Seismic site classification and site period mapping in the Ottawa area using geophysical methods
Authors: Hunter J A , Crow H L , Brooks G R , Pyne M , Motazedian D , Lamontagne M , Pugin A J
-M , Pullan S E , Cartwright T , Douma M , Burns R A , Good R L , Kaheshi-Banab K , Caron R , Kolaj M , Folahan I , Dixon L , Dion K , Duxbury A , Landriault A , Ter-Emmanuil V , Jones A , Plastow G , Muir D 
Published: January 2011

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.