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Temblors in region are rare, but shook Boston centuries ago

Mass. engineers try to prepare

By Carolyn Y. Johnson
Globe Staff / March 20, 2011

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Tremors rumbled through Boston on a winter day in 1663, causing chimneys to tumble, a tavern wall to crack, and objects to fall off shelves. The Rev. S. Danforth of Roxbury described the event in his church record.

“About 6 o’clock at night there happened an earthquake, wch shook mens houses and caused many to run out of their houses into the streets,’’ he wrote. “And ye tops of 2 or 3 chimnyes fell off.’’

The historical accounts of the quake “which made all New England tremble,’’ according to another telling, are a reminder that the region is not immune to earthquakes. And for John Ebel, director of Boston College’s Weston Observatory, they are a scientific clue, which may give insight into earthquake risk in New England today.

Using historical records and seismological models, Ebel estimates in a paper to be published soon in the Bulletin of the Seismological Society of America that the tremors reported in Boston were from a 7.5-magnitude quake that struck about 400 miles away, in the Charlevoix region of Quebec. Previous estimates have put the quake at 7.0.

Earthquakes are relatively infrequent in the Northeast — it would take about 100 years to get the same number California gets in one year.

But they occur, and engineers and emergency planners thinking about quake risk frequently reference a 6.2-magnitude quake that struck Cape Ann in 1755 and toppled the weathervane on Faneuil Hall.

If a major quake like that occurred close to Boston today, models indicate it could cause deaths and major damage to the city’s large collection of historic brick buildings, old infrastructure, and neighborhoods built on landfill or loose soils, such as the Back Bay. But engineers say the state has been a leader, among areas with low to moderate earthquake risk, in planning for one.

“Obviously, it’s a very low-probability but high-risk type of event,’’ said Peter Judge, a spokesman for the Massachusetts Emergency Management Agency, who said the agency routinely holds drills to model response to an earthquake disaster.

According to Edward Fratto, executive director of the nonprofit Northeast States Emergency Consortium, if a Cape Ann-sized earthquake were to occur in the same spot today, a hazard model predicts about $1.6 billion in losses and 10 deaths in Massachusetts.

There is no evidence that there’s ever been a quake of that magnitude centered in Boston, but Fratto said that modeling indicates one could cause $42 billion in damage and about 1,300 deaths in the state.

AIR Worldwide, a Boston company that provides catastrophic risk modeling to insurance companies, governments, and corporate clients, has calculated that an earthquake strong enough to cause $10 billion in losses is expected in the Northeast once every 500 years, according to Jayanta Guin, a senior vice president.

In Massachusetts, an earthquake that could cause $1 billion in losses is expected once every 1,000 years.

Such worst-case calculations are useful for planning and risk-assessment purposes, but they are educated guesses. Even in earthquake-prone Japan, where officials and the public are attuned to the high threat of destructive quakes, the 9.0-magnitude temblor came as a surprise to many.

The last hint in Japan of a tsunami and massive earthquake of comparable size are in written records from the year 869, according to Brian Atwater, a geologist with the US Geological Survey.

“As you get to larger and larger magnitudes, you get into sizes that more and more scientists would be skeptical that such an earthquake could occur and they’d have good scientific reasons to argue against it,’’ Ebel said. “We run into the same problem here in New England — if the largest we’ve seen is 6.2 . . . Could we have 7? Could we have 7.5? Could we have 7.8?’’

Because seismic instruments are only a relatively recent development, geologists trying to estimate earthquake hazards have limited data from which to draw conclusions. Ebel used various tools, including geological models and estimates of the shaking needed to topple a 17th-century chimney, to calculate the likely magnitude of the Quebec earthquake.

John Adams, a seismologist with the Geological Survey of Canada, said that Ebel’s analysis — greater than the current estimate of 7.0 for the 1663 earthquake — is a valuable addition to the ongoing scientific debate about that quake, although he noted that not all geologists would find it compelling.

While the risk of a major quake is much less here than in Japan or California, seismic provisions have been incorporated into the state building code for new construction since 1975, and since about 1997, major renovations to older buildings have required some measure of seismic upgrade, according to Joseph Zona, chairman of the state’s structural advisory committee.

Guidelines in the building code tell engineers how much shaking a building in a given area must be able to withstand. The building code, however, accepts a certain amount of damage if an earthquake is to occur, setting a standard that buildings must be designed to keep occupants from being killed.

“We design for buildings to be damaged, maybe even to point they need to be torn down — but safe to get people out,’’ Zona said. “We’re willing to absorb financial loss for earthquakes because it’s so infrequent.’’

Zona said that newer buildings and the wood-framed houses typical of the region should withstand an earthquake. Buildings made of brick or concrete block without reinforcement, however, are especially vulnerable because they are rigid and can’t move without cracking and breaking up.

And areas with structures built on loose soil are at risk because the ground can amplify shaking.

Guin noted that liquefaction is also a risk, in which soils saturated with water behave like a fluid when shaken, causing structures on top to topple or sink.

In the 1989 Loma Prieta earthquake in San Francisco, the Marina District, which is built on fill, was badly damaged.

Boston’s gas pipes, some of which are a century old, are another possible concern.

John Flint, National Grid’s vice president of gas operations and construction for New England, said the company does regular emergency drills, and has the capacity to shut off service to a street, a neighborhood, or even Boston.

John Christian, a consulting engineer in Waltham, noted that seismic hazards were considered in the engineering of the Big Dig tunnels.

“I would worry less about the large structures like the Central Artery than about all the brick buildings in the Back Bay,’’ he said. “It’s the thousands of little things you have to worry about.’’

Carolyn Y. Johnson can be reached at cjohnson@globe.com.

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