It was a startling reminder that Central Canada is an earthquake zone.
Shortly after 1:40 p.m. on Wednesday, a 5.0-magnitude earthquake was felt across Ontario and Quebec, rattling homes and buildings from Sudbury to Quebec City, and as far south as New York City. Closer to the quake's epicentre, about 55 kilometres north of Ottawa, workers evacuated downtown buildings, pouring into city streets by the thousands, dodging falling objects and broken glass.
While no major injuries were reported, earthquake experts said geological faults throughout Quebec and in Ontario - near nuclear plants - could bring disaster in the future to a region that is not ready for a big quake.
"In the west, they're prepared: they design their buildings, they teach children in schools what to do," said Arsalan Mohajer, a professor of geology at the University of Toronto. "But in the east, we don't think it's going to happen. We'll be caught by surprise."
Dr. Mohajer said that faults run across Ontario and Quebec, and are potentially disastrous. He said a fault that runs along the St. Lawrence river valley is particularly threatening, meaning areas near Montreal, Cornwall and Quebec City could have large earthquakes in the future. Similar faults exist in the Niagara-Pickering area, he said, "inconveniently close to Toronto and safety-related nuclear facilities east of the city."
Wednesday's quake happened in the western Quebec seismic zone, which runs from Montreal to Timiskaming, Alan Baird of Queen's University said. Generally, earthquakes occur at plate boundaries where the rigid masses that make up the earth's outer shell rub and push at one another. The areas of the world best known for earthquake activity are found where plates bump and slide under one another. But Wednesday's quake, Dr. Baird said, was an "intraplate quake," which means it happened within a plate, as opposed to at a plate boundary.
The area is a "weak zone," he explained, where low to moderate seismic activity is common, and the result of stress built up within the plate from constant pushing at the boundaries.
Across central Canada, reports flooded in from those startled by the tremor.
Samantha Clement of L'Ange Gardien, Que., had just got her daughter to fall asleep when the shaking started. "At first, it felt like there was a big truck coming down the road," she said. "Then it was like an explosion from afar."
Ms. Clement gathered up her crying daughter and ran out to join her neighbours on the street. When she returned, she said, she found a two-inch crack on the ceiling of her upstairs hallway.
Meanwhile, the mayor of Gracefield, Que., declared a state of emergency after the earthquake damaged several buildings, including a hotel and the town church. Eric Alie, a mechanic for the town, said the church is closed because the roof was badly damaged.
On Parliament Hill in Ottawa, where the Senate was still in session, Senators were sent running onto the Parliament lawn after seeing chandeliers swinging and hearing the sound of glass windows crunching.
"We're having some contentious debates, so we were wondering if it was a message from the gods," joked Sen. Marjory LeBreton.
And for a city already on edge due to the upcoming G20 summit, Wednesday afternoon's tremors had many in Toronto panicking.
"We had a few conspiracy theories going," said James Lorimer, who works on the 37th floor of the First Canadian Place, the country's tallest office building. Just that morning, one of the city's subway lines had been shut down due to a "suspicious package," and some initially thought the shaking could be an act of terrorism or protest.
With reports from Campbell Clark, and Erman Boyd