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© Northeast States Emergency ConsortiumSince 1975 moderate size earthquakes have occurred in New Brunswick, New Hampshire, northern New York and Quebec.
"Drop, cover and hold on."

It's a message that likely hasn't reached most New Englanders who rarely worry about the dangers of a major earthquake happening here. But the threat is real.

"Definitely the risk in this area is significantly greater than most people think," said Margaret Boettcher, an assistant professor of geophysics at the University of New Hampshire who has researched the physics of earthquakes and the mechanics of fault slip.

Seacoast emergency officials agree there is a danger and people should take it seriously. While strong earthquakes are rare here, history has shown that they can happen and could pose a significant threat to many of the region's historic buildings that were constructed long before codes were put in place to protect against seismic activity. The old brick buildings in places like Exeter and Portsmouth are at greatest risk, officials say.

"Our biggest area of concern is always the downtown because of the construction," Exeter Fire Chief Brian Comeau said.

The threat of a significant quake occurring at any time, and without warning, has the New Hampshire Division of Homeland Security and Emergency Management encouraging residents, schools and others to participate in this week's "2014 Great Northeast ShakeOut." The event is part of a global earthquake drill designed to raise awareness about the threat and the importance of being prepared. The drill is being held Thursday, Oct. 16 at 10:16 a.m. The state hopes people will heed the message to "drop, cover, and hold on."

Awareness is key, according to Perry Plummer, director of New Hampshire's Homeland Security and Emergency Management.

"Earthquakes do happen in New Hampshire," he said. "In fact, the U.S. Geological Survey's most recent study suggests that earthquakes along the East Coast have the potential to be larger and more damaging than previously thought. The greatest risk is projected for the southeast portion of the state."

Plummer said many of the buildings here are simply not earthquake proof. "These are beautiful buildings that hold much of the character of our state," he said, "but they were built well before many of the modern safety features that can be incorporated today existed. Older brick buildings are particularly vulnerable."

A look through the history books shows the region has been jolted by a number of significant earthquakes. A strong 6.0 quake hit Exeter on Nov. 9, 1810, and created what sounded like a large explosion in the ground, according to information from the U.S. Geological Survey.

"Windows were broken in Portsmouth and a vessel in the harbor seemed to strike bottom. The shock was also felt in Maine at Kennebunk and Portland," the USGS reported.

Central New Hampshire has experienced several quakes over the years, but historians often refer to the major quake that struck off the coast of Cape Ann, Mass., in 1755. The 6.0 quake was felt across much of New Hampshire and caused extensive damage in Boston.

"All of these earthquakes are likely to reoccur, but we don't have a sense of when that would be," Boettcher said.

More recently, New Hampshire and Maine residents got quite a jolt on Oct. 16, 2012 when a 4.0-magnitude quake struck near Hollis Center, Maine. The quake rattled buildings and nerves, and even caused some damage to Epping's historic Watson Academy, a 130-year-old building that had most recently housed the town's recreation center until it was closed after cracks and other problems were discovered following the quake.

Engineers concluded that some of the damage likely resulted from the quake, but also noted that the aging building was already compromised before the quake. Among other things, the quake caused the building's bell tower to rock, and the heavy bell above the roof level is believed to have contributed to the bowing of wood panels and the shifting of door frames.

Boettcher said earthquakes on the East Coast are often felt over a much larger area than those on the West Coast. "Our rocks are stronger and propagate shaking more efficiently," she said.

Portsmouth Fire Chief Steve Achilles, who's also the city's emergency management coordinator, doesn't take the quake threat lightly.

"I think we recognize that Portsmouth has a number of historic and older buildings that weren't built to withstand any seismic activity," he said. "We realize that if we have something more substantial in the range of a (magnitude) 4, 5 or even a 6, we would have significant damage to a lot of our buildings. In our business district, I would be concerned a serious earthquake would cause a lot of damage."

Achilles said his biggest concern would be building collapses and people becoming trapped. Damage to infrastructure such as electricity and natural gas lines could also pose fire hazards.

Robert Marsilia, Portsmouth's chief building inspector, said new construction must meet seismic requirements, but older buildings aren't required to be retrofitted. "If there's a major earthquake, there's going to be major damage," he said. "There's only so much you can do. Fortunately, we don't have a lot of buildings that are in poor shape."

Marsilia has inspected some old masonry buildings and asked owners to bring in structural engineers to make sure they're sound, but he said he doesn't have any "major concern" about any structures in the city that are in immediate danger of collapsing.

Newmarket Fire Chief Rick Malasky said the old mill buildings in his town, which are made mostly of brick and granite, would be of most concern. Cracked chimneys would also be an issue, he said.

Despite the threat, Achilles said there's little that can be done in advance of a large earthquake. "We can have on our minds how we're going to respond and prepare for it," he said, "but it's highly difficult to mitigate it beforehand or implement measures."

Emergency officials said the October 2012 quake raised the awareness that stronger earthquakes can occur. Now they hope people take steps to prepare.

"Unfortunately, that's the nature of disasters and emergencies - they're often unexpected with little or no time to react," Plummer said. "No notice events are some of our biggest challenges because they don't give you time. That's why we're always focusing on preparedness. The time to prepare for a natural disaster is now. The ShakeOut is an opportunity to practice for an earthquake, but also an opportunity to look at your overall preparedness for any emergency or disaster."