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Three earthquakes in three days. More than coincidence?

First came an earthquake in Colorado. Then Virginia's quake shook the US from South Carolina to New England. Finally, San Francisco had a rattler as well. Are they connected?

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This map shows the earthquakes around the world in the past week, and this map focuses on North America. It's clear that most seismic activity is around the Pacific Rim.

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More than 700 earthquakes a year around the world “may be sufficiently strong to cause property damage, death, and injury,” according to the USGS, “but fortunately, most of these potentially destructive earthquakes center in unpopulated areas far from civilization.”

One of the largest recent earthquakes (magnitude 6.8) occurred July 31 near the north coast of Papua New Guinea. (Because they’re figured logarithmically, a 6.8 quake has ten times the amplitude of a 5.8 one.)

[UPDATE: A magnitude 7.0 earthquake hit a remote area of Peru near the border with Brazil Wednesday. There were no reports of damage or injuries, according to the Associated Press.]

Though aftershocks can be expected to occur, even larger earthquakes are unlikely to trigger major events elsewhere, according to a report in the journal Nature Geoscience last year.

“Based on the evidence we’ve seen in our research, we don't think that large, global earthquake clusters are anything more than coincidence,” said Tom Parsons, USGS geophysicist and author of the study.

Earthquakes in the central and eastern United States, although less frequent than in the West, typically are felt over a much broader region.

“East of the Rockies, an earthquake can be felt over an area as much as 10 times larger than a similar magnitude earthquake on the West Coast,” according to the USGS. “A magnitude 5.5 eastern US earthquake usually can be felt as far as 300 miles from where it occurred, and sometimes causes damage as far away as 25 miles.”

As San Diego State University's Dr. Rockwell explains it, that’s because of the difference in earth’s crust on the two US coasts: the West is mushier and more broken up, the East is old granite.

Using a sound metaphor, Rockwell likens the West’s crust to a block of wood, the East to a brass bell with the sound of the bell heard much farther away.

“Seismic energy gets transmitted farther and stronger with high-quality rock,” he says.

Related to this, Columbia University's Dr. Nettles notes that it probably was a good thing that the Virginia quake was located between populated areas. “If you’re going to have an earthquake of this size on the East Coast, that was a pretty good place to have it.”

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