Classification of the seismic characteristics of soils has become an important issue for major Canadian cities, including Ottawa and Gatineau in the National Capital Region (NCR), following the publication of the 2005 National Building Code of Canada (2005 NBCC).
As in other areas of eastern Canada, earthquakes do occur in the Ottawa-Gatineau area. Large events are infrequent, although should they happen, their potential impact may be significant.
Earthquake motion varies greatly across the region, depending on underlying geology and soil thickness, and the level of ground motions is amplified by soil overlaying bedrock. Reflecting that subsurface conditions play a major role in the damage potential of incoming earthquakes, amplification measurement methodologies have become an important part of the latest Canadian building code, 2010 NBCC.
To provide a valuable tool to the engineering community for mapping seismic soil classes, a Working Group on Seismic Hazard Microzonation in the Ottawa Region was established in 2005. Based on collaboration between Carleton University and the Geological Survey of Canada (GSC), the program seeks to address problems directly related to seismic microzonation for earthquake hazard mitigation and the research is providing microzonation tools for the development of earthquake hazard maps in eastern Canada.
IN THE ZONE
Available online for both Ottawa and Gatineau, the maps represent the fruition of recent joint research by Carleton University and the GSC funded by the “Assessing Earthquake Geohazards” Project of Natural Resources Canada and the Canadian Seismic Research Network (CSRN). The work was led jointly by Carleton’s Dr. Dariush Motazedian and GSC’s Dr. Jim Hunter.
The primary map of the Ottawa-Gatineau area displays the locations of interpreted NEHRP — National Earthquake Hazard Reduction Program — zones required for estimating earthquake motions at building sites, as defined by the 2010 NBCC. With approximately 800 sites in the two cities occupied by geophysical crews consisting of both Carleton University students and GSC staff, shear wave velocities of the materials were measured from the ground surface down through the soils to the bedrock.
Velocity correlations were made with various unconsolidated soil units and with bedrock. These correlations were then applied to adjacent borehole information (~21,000 water wells and geotechnical borings) previously compiled by the GSC for the NCR.
NEHRP zones, as described in the 2010 NBCC, can be derived from measurements of the average shear wave velocities from the ground surface to a depth of 30 metres (defined as Vs30). As shown, the Vs30 map demonstrates the presence of five of the six NEHRP zones, ranging from Zone A (hard rock) to Zone E (very soft soil). The sixth zone, Zone F, describing special geotechnical properties of soils (sensitive clay, thick organic layers and artificial fill) cannot be distinguished by geophysical techniques alone and may exist within the mapped soil zones: Zone A (hard rock); Zone B (rock); Zone C (soft rock or firm soil); Zone D (soft soil); and Zone E (very soft soil).
Throughout the survey area, GSC and Carleton University site investigations probed much deeper than 30 metres. In most cases, they probed to the top of firm bedrock to estimate the resonance effects that occur for sites where soft soils overlie firm bedrock and to provide regional geophysics data, which may be of use in future earthquake research.
In the NCR, and throughout much of the Ottawa and St. Lawrence valleys, large shear wave velocity contrasts between soils and bedrock have been measured. These contrasts may lead to large ground resonance amplifications during earthquake shaking at specific frequencies (0.5 to 10 Hertz, Hz) or over long periods (0.1 to 2 seconds), which may vary in line with the thickness of the soft soil. This information is of interest for the construction of tall structures, where resonant periods may be similar to ground resonance.
The fundamental site periods map, available both on the GSC and the Carleton University websites, provides the variation of ground resonance within the Ottawa-Gatineau region based on the recommended “single overburden soil layer” analysis procedure, as given in the current NBCC.
The Ottawa-Gatineau maps are offered as a regional guide to soil-rock geotechnical conditions; they are not meant to be utilized in lieu of direct on-site geotechnical measurements for NEHRP zone determination, as may be required for things such as building permits.
Recent research suggests the resonance periods may alter from those provided on the region map during significant strong motion earthquake events.
Other earthquake hazard mapping is under way as part of CSRN’s five-year, eight-university research project and includes Vancouver and Montreal. The GSC is also developing maps for North Vancouver and the Quebec City area. Final products could be available in the next year or so.
These continuing government-university research programs have a direct impact on the mitigation of earthquake hazards and, as such, the quality of life of Canadian citizens. The general benefits of seismic microzonation include a better understanding of the potential for and spatial distribution of seismic hazards, providing a rational basis for planning and policy-making regarding the mitigation of these effects.
The microzonation maps generated over the course of the Ottawa-Gatineau research program are an essential tool for making earthquake emergency plans and for urban planning. They give authorities relevant and concrete information for focusing mitigation efforts, such as on schools and critical infrastructure, as well as on lifelines. They provide the framework to facilitate ranking of priority areas or structures by seismic vulnerability, thereby offering a reasonable basis for distribution of funds for upgrading, retrofitting and other remediation efforts.
Organizations that benefit from having access to microzonation information include private industry, financial and insurance agencies and emergency services, as well as community and regional planners who address utilities designs, transportation networks, high-occupancy structures and hazardous goods transportation and storage (for example, toxic waste repositories and nuclear power stations).
In eastern Canada, where there is a propensity for the occurrence of soft soils (the so-called Leda Clay), microzonation mapping has triggered joint government-university research on additional 3D amplification of earthquake shaking in soft-soil basins within urban areas.
In the long term, the seismic microzonation of earthquake-prone areas throughout Canada is expected to provide a better overall understanding of the potential for and spatial distribution of seismic hazards and will contribute substantially to the health and safety of Canadians.
Notes and References: This article makes reference to information included in Earthquake Hazard Maps of the City of Ottawa, Ontario, Canada using Near-Surface Geophysical and Geological Methods (Hunter et. Al, 2010), and Development of a Vs30(NEHRP) Map for the City of Ottawa, Ontario, Canada (Motazedian et al., 2011); maps providing information for 25,000 data points are accessible at the website of the Working Group on Seismic Hazard Microzonation in the Ottawa Region; and free downloads of GSC Open Files 6191, 6273, 7067 and 7078 are available online at the Natural Resources Canada publications database.