ABSTRACT: Earthquake hazard assessment in a low seismicity intraplate region of Ontario - potential impacts of uncertainties and under-estimation of Neotectonic activity.

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Abstract

The prediction of neotectonic hazards is a relatively new field of endeavour in southern Ontario. Seismic hazard assessment in eastern Canada is faced with a relatively wide range of uncertainties, however this is mostly due to the presence of an inadequate seismological database. The official record of earthquake events is considered to be too short and incomplete to provide a representative picture of where moderate or large earthquakes have occurred in the geologically recent (i.e., postglacial) past. Recognized seismichazard estimates for public safety issues in infrastructure engineering and development such as siting of engineered industrial structures have generally been provided on the basis of this database. It is considered that many of these estimates have been made without adequate inclusion of geological and geophysical data and information determined from known faults, postglacial structures and other deformations of earthmaterials. Thus, areas apparently devoid of well-documented earthquakes, small, moderate or large, may have been under-estimated with respect to potential seismic hazards.Factual information from all sources, published and unpublished, is required in improving the evaluation ofneotectonic hazard potentials. Risk assessment methodologies require the input of substantive natural hazardestimates if the risk estimates are to be considered valid. Independent efforts to estimate seismic hazard potentials in southern Ontario have been undertaken through integration of various geological and geophysical databases, recent microearthquake monitoring and remote sensing. Ground level and lake bottom examinations of spatially related geological features have been undertaken. These efforts represent an example of a multi-disciplinary and complementary approach where relevant seismological data is scarce. Occurrence of a magnitude 6.5 to 7 event in the western Lake Ontario region is a credible scenario with varying estimates for annual probability of exceedance ranging from 10 -4 to 10 -5 . These conclusions, together with the current debate on the wide uncertainties in statistical parameters used in probabilistic estimation of seismic hazard, suggest that there may be a higher level of seismic hazard and hence risk for urban infrastructure in major urban centres such as Toronto and Hamilton and for the surrounding communities. For example, there are several examples of recently discovered geological structures found inconveniently close to nuclear power generating stations sited on the north shore of Lake Ontario, east of Toronto. Thesegeological structures range from small localized fault displacements of Quaternary soil and Paleozoic bedrock rock to large bedrock fault segments of what is interpreted to be a major extension of the St. Lawrence Rift structures into the lower Great Lakes, to the Precambrian Central Metasedimentary Belt Boundary Zone or to other geologically significant North American continental structural systems. The larger structural systems extend to distances of several hundred km and are thought to be spatially associated with historical earthquakes of small to moderate magnitude in the Niagara Peninsula, western New York, Ohio and larger events from the St. Lawrence Valley. The geological structures are considered as potential sources of significant threat toseismic safety if proven active. The nuclear facilities were licensed, constructed and continue to be operated under consideration of original seismic design parameters prepared and provided to the Atomic Energy Control Board. It is often repeated by non-geoscientists that earthquakes are random events, and therefore recurrence of the hazard can be treatedwith probabilistic equations. Earthquakes are not random events but occur because oriented ambient stresses are applied to and resolved by a fault until the stored strain energy exceeds the ability of the earth materials to resist slip. An earthquake occurs for a reason, and at a location. When the earthquake occurs, it indicates that the fault is active. On November 26, 1999, a magnitude 3.8 earthquake occurred under Lake Ontario within about 25 km of both the Pickering and Darlington nuclear sites. The event was felt in an area of about 100 km surrounding the epicentre. The statement from a Geological Survey of Canada spokesperson, widely reported in the local media on November 26 and 27, was that there were no active faults in the area. More than 50 small to moderate earthquakes have been reported to have occurred in the immediate Lake Ontario and surrounding seismotectonic region. Deterministic approaches to the integration and analysis of new geoscientific data are required in the assessment of neotectonic activity. Factual information rather than assumptions must be included in probabilistic methodologies used in assessing risks from natural hazards. It is increasingly necessary to look for improved results in technical terms. Communication of knowledge of the seismic risk that may be attributed to neotectonic conditions in an area has become an important task for the geoscientist. Such communication of site-specific risks must be made in terms that can be understood by the user community including engineering designers, insurers, financial institutions, public officials and the public. Seismic hazard estimation in areas of urban concentration highlights the need for professionalregistration of geoscientists when the opinions of geoscientists affect both public interest and public safety. It has become increasingly necessary to protect the public from unskilled orunethical geoscientific practice in many areas of practice. This is perhaps even more important in areas such as neotectonic hazard estimation which can directly affect more persons, livelihoods and infrastructure within an area of influence than for a prospectus prepared for a mining or oil or gas stock issue which may only affect investors financial positions. External pressures to provide economic ‘solutions’ when providing statements of seismic parameters for use in design andoperation of engineered structures and facilities must not be permitted to take precedence over the careful identification of hazardous or dangerous conditions that may threaten economic interests, life, limb and property of the public or the natural environment. All jurisdictions in Canada already have or are moving to license the practice of professional geoscience. The Canadian Council of Professional Geoscientists, representing all jurisdictions, is working to achieve agreement on interprovincial mobility for geoscientists through whichprofessional geoscientists will recognize that they may be held personally accountable to the public and to their peers for misconduct or failure to report, wherever they work in Canada.

Bibliography of Canadian Geomorphology