation of Vs30 is observed and site class agreement varies between 18 and 36% across the regions. Secondly, an approach is proposed to improve regional estimates of Vs30 where detailed site characteristics are not available other than the local topography and surface geology information. The surface deposit types from the geological map of Quebec are compared to Vs30 data previously obtained for zonation maps of Montreal, Saguenay and Gatineau in order to estimate Vs30 as a function of sediment deposit types as an alternative to the slope approach. A site class map for the province of Quebec is then proposed.