A large earthquake near America's northern border has rattled millions in the southern part of Canada and sent shockwaves across multiple US states.
The magnitude 4.0 earthquake struck at 12.36pm ET on Tuesday just 40 miles north of Ottawa, Ontario - a city of more than 1.1 million people.
According to the US Geological Survey (USGS), the shockwaves of this seismic event were felt by people in several nearby cities, including Montreal and its 4.4 million residents, and in northeastern US states such as New York and Vermont.
The Canadian counterpart of USGS, Earthquakes Canada, confirmed that the quake spread seismic waves across an area roughly 300 miles in diameter, generating thousands of reports of mild to moderate shaking.
The agency added that quakes of this magnitude are often felt across the region, but rarely cause significant damage.
A resident in Ottawa's Constance Bay told CTV News: 'The building was vibrating and it sounded like a large truck was going by at the beginning. It increased to where we felt the vibration in buildings.'
'The house was shaking and it scared us,' another person in Quebec's Otter Lake area added.
There have been no reports of injuries at this time.
Canadian officials have reported that the quake was slightly weaker than what US monitoring stations have said, calling Tuesday's event a 3.9 magnitude tremor.
Canada's National Capital Region, which includes the provinces of Ontario and Quebec, has not suffered an earthquake stronger than magnitude 5.6 since officials in the country started keeping accurate seismic records after 1899.
That record-setting earthquake struck both Cornwall, Ontario and Massena, New York in the US in 1944. A pair of Canadian quakes in 2010 and 2013 were measured at around magnitude 5.0.
Tuesday's earthquake struck in the heart of one of the most active seismic zones in eastern Canada.
The Western Quebec Seismic Zone includes a massive swath of land, including the Ottawa Valley from Montreal to the city of Temiscaming.
The Laurentian Mountains and Eastern Ontario are also part of this seismic zone, covering cities such as Ottawa and Cornwall.
This zone regularly produces small to moderate earthquakes and is the main source of seismic hazards for Ottawa, Gatineau and nearby areas.
The shaking happens because the ground here has several ancient faults below the surface, and ongoing pressure from deep inside the Earth keeps pushing on them, causing these cracks to occasionally slip and release energy.
There has been widespread seismic activity throughout North America this week, from the Pacific Northwest to the Northeast.
On Sunday, a swarm of 18 earthquakes was detected right off the coast of Washington State, including a magnitude 4.2 eruption in the Pacific Ocean.
The swarm took place right along the Cascadia Subduction Zone, an ancient 'subduction' zone where a tectonic plate slides underneath the North American plate.
Geologists fear this region could be 'overdue' for a massive earthquake stronger than magnitude 7.0.
On Monday, a strong magnitude 5.7 earthquake was felt in Western Nevada and set off a swarm of aftershocks in the area.
Multiple earthquakes in Silver Lake can be caused by several factors, but the most common reason is movement along faults, which are fractures in the Earth’s crust where blocks of rock slip past each other, just like in Canada.
