Abstract

Seismic techniques have been used to supplement geological and geophysical borehole data for assessing earthquake hazard in the Ottawa Valley near Ottawa, Ontario, Canada. The methodology used to obtain critical parameters for site effect studies (depths of major contacts, shear-wave velocity-depth function) is presented in this paper. Bedrock depth values and basic stratigraphic information were obtained from water well descriptions and three geological units were defined: post-glacial marine, deltaic and fluvial sediments (mainly silt, clay and sand), glacial sediments (till, glaciofluvial sand and gravel), and Paleozoic (limestone, shale) and Precambrian bedrock. In order to augment existing knowledge of bedrock depths and overburden stratigraphy, 100 P-wave reflection soundings (test sites) were acquired. Good quality, high-frequency data have allowed identification of the reflections associated with the glacial-post-glacial boundary as well as the top of bedrock at each site. Subsequently, P-wave and S-wave velocities measured from high-resolution downhole logging in boreholes in the Ottawa Valley area have been used to establish P- and S-wave velocity-depth functions for each stratigraphic unit. We have used these results to: 1) obtain thickness of post-glacial and glacial sediments and bedrock depth at the P-wave reflection sites, 2) calculate Vs30 for all borehole and seismic test site locations and plot NEHRP site classifications over the study area, and 3) combine shear-wave velocity and depth/thickness information to obtain a fundamental resonance period map. We demonstrate that large lateral variations in Vs30, as well as resonance effects, occur within the project area, and that the Vs30 criterion used in NEHRP site classifications is sensitive to the presence and depth of a high-velocity contrast within 30 m of the ground surface. Using the 30-m criterion may not provide an adequate description of the site effects in this environment.

You do not currently have access to this article.