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PHOTO Courtesy of Richard Little

A roadside in Gill is bordered by sedimentary layers formed in the Jurassic period that were tilted by the Eastern Border Fault that runs through the eastern Connecticut River Valley.

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map courtesy of the u.S. GeologicAL Survey

The location of earthquakes in Massachusetts from 1990-2006.

When the earth shook just before 2 p.m. Tuesday, Valley residents may have thought it was a tremor caused by a passing vehicle, temporary dizziness or a colleague's prank - anything but a quake.

But the magnitude 5.8 earthquake was real enough. While no deaths or serious injuries were reported, it reminded East Coast residents from Georgia to Maine that quakes are not just a western phenomenon.

"It was a nice wake-up call," said Stephen Mabee, the state geologist for Massachusetts, speaking from his office in the Geoscience Department at the University of Massachusetts Amherst, where he is on the research faculty.

While the quake was "fairly minor," he said, it was felt by a huge number of people on the East Coast where, due to the rarity of quakes, many older structures are ill-equipped to withstand them.

"Even minor quakes can cause damage," said Mabee. Hopefully, he added, the earthquake will prompt officials "to take the few bucks that we have" to map those areas where quakes are likely to happen.

Richard Little of Easthampton, a geologist who has written two books and created two videos about Connecticut River Valley's geology, said area residents likely felt Tuesday's earthquake differently, depending on where they were when it passed through.

People living on bedrock or older metamorphic rock don't get as much shaking, he said, while residents living in the Valley may have felt the quake much more. That's because the ground there is composed of looser sediment deposited on the shoreline of the Glacial Lake Hitchcock that filled what is now the Connecticut River Valley - and beyond - some 15,000 years ago.

Of course, many people might not have felt the shaking at all because they were concentrating too much on tasks at hand.

Little said he was at home, probably working at his computer, when the quake rolled through. "I didn't feel it," he said.

Eastern quakes

Mabee, the UMass geologist, said most earthquakes that make headlines happen at "plate boundaries," like those that meet at California's San Andreas Fault or in Japan, where the massive 8.9 quake in March resulted in the deaths of more than 15,000 people.

The East Coast, by comparison, is located in the middle of a plate, said Mabee, with one end in the middle of the Atlantic and the other on the West Coast. In geological terms, the area is known as a "passive margin," he said.

Still, stresses do build up in the middle of the plate that result in "little adjustments," he said. These produce plenty of quakes, most of them too minor to be felt.

Tuesday's quake was fairly large for the East Coast, said Mabee, although there have been 6.0 magnitude quakes.

Why did Tuesday's quake extend over such a large area? Mabee explained that seismic waves tend to travel further in the eastern part of the country because the ground is made up of colder, denser rock, compared to the earth's surface in the West, which tends to be less solid and more fragmented.

"It's like a solid piece of concrete compared to one made up of bricks and cracks," he said.

There are numerous faults up and down the eastern seaboard, said Mabee, including a major one called the "East Border Fault" which runs east of Springfield through Amherst and north into Vermont. Formed when the supercontinent known as Pangea split apart into what is now North and South American and Africa, the fault has not been active for some 200 million years. Other faults are located in upstate New York, the Saint Lawrence River Valley and the eastern part of the Massachusetts near Littleton.

"These old faults are pretty much dormant, but occasionally they get reactivated," said Mabee.