A. A. Mohajer

Université du Québec à Montréal

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Seismicity and Seismotectonics of the Western Lake Ontario Region

Géographie physique et Quaternaire (2007)

A. A. Mohajer

The western Lake Ontario region, a traditionally perceived area of low seismic risk, is densely populated and is home to, among other critical facilities, the nuclear reactors of Pickering and Darlington. These and other characteristics of the region call for improved estimates of seismic hazard. Due to a lack of understanding of the causative geological sources and recurrence characteristics of the reported seismic activity, there is considerable uncertainty regarding estimated ground motion parameters, a fundamental component of seismic hazard assessments. To attempt to improve the definition of the seismic source zones and, consequently, seismic hazard assessments, the hypocentres of about 30 local earthquakes were recomputed. A new data compilation, based on the revised locations or those with the least travel-time residuals, shows that local microearthquakes (ML"3.5) generally occur along, or at the intersection of, prominent aeromagnetic or gravity anomalies. A notable seismicity trend extends in a northeast-southwest direction between Toronto and Hamilton, and is bounded by magnetic lineaments. A major geological structure, the Central Metasedimentary Belt Boundary Zone (CMBBZ), coincides with a strong aeromagnetic anomaly which extends to the northeast into the Western Québec Seismic Zone. This magnetic lineament also extends to the south, across Lake Ontario, to join the Akron (Ohio) magnetic boundary that was associated with several historical earthquakes and with a mb=4.9 earthquake in 1986. Most of the seismic events recorded instrumentally in the 20th century have occurred within a depth range of 5 to 20 km. This observation supports the correlation of local earthquakes with deep geophysical and geological features, suggesting contemporary reactivation of basement structures. This may imply that a more conservative deterministic hazard estimate is needed to verify the probabilistic approach currently used to assess seismic hazard in southern Ontario.

Lineament

Seismotectonics

10.7202/032963ar

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Citations (18)

Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region

Geology (1992)

A. A. Mohajer N. Eyles C. Rogojina

Research Article| November 01, 1992 Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region A. Mohajer; A. Mohajer 1Seismican Geophysical Ltd., 239 Dunview Avenue, North York, Ontario M2N 4J3, Canada Search for other works by this author on: GSW Google Scholar N. Eyles; N. Eyles 2Glaciated Basin Research Group, Department of Geology, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar C. Rogojina C. Rogojina 2Glaciated Basin Research Group, Department of Geology, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information A. Mohajer 1Seismican Geophysical Ltd., 239 Dunview Avenue, North York, Ontario M2N 4J3, Canada N. Eyles 2Glaciated Basin Research Group, Department of Geology, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada C. Rogojina 2Glaciated Basin Research Group, Department of Geology, Scarborough Campus, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1992) 20 (11): 1003–1006. https://doi.org/10.1130/0091-7613(1992)020<1003:NFIMTI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Mohajer, N. Eyles, C. Rogojina; Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region. Geology 1992;; 20 (11): 1003–1006. doi: https://doi.org/10.1130/0091-7613(1992)020<1003:NFIMTI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Persistent small-scale seismicity around the western end of Lake Ontario, together with larger events in eastern North America, have focused attention on the seismotectonic and neotectonic conditions of southern Ontario and adjacent New York State. Newly discovered normal faults that offset both bedrock and overlying Quaternary sediments by as much as 1.25 m were discovered along the Rouge Valley in metropolitan Toronto. This location is within a zone marked by the intersection of three major lineaments and an elevated level of seismicity. These faults result from reactivation of east-west-oriented fractures in Ordovician shales. Structures are exposed in section only and have no surface expression, suggesting that others may have gone undetected. The origin of the faults is not yet well established, although they may represent the effects of uplift consequent upon isostatic rebound aided by high horizontal compressive stress. Nevertheless, their development in the recent geologic past has important implications for regional seismic hazard assessment, because they occur within 7 km of the Pickering nuclear power plant. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

10.1130/0091-7613(1992)020<1003:nfimti>2.3.co;2

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Citations (28)

Discussion of "Analysis and reinterpretation of deformation features in the Rouge River valley, Scarborough, Ontario"

Canadian Journal of Earth Sciences (2003)

N. Eyles A. A. Mohajer

Structurally deformed Pleistocene sediments along the Rouge River are located < 7 km from Pickering Nuclear Generating Station (PNGS). They rest on structurally disturbed Paleozoic sedimentary strata and lie within an area of poorly understood geophysical and structural complexity in the underlying Proterozoic basement (Wallach et al. 1998). PNGS was constructed adjacent to a major population centre (now more than five million people) in the late 1960s largely in ignorance of local and regional geological conditions and well before the plate tectonic paradigm provided a model for basement evolution. The presence and significance of major bedrock lineaments, such as the Central Metasedimentary Belt Boundary Zone (CMBBZ) that passes directly under PNGS, together with several other structures that intersect below Pickering, was not then known. Today, such structures are recognised as being defined by persistent earthquake activity (Mohajer 1991, 1993, 1995; Wallach et al. 1998). A magnitude 3.1 earthquake occurred within 3 km of PNGS on May 24, 2000. Ten smaller magnitude earthquakes have been recorded in the last decade along the structure between Niagara and Pickering by the seismic networks of the Geological Survey of Canada and the United States Geological Survey. The more recently constructed Perry nuclear plant in the U.S.A. was temporarily closed in 1986 by a magnitude 5 temblor along the same CMBBZ structure. The local community has every right to be concerned about the presence of an aging nuclear reactor in their midst. The four Pickering “A” reactors were shutdown in 1997 as a result of “long standing management, process and equipment problems” (NPAG, Nuclear Performance Advisory Group 1997). As part of the recent Environmental Assessment regarding …

Reinterpretation

River valley

10.1139/e03-036

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Citations (5)

Abstract: Surficial pop-ups as geological indicators of major seismicity in eastern North America

Joe Wallach J. R. Bowlby G. H. McFall D. McKay A. A. Mohajer

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Citations (1)

Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region: Comment and Reply

Geology (1993)

J Adams L A Dredge Clark Fenton Douglas Grant W. W. Shifts

Research Article| September 01, 1993 Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region: Comment and Reply J. Adams; J. Adams 1Geophysics Division, Geological Survey of Canada, 1 Observatory Crescent, Ottawa K1A 0Y3 Canada Search for other works by this author on: GSW Google Scholar L. Dredge; L. Dredge 2Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar C. Fenton; C. Fenton 3Mineral Resources Division, Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar D. R. Grant; D. R. Grant 4Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar W. W. Shifts; W. W. Shifts 5Mineral Resources Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar A. A. Mohajer; A. A. Mohajer 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar N. Eyles; N. Eyles 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar C. Rogojina C. Rogojina 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. Adams 1Geophysics Division, Geological Survey of Canada, 1 Observatory Crescent, Ottawa K1A 0Y3 Canada L. Dredge 2Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada C. Fenton 3Mineral Resources Division, Geological Survey of Canada, 601 Booth Street, Ottawa K1A 0E8, Canada D. R. Grant 4Terrain Sciences Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada W. W. Shifts 5Mineral Resources Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada A. A. Mohajer 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada N. Eyles 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada C. Rogojina 6Environmental Earth Sciences, Scarborough Campus, University of Toronto, 1265 Military Tram Scarborough, Ontario M1C 1A4, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (9): 863–864. https://doi.org/10.1130/0091-7613(1993)021<0863:NFIMTI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Adams, L. Dredge, C. Fenton, D. R. Grant, W. W. Shifts, A. A. Mohajer, N. Eyles, C. Rogojina; Neotectonic faulting in metropolitan Toronto: Implications for earthquake hazard assessment in the Lake Ontario region: Comment and Reply. Geology 1993;; 21 (9): 863–864. doi: https://doi.org/10.1130/0091-7613(1993)021<0863:NFIMTI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geological survey

10.1130/0091-7613(1993)021<0863:nfimti>2.3.co;2

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Citations (9)

Linear zones, seismicity, and the possibility of a major earthquake in the intraplate western Lake Ontario area of eastern North America

Canadian Journal of Earth Sciences (1998)

Joe Wallach A. A. Mohajer R. L. Thomas

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 m b = 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.

Lineament

Paleoseismology

Outcrop

Seismotectonics

10.1139/e98-027

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Citations (55)