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Fault friendly? Quakes in NH

History shows New Hampshire not immune to earthquakes

Staff Writer

Published: Thursday, January 28, 2010

Updated: Wednesday, February 13, 2013 15:02

quake pic resized

Courtesy Photo


The top story of winter break was without a doubt the magnitude 7.0 earthquake that devastated the capital city of Haiti and the surrounding region, causing a death toll that had topped 150,000 as of Sunday.

And although California may be the region that seems most prone to earthquakes in the United States, New England, and New Hampshire itself, is not immune to seismic activity.

"If the past is any key to the future, there is the possibility of a pretty significant earthquake in New Hampshire," said David Wunsch, New Hampshire State Geologist and adjunct professor in the Department of Earth Sciences.

That's because the best predictor of where earthquakes are going to happen continues to be, well, places where earthquakes have already happened.

"All risk is based on previous earthquakes and doing statistical analysis," said Wunsch.

And though it may not be well known for it, the region does have a history.

"There certainly have been some relatively large earthquakes [in New England] over the past 400 years," said Margaret Boettcher, assistant professor of Earth Sciences.

The largest earthquake in New England's history is a magnitude 6.2 quake that struck off the coast of Massachusetts' Cape Ann in 1755. The largest quake to strike New Hampshire was a magnitude 5.5 that struck near Ossipee Lake in 1940. The most recent sizeable quake in the state was a magnitude 4.4 that struck in Gaza, New Hampshire in 1982.

"Those are getting into the moderate range, spaced 40 years apart," said Wunsch.

Although it has been a while since the last major occurrence, that shouldn't be a cause for comfort.

Boettcher noted that areas that have had large quakes in the past are "extremely likely" to have them reoccur. However, scientists don't yet have a good sense of the reoccurrence interval, or length between major quakes.

"There's a lot more research that really should be done in New England," said Boettcher.

However, scientists do know that New England's unique geology actually makes damage more likely than in other areas.

"If there was to be a large earthquake, we would have more damage from the same amount of shaking than California," said Boettcher.

According to Boettcher, this is due to the fact that the rocks that form the base of the region are denser than the rocks in many other areas, including California. This means that the shaking is transmitted more efficiently so that, all things being other equal, a magnitude 6.0 quake in New England would cause more damage than a magnitude 6.0 quake elsewhere. Or, put another way, major damage would occur at a lower magnitude here than elsewhere.

The dense rocks also mean that the region could be more vulnerable to damage from faraway quakes. Once again, history backs the record up. In 1663, a magnitude 7.0 earthquake struck near Charlevoix, Quebec, and was felt as far away as Boston.

The exact magnitude at which damage begins depends on the location and depth of the quake, but damage often occurs when the magnitude is around 5.0 or 5.5.

"The rule of thumb is that is when serious damage will occur," said Pedro de Alba, Professor of Civil Engineering.

The fact that New England can have earthquakes dispels the oft-believed notion that quakes only occur along the fault lines of the tectonic plates. The nearest fault lines to New England are the Mid-Atlantic Ridge to the east and the San Andreas Fault to the West.

The quakes that strike New England are known as intraplate earthquakes. These quakes occur along small faults in the normally stable middle of tectonic plates. According to de Alba, the North American plate is constantly being stretched, which includes the majority of the continental United States, and its brittle nature allows for some seismic activity.

De Alba said that, even in areas with well-known fault lines, predicting the where and when of an earthquake is very tough. Scientists have projected a 99.7 percent chance of a magnitude 6.7 or greater quake striking California, but it could happen any time within the next 30 years. When seismic activity is due to hard-to-identify faults within plates, as opposed to faults between plates, the job is ever harder.

"It's very difficult to predict where it [an earthquake] will happen and when," said de Alba.

But just how at risk is New England? The region's vulnerability can be looked at two ways: seismic hazard and seismic risk. Seismic hazard refers simply to an area's likelihood of having a major earthquake. The majority of New England has low to moderate seismic hazard.

Seismic risk builds on seismic hazard to include both probability and consequences of a quake. It factors in the economic, social, and environmental consequences of a quake, essentially detailing just how bad the damage would be. Using this method, several areas in New England are labeled high risk.

Part of New England's vulnerability to earthquakes is the fact that we don't think we've vulnerable, despite the data.

"We're not earthquake conscious," said de Alba.

Wunsch did note that the New Hampshire Department of Safety does have an earthquake education program that is sent to schools around the state.

The region's again infrastructure also contributes to its higher seismic risk.

"Some of our old buildings are quite vulnerable," said de Alba, noting that many historic buildings in the region have tall chimneys and parapets, both of which often collapse during quakes.

However, de Alba did note that the majority of new buildings in New Hampshire are built to conform to Massachusetts' building codes, which require buildings to have a degree of earthquake resistance.

One possible chance to learn more about the region's earthquake risk may be slowly working its way across the country. As part of the EarthScope program, sponsored by the National Science Foundation, a set of several thousand censors, which detect seismic activity, is being set up in bands stretching from the top to bottom of the continental United States. The censors were originally set up on the west coast, stretching from the Canadian to the Mexican borders, and are making its way eastward after being in a location for several years.

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