Why Was the Ottawa Earthquake Felt So Widely?

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Online reports from people who felt the 2013 Ottawa earthquake.

The moderate magnitude-4.4 earthquake that rattled Canada and the Northeast this morning (May 17) made a big impact thanks to old bedrock.

Quakes on the East Coast are generally more widely felt than out West because of differences in the Earth's crust between the two regions, according to the U.S. Geological Survey (USGS). East of the Rockies, earthquakes of the same size are often reported as felt across an area 10 times larger than in the West.

The East Coast has been a pretty quiet place, geologically, since the Atlantic Ocean opened 160 million years ago. Compared to the West Coast, the Earth's crust out East is older, denser and more uniform. That helps earthquake waves zip through the crust.

Along the Pacific, a conveyor belt called a subduction zone (a collision between two tectonic plates), as well as the strike-slip San Andreas Fault, have plastered hundreds of miles of new crust onto North America. These active plate boundaries created a mishmash of rocks and faults that hinder earthquake waves.

Today's earthquake hit in the Western Quebec Seismic Zone, the site of a 2010 magnitude-5.0 earthquake felt as far away as Ohio, the USGS said. (A magnitude-5.0 earthquake releases eight times more energy than a magnitude-4.4 earthquake.) Damaging earthquakes occur in the zone about once a decade, while a smaller earthquake strikes three to four times a year, according to the USGS.

The biggest historic earthquake in the Western Quebec Seismic Zone was an estimated magnitude-6.2 in 1935 that toppled chimneys in Northeastern Ontario.

Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook & Google+. Original article on LiveScience's OurAmazingPlanet.

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