Natural Resources Canada recorded a Magnitude 4.6 Earthquake slightly before 10.50 am local time (slightly before 2.50 pm GMT) on Saturday 21 September 2013. This was a moderately large quake, and was felt on both sides of the estuary, though there are no reports of any damage or casualties.
The approximate location of the 21 September 2013 St Lawrence River Estuary Earthquake. Google Maps.
The Lower St. Lawrence Seismic Zone lies largely beneath the St. Lawrence River Estuary. Small quakes are very common here, averaging roughly one per week, but larger quakes are all but unheard of, with only two quakes in excess on a Magnitude 5.0 ever being recorded. The Seismic Zone is thought to be connected to a graben-structure (area of tectonic plate expansion) beneath the river, the formation of which relates to the opening of the Iapetus Ocean in the late Proterozoic to early Paleozoic, which has become reactivated during the breakup of Pangea and the formation of the Atlantic Ocean, around 150 million years ago. In a graben structure the Earth's crust is stretched and becomes thin, then faulting occurs allowing the central part to sink.
Witness reports can help geologists to understand the processes going on in Earthquakes and the structures in the rocks that cause them. If you felt this quake you can report it to Natural Resources Canada here.
See also Earthquake shakes Cleveland, Ohio, Earthquake under Maine shakes New England, Earthquake in Labrador, Canada and Quebec Earthquake, 27 August 2011.
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