Edwin L. Aguirre
The magnitude 4.0 earthquake that struck near Hollis Center, Maine, on the evening of Oct. 16 was felt all over New England and eastern New York.
What exactly triggered this rare, fairly strong seismic event? Is there a specific fault, or fracture within the bedrock, that is responsible for it?
“There isn’t one you can put your finger on,” says Prof. Nelson Eby, chair of the Department of Environmental, Earth and Atmospheric Sciences.
“New England has a lot of faults, many of them have been inactive for hundreds of millions of years,” says Eby. “These are trending roughly northeast-southwest, and the fault that was recently activated is probably perpendicular to what we see on the ground, so it’s not visible.”
The quake is most likely caused by the buildup of stress in the bedrock due to crustal movement along the Mid-Atlantic Ridge, he explains.
The ridge, located about a thousand miles away on the floor of the Atlantic Ocean, marks the tectonic boundary where the North American Plate and the Eurasian Plate in the North Atlantic and the South American Plate and the African Plate in the South Atlantic are moving gradually away from each other.
“Movement along this ridge is not uniform, and this differential motion causes stress to build up over time, eventually causing a previously existing known fault — or an undetected, deeply buried one — to suddenly slip and trigger an earthquake,” explains Eby.
These “far-field stress quakes” can occur randomly but, fortunately, they tend to be relatively small compared to earthquakes on the West Coast, he notes.
When the Oct. 16 quake struck, many New Englanders reported feeling a jar or a jolt, as if a train or big truck had rumbled though, that lasted for several seconds.
“We’re sitting on ancient crust, mainly igneous and metamorphic rocks that had been eroded, and all these crystalline materials tend to be rigid and transmit earthquake waves faster, over a much broader region. In California, where the ground has more sediment and therefore is less rigid, the earthquakes typically tend to be confined to a smaller area,” Eby says.
Earthquakes on the West Coast are likely to produce a rolling feeling, like being on a ship out at sea, he adds.
When Is the Big One Happening?
“It’s hard to tell what will happen tomorrow, next year or the next thousand years,” says Eby. “Unlike the West Coast, the Northeast has no good seismic model so it’s a challenge to come up with an accurate prediction. The events are so random. That’s why builders of nuclear power plants had a hard time assessing the region’s seismic risk.”
According to the U.S. Geological Survey, moderately damaging earthquakes strike the region every few decades, and smaller quakes are felt roughly twice a year. The most powerful New England earthquakes occurred in 1638 (estimated at magnitude 6.5) in Vermont or New Hampshire, and in 1755 (magnitude 5.8) offshore from Cape Ann, northeast of Boston.
The 1755 temblor caused severe damage to Boston. Chimneys were leveled and stone fences were knocked down. New springs formed, and old springs dried up. Ground cracks were reported in Scituate, Pembroke and Lancaster, and the shaking was felt from Halifax, Nova Scotia, to Chesapeake Bay in Maryland.
Eby says New England has the same seismic risk as San Francisco in terms of damage to infrastructure.
“A lot of Boston, especially the Back Bay, was built on landfill, so the potential for structural damage is high, but the probability of a devastating quake happening is low,” he says.
If a major earthquake does happen today, the city’s tall, modern buildings like the Prudential or the Hancock Tower would sustain substantial damage, but they won’t necessarily come down or collapse, he notes.
“The foundations for these high-rise buildings were driven all the way down to solid bedrock,” says Eby.
“But older houses would suffer severe damage, with brick facades and chimneys coming down, so you don’t want to be outdoors, where you might get hit by falling debris,” he says. “Our bridges should be OK.”
For more information about what to do before, during and after an earthquake, visit the website of the Federal Emergency Management Agency (FEMA).