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Correlating Seismic Microzonation and Intensity Data in Three Urban Areas of Eastern Canada

Session: Tectonics and Seismicity of Stable Continental Interiors I

Type: Oral

Date: 4/23/2021

Presentation Time: 10:45 AM Pacific

Description: 

Seismic hazard assessments generally result in equal hazard values across any given community. Seismic risk analyses, however, paint a different picture where risk within some areas of a community may be elevated due to amplification of shaking levels by factors such as soft soils. Leda clay deposits from the old Champlain Sea are of particular concern for amplification in many parts of Quebec and Ontario. Seismic microzonation maps characterizing the upper 30 m of soil in terms of average shear wave velocity and predominant resonance frequency have been developed for Montreal, Ottawa and Quebec City. All three cities are in or near active seismic zones. The maps, derived from seismic and borehole measurements, have been used to develop Shakemap 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. Felt reports, although less precise 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 an internet “Did You Feel It” page, we systematically compare reported shaking levels to soil conditions indicated by the microzonation maps. The work for Ottawa and Quebec City is ongoing. The study for Montreal has been completed and shows a strong correlation between high reported intensities and soft soils. The results suggest that intensity data collected via the internet and social media provide a viable method for validating microzonation maps and shaking scenarios.

Presenting Author: Allison L. Bent

Student Presenter: No

Authors