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TitlePositive correlation between DYFI intensity data and microzonation site classes for Ottawa, Quebec City, and the Metropolitan Area of Montreal
AuthorRosset, P; Bent, A L; Halchuk, S; Chouinard, L
SourceSeismological Research Letters 2022 p. 1-13,
Alt SeriesNatural Resources Canada, Contribution Series 20210479
PublisherSeismological Society of America
Mediapaper; on-line; digital
File formatpdf
ProvinceQuebec; Ontario
NTS31F/05; 31G/06; 21L/14
AreaOttawa; Montreal; Québec
Lat/Long WENS -78.0000 -69.0000 49.0000 44.0000
Subjectstectonics; soils science; seismic risk; earthquakes; LEDA; clays; seismic zones; seismicity; seismology; Champlain Sea
Illustrationslocation maps; tables; histograms; distribution diagrams; plots
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 09 21
AbstractAt 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 forMontreal, Ottawa, and Quebec City. Themaps 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 highreported 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.
Summary(Plain Language Summary, not published)
Seismic hazard may be described as the probability of ground shaking exceeding a certain level. Seismic risk is the consequences of that shaking. Seismic hazard is generally assumed to be uniform across a given community but risk might not be. Shaking from a single earthquake may vary across a community for many reasons, one of which is variations in surface geology. Soft soils tend to amplify shaking. Microzonation maps, which show variations in surface geology, have been developed for Montreal, Ottawa and Quebec City. They cannot be used to verify earthquake scenarios because the instrumental coverage is insufficient. Felt reports describing earthquake effects are less precise than instrumental data but are numerous and provide much better sampling of variations within a community. In this paper we compare felt reports from 26 earthquakes to site conditions in Montreal, Ottawa and Quebec City. In general, we found that the highest shaking corresponded to areas underlain by soft soils. A few exceptions need further investigation. This study provides proof of concept for the method, which could be applied to other regions.

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