Seismic hazard assessments conducted in eastern Canada rely on seismological data, which are essential, but alone, inadequate. That is because the earthquake record is too short to provide a representative picture of where large earthquakes have occurred in the past. Consequently, seismic hazard may be underestimated in areas, such as that encompassing western Lake Ontario, that are devoid of documented large earthquakes. To attempt to ascertain the likelihood of a major earthquake occurring in that highly populated and industrialized area, three regionally extensive geophysically expressed lineaments were investigated and the results were combined with available seismological information. The most conspicuous is the Niagara-Pickering linear zone, within which there have been at least two, if not three, periods of brittle faulting, including displacements compatible with the current stress field. It also appears to represent the same major structure as the Akron magnetic boundary in eastern Ohio, the site of the mb = 4.9 Leroy earthquake. The second is the Georgian Bay linear zone, which extends from Georgian Bay to New York State. It displays evidence of recent outcrop-scale faulting, an alignment of earthquakes along its southwestern boundary, and a possible spatial relationship to other earthquakes, including two of M >= 5. Lastly, there is the Hamilton - Lake Erie lineament, which is parallel and proximal to a possible fault and coincides with a linear array of small to moderate earthquakes. All three converge on the western Lake Ontario area, which has a higher frequency of seismicity than the adjacent areas. Thus, the western Lake Ontario area might have a greater potential to experience a major earthquake than heretofore believed.

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