Earthquake Hazard Maps Of The City Of Ottawa, Canada, Using Near-Surface Geophysical And Geological Methods

Abstract

The city of Ottawa, Ontario, Canada, is in an area of elevated earthquake shaking hazard and is considered to be the Canadian city third-most at risk to damage (after Vancouver and Montreal). Maps showing regional variation of Vs30, fundamental site period, and an example map of the spectral accelerations at Sa=0.2s have been completed for the city at a reconnaissance scale. These have been developed from combined geological information from ~21,000 boreholes (water wells and geotechnical borings) within the city limits as well as ~700 surface and borehole shear wave seismic measurement sites. Borehole and surface shear wave measurement techniques developed for the operational area included: surface reversed refraction and reflection sites, MASW, downhole shear wave and horizontalto-vertical spectral analyses of ambient noise. The surficial geology was subdivided into three basic units based on their geotechnical properties: (i) soft, fine-grained, post-glacial sediments, (ii) glacial till and coarse-grained, till-derived sediments, and (iii) firm Paleozoic and PreCambrian bedrock. A series of shear wave velocity-depth functions were assigned to each borehole based on interpolation from proximal geophysical sites. The resulting three-dimensional shear wave velocity database was then used to determine parameters such as Vs30 and fundamental site period. The Vs30 map of the city is subdivided in terms of the U.S. National Earthquake Hazard Reduction Program (NEHRP) zones which have been adopted by the 2005 National Building Code of Canada. Since the City of Ottawa has a large areal extent and is close to the West Quebec Seismic zone, the base accelerations for the 2% in 50 year earthquake event vary extensively within the city boundaries. Hence an example map of the spectral accelerations at Sa=0.2s has been developed to demonstrate regional variations and the effects of NEHRP zones.