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The 2005 National Building Code of Canada (2005NBCC) recommended new guidelines for construction in earthquake hazard areas. Estimated horizontal shearing forces at ground surface, resulting from a 1:2475 year return period earthquake, are in part based on the geotechnical/geophysical properties of the near surface at a particular building site. This project has demonstrated the use of geophysical techniques to map the variation of nearsurface geotechnical properties required to establish the seismic shaking properties within the City of Ottawa. Ottawa was chosen for this demonstration since: 1) it is an area of higher seismic hazard under the influence of the West Quebec and Ottawa Valley seismic zones, 2) a wide variation in soil and rock properties exist within the city limits, 3) similar site conditions occur throughout much of the Ottawa-St Lawrence Valley area, and, 4) GSC and university specialists were available locally, thus reducing field costs. Current 2005NBCC guidelines suggest estimates of average shear wave velocity to a depth of 30 m (Vs30) as a primary parameter required to estimate seismic shaking levels. Shear wave velocity measurement techniques were tested and modified for soil and rock conditions within the project area. These included: seismic refraction and reflection site evaluation (685 sites), multichannel analyses of surface waves (MASW-33 sites), downhole seismic measurements (16 boreholes), and multichannel towed seismic reflection profiling techniques (Landstreamer ~25 line-km). Shear wave velocities were measured throughout the soil column (to depths of 100+ m); in addition, shear wave velocities were measured for the differing rock types within the city. Within the survey area, a borehole geology data bank consisting of approximately 21900 entries was re-analysed in terms of shear wave velocity-depth structure based on the field shear wave velocity measurements. A map of the variation of Vs30 was developed from the integration of all borehole and shear wave velocity data. The map shows that low Vs30 values (associated with higher shaking levels) occur in areas where thick soft soils occur. A second map of fundamental site periods was developed from the measurement of both soil seismic properties and depth to bedrock. Estimates of such natural resonant periods of the ground are also required as an aid in estimating the response of structures to seismic shaking. The fundamental site period map indicates that significant long period resonance is associated with areas where thick soft soils occur. Comparison of this map with approximately 200 passive seismic noise measurements (horizontal to vertical spectral ratios HVSR) indicated a well-documented systematic variance. An empirical relationship between the two measurement techniques has been developed. The main products of this research are: 1) A map of Vs30 , following the guidelines of the current 2005NBCC, 2) A map of fundamental resonance period of the soil based on measured shear wave velocities and depths to resonant impedance boundaries, and 3) A data bank (approximately 22000 points) of average shear wave velocity-depth functions including depth to bedrock, bedrock shear wave velocities and fundamental site periods. It is hoped that this information will be a useful guide for city planners, geotechnical engineers, as well as emergency planning organizations. Similar geophysical applications are possible in other areas of the Ottawa- St. Lawrence valleys in the future.

Why do stochastic point-source and finite-fault simulation models not agree on the predicted ground motions for moderate earthquakes at large distances? This question was posed by Ken Campbell, who attempted to reproduce the Atkinson and Boore (2006) ground-motion prediction equations for eastern North America using the stochastic point-source program SMSIM (Boore, 2005) in place of the finitesource stochastic program EXSIM (Motazedian and Atkinson, 2005) that was used by Atkinson and Boore (2006) in their model. His comparisons suggested that a higher stress drop is needed in the context of SMSIM to produce an average match, at larger distances, with the model predictions of Atkinson and Boore (2006) based on EXSIM; this is so even for moderate magnitudes, which should be well-represented by a pointsource model. Why? The answer to this question is rooted in significant differences between pointsource and finite-source stochastic simulation methodologies, specifically as implemented in SMSIM (Boore, 2005) and EXSIM (Motazedian and Atkinson, 2005) to date. Point-source and finite-fault methodologies differ in general in several important ways: (1) the geometry of the source; (2) the definition and application of duration; and (3) the normalization of finite-source subsource summations. Furthermore, the specific implementation of the methods may differ in their details. The purpose of this article is to provide a brief overview of these differences, their origins, and implications. This sets the stage for a more detailed companion article, "Comparing Stochastic Point-Source and Finite-Source Ground-Motion Simulations: SMSIM and EXSIM," in which Boore (2009) provides modifications and improvements in the implementations of both programs that narrow the gap and result in closer agreement. These issues are important because both SMSIM and EXSIM have been widely used in the development of ground-motion prediction equations and in modeling the parameters that control observed ground motions.

The average shear-wave velocity to a depth of 30 m (Vs30) has been obtained for 73 sites in the Orleans area in the northeast part of the City of Ottawa. Measurements of Vs30 were made using both ground surface reflection and refraction methods. In addition, borehole data was used to estimate Vs versus depth profiles using average Vs values assigned to distinct geological units. High values of Vs (>1500 m/s) were obtained in areas of thin surficial sediments overlying Paleozoic bedrock, and low Vs values (<180 m/s) were calculated in areas of thick late–post-glacial clay. The Vs30 values have been used to prepare an NEHRP map for the study area. Much of the suburb of Orleans is classified as NEHRP zone E, whereas the perimeter areas and some isolated central areas are classified as zones ranging from zone D to zone A. The presence of thick unconsolidated late–post-glacial sediments deposited in the Champlain Sea is the main contributing factor to the wide range of average shear-wave velocities in the study area.