Abstract

Based on earthquake hazard and population, the seismic risk of the Ottawa-Gatineau region ranks third in Canadian urban areas. As part of a seismic microzonation project, the impact of earthquakes was documented for the 18 strongest events that affected the region. These events were 12 moderate earthquakes (magnitude between 5.0 and 6.1) at less than 300 km epicentral distance and five more-distant earthquakes (350 to 525 km epicentral distance) with epicenters in the Charlevoix seismic zone and in the Saguenay region. It was found that between 1830 and 2008, 13 of these earthquakes had an impact in Ottawa of at least Modified Mercalli Intensity (MMI) level V. Most earthquakes occurred in the first half of the 20th century when Ottawa occupied only a small proportion of its current urban area. Consequently, most damages occurred in the city's historical downtown, where most of the buildings were located. At a very local level, certain city districts (wards) were affected more than others, suggesting higher levels of ground motions due to unconsolidated deposits. Historically, a 10–50-m thick basin filled with unconsolidated deposits in downtown Ottawa had most instances of MMI VI and VII. In contrast, other areas with a thin veneer of unconsolidated material had very little damage. Considering the rapid development of the Ottawa region during the past few decades, other areas that were weakly populated during past earthquakes could suffer damage in future earthquakes.

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