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TitleEarthquakes and geological structures of the St. Lawrence Rift System
AuthorLamontagne, M; Ranalli, G
SourceIntraplate earthquakes; by Talwani, P (ed.); 2014 p. 1-25
Alt SeriesEarth Sciences Sector, Contribution Series 20130049
PublisherCambridge University Press
File formatpdf
NTS21L/05; 21L/11; 21L/12; 21L/13; 21L/14; 21L/15; 21L/16; 21M; 21N/09; 21N/10; 21N/11; 21N/12; 21N/13; 21N/14; 21N/15; 21N/16; 22B/13; 22B/14; 22C; 22D/01; 22D/02; 22D/07; 22D/08; 22F/01; 22F/02; 31I/01; 31I/02; 31I/07; 31I/08; 31I/09; 31I/10; 31I/16
AreaSt. Lawrence River; Gulf of St. Lawrence; Betsiamites River; Saguenay River; Trois-Rivières; La Malbaie; Quebec City; Baie-Comeau
Lat/Long WENS-73.0000 -67.5000 49.5000 46.0000
Subjectsmarine geology; surficial geology/geomorphology; geophysics; landslides; seismic risk; seismicity; seismic zones; earthquake risk; earthquakes; earthquake catalogues; estuaries; estuarine deposits; estuarine studies; hydrography; marine environments; marine deposits; submarine features; sediment transport; fluvial systems; bathymetry; slope deposits; slope failures; slope stability; slope stability analyses; Charlevoix Seismic Zone; St. Lawrence Seismic Zone; St. Lawrence Rift System; Cenozoic; Quaternary
Illustrationslocation maps; tables; block diagrams
ProgramReducing Risk Program Management, Public Safety Geoscience
LinksGoogle Books
AbstractThe St. Lawrence Rift System (SLRS), which includes the Ottawa-Bonnech`ere and Saguenay grabens, is located well inside the North American plate. Most historic and the some 350 earthquakes recorded yearly occur in three main seismically active zones, namely Charlevoix (CSZ),Western Quebec (WQSZ), and Lower St. Lawrence (LSLSZ). Outside these areas, most of the Canadian Shield and bordering regions have had a very low level of earthquake activity. In the SLRS, moderate to large earthquakes (moment magnitude, M, 5.5 to 7) are known to have occurred since 1663, causing landslides and damage mostly to unreinforced masonry elements of buildings located on ground capable of amplifying ground motions. Most earthquakes in these seismic zones share common characteristics such as mid- to upper-crustal focal depths, no known surface ruptures, and proximity to SLRS faults. Variations also exist such as vast seismically active regions (WQSZ and LSLSZ), the presence of a large water body (CSZ and LSLSZ), and absence of SLRS faults near concentration of earthquakes (WQSZ). The CSZ is the best studied seismic zone; there, earthquakes occur in the Canadian Shield, mostly in a 30 × 85 km rectangle elongated along the trend of the St. Lawrence River with local variations in focal depth distribution. Faults related to the SLRS and to a meteor impact structure exist, and earthquakes occur along the SLRS faults as well as in between these faults. Overall, the SLRS faults are probably reactivated by the larger earthquakes (M 4.5) of the twentieth century (CSZ in 1925; WQCSZ in 1935 and 1944; Saguenay in 1988) for which we have focal mechanisms. We propose that caution be exercised when linking historical events that have uncertain epicentreswith SLRS faults. Similarly, SLRS faults should not necessarily be considered to be the reactivated structures for most small to moderate earthquakes (M<4.5).Agood example of this is the earthquakes of theWQSZ, which tend to concentrate in a well-defined NW-SE alignment with no obvious geological control, except perhaps a hypothetical hotspot track. Two local factors can lead to the occurrence of SLRS earthquakes: weak faults or enhanced stress levels. We propose that local conditions, concentrated in a few seismic zones, can alter these factors and lead to the occurrence of earthquakes, especially those with M < 4.5. At a continent-wide scale, the correlation between the SLRS and earthquakes is appealing.We suggest, however, that pre-existing faults related to the SLRS do not explain all features of the seismicity. Seismicity is concentrated in more active areas, some with conspicuous normal faults and some with suspected weakening mechanisms, such as intense prefracturing (e.g., due to a meteorite impact), the passage over a hotspot, or the presence of intrusions and lateral crustal density variations.
Summary(Plain Language Summary, not published)
The last few decades have brought to light numerous geological and geophysical characteristics of the St. Lawrence Rift System (SLRS) and its links with the seismicity. At a continent-wide scale, the correlation between the SLRS and earthquakes is appealing. When a more local scale is looked at or when smaller magnitude earthquakes are considered, questions arise and the picture becomes more complex. We conclude that the sole presence of normal faults related to the SLRS does not explain all features of the seismicity of eastern Canada. Seismicity is concentrated in more active areas, some with conspicuous normal faults and some with suspected weakening mechanisms such as the passage of a hot spot or the presence of intrusives.