Positive Correlation between DYFI Intensity Data and Microzonation Site Classes for Ottawa, Quebec City, and the Metropolitan Area of Montreal

Abstract

Abstract At the local scale, seismic risk is often poorly estimated when considering equal hazard values across any given community. Indeed, past damaging earthquakes have shown that site conditions, which may amplify or deamplify ground shaking, have an influence on the spatial distribution of damage in urban areas. In eastern Canada, Leda clay deposits from the old Champlain Sea are of particular concern for strong site effects in many parts of Quebec and Ontario. To capture the variability in seismic site response, microzonation maps characterizing average shear wave velocity for the upper 30 m of soil, and predominant resonance frequency have been developed for Montreal, Ottawa, and Quebec City. The maps derived from seismic and borehole measurements have been used to develop shake map scenarios but have not been validated, because there have not been any significantly large, close earthquakes in recent years, and because the seismograph network coverage is not adequate to provide a detailed picture of variations in shaking across a city. Nevertheless, all the three cities are in or near active seismic zones, and felt reports, although less accurate than instrumental data, are numerous and provide a dense dataset showing relative shaking levels across a region. Using intensity data for several moderate earthquakes collected largely via the Canadian internet “Did You Feel It?” page, we systematically compare reported shaking levels to soil conditions indicated by the microzonation maps. This study shows a clear correlation between high-reported intensities and soft soils for Montreal where the number of observations is the largest. The results suggest that intensity data collected via the internet and social media could provide a viable method for validating microzonation maps and shaking scenarios.