Saint Lawrence rift system

{{short description|Seismically active zone paralleling the Saint Lawrence River}} The '''Saint Lawrence rift system''' is a seismically active zone paralleling the Saint Lawrence River. The rift system trends northeast and southwest and forms a half-graben that links the Ottawa-Bonnechere and the Saguenay grabens. The rift system extends more than {{convert|1000|km|abbr=on}} along the Saint Lawrence valley from the Ottawa – Montreal area. Within the system, fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of the Iapetus Ocean.<ref name="SI">{{cite book |first1=Alain |last1=Tremblay |first2=Yvon |last2=Lemieux |chapter=Supracrustal faults of the St. Lawrence rift system between Cap-Tourmente and Baie-Saint-Paul, Quebec |title=Current Research 2001 |publisher=Natural Resources Canada |isbn=978-0-662-29890-8 |id=M44-2001/D15E-IN |chapter-url=http://dsp-psd.pwgsc.gc.ca/Collection/M44-2001-D15E.pdf |year=2001}}</ref>

Two significant historically active seismic zones occur along this system associated with northwest trending intersecting graben structures. The Charlevoix region has been the location of at least five magnitude six or larger earthquakes over the last 350 years, including the 1925 Charlevoix–Kamouraska earthquake. At the Lower St Lawrence zone the largest recorded earthquakes are about magnitude five. Seismic studies indicate a crustal convergence across the Saint Lawrence valley of about {{convert|0.5|mm|abbr=on}} per year.<ref>{{cite journal| last1=Mazzotti |first1=S. |last2=Henton |first2=J. |last3=Adams |first3=J. |title=Crustal strain rates and seismic hazard from seismicity and GPS measurements along the St Lawrence Valley, Quebec |journal=AGU Spring Meeting Abstracts |volume=2004 |pages=S14A–02 |date=Spring 2004 |id=Abstract #S14A-02 |bibcode=2004AGUSM.S14A..02M}}</ref>

The earthquakes of the Charlevoix seismic zone are thought to be related to the re-activation of ancient fault structures by the Charlevoix impact event.<ref>{{cite journal |first1=F. |last1=Anglin |first2=G. |last2=Buchbinder |title=Microseismicity in the mid-St. Lawrence Valley Charlevoix zone, Québec |journal=Bulletin of the Seismological Society of America |date=October 1981 |volume=71 |issue=5 |pages=1553–60 |doi=10.1785/BSSA0710051553 |bibcode=1981BuSSA..71.1553A |url=http://bssa.geoscienceworld.org/cgi/content/abstract/71/5/1553|url-access=subscription }}</ref>

Post-glacial rebound is also a cause of earthquakes in the St. Lawrence lowlands.<ref>{{cite web|title=Earthquakes in southeastern Canada|url=https://earthquakescanada.nrcan.gc.ca/pprs-pprp/pubs/GF-GI/GEOFACT_earthquakes-SE-Canada_e.pdf|website=Earthquakes Canada|publisher=Natural Resources Canada|access-date=17 April 2020}}</ref>

==See also== * {{annotated link|1944 Cornwall–Massena earthquake}} * {{annotated link|List of earthquakes in Canada}}

==References== {{reflist}}

Category:Rifts and grabens Category:Seismic faults of Canada Category:Structural geology Category:Plate tectonics Category:Geology of Quebec