ABSTRACT: Improving the assessment of seismic hazards in the City of Ottawa.

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Abstract

A demonstration project by the Geological Survey of Canada and Carleton University recently published two microzonation maps of the City of Ottawa. One map depicts the 2005 National Building Code of Canada (NBCC) seismic site categories, based on an average stiffness (or shear wave velocity) of the upper 30m of the ground. The second shows the distribution of fundamental site periods across the city. To compile the seismic site class map, shear wave measurements were made at over 700 locations. These geophysical data were used to develop representative velocity-depth functions for each of the three generalized material types found in the upper 30m of the ground surface: bedrock, glacial and glaciofluvial deposits, and Champlain Sea–post-glacial deposits. These relationships were applied to an existing database of 20,000 boreholes, which allowed for each stratigraphic record to be converted into a shear wave velocity profile, whereby the seismic site class could be determined for each site. Fundamental site periods were calculated using the 2005 NBCC equation of 4h/Vs, where h is the depth to the main acoustic impedance layer, and Vs is the average shear wave velocity to this depth. GIS and geostatistical techniques were used to interpolate the complex regional datasets. The seismic site class map revealed that all five seismic site classes (A through E) are present in the City of Ottawa. Abrupt changes in site class (and thus in expected shaking intensity) occur over distances as short as 100m, reflecting variations in subsurface geology. Thick pockets of soft soil, corresponding to site category E, are present in downtown and suburban areas of the city, making them particularly vulnerable to amplified ground motions during significant earthquake events. The fundamental site period map indicates periods can be as high as 2.6 seconds, corresponding to 140m of soft soil over bedrock. The map has implications for urban environments, as structures with periods matching those of the subsurface are at risk of increased shaking. The maps have been well received by the City of Ottawa and the geotechnical engineering community. They have had a major impact on the geotechnical reporting requirements for the issuing of building permits, and City officials are using the maps as a guide to make more informed decisions regarding land use and emergency response planning. The datasets are also being used in hazard and risk assessment studies by academic partners in the Canadian Seismic Research Network (CSRN).

Bibliography of Canadian Geomorphology