November 23, 2022

Lakes Region not immune to seismic activity

By Abby Wilson

This summer, a series of about 10 earthquakes hit Jonesboro Maine, but because the magnitude was less than 3, not many people noticed.

Maine is not immune to earthquakes and each year about
20 earthquakes are recorded but because they are so small
they are usually not noticed. But major earthquakes have
struck New England in years past and might again 
sometime in the future.  
New England experiences around 20 earthquakes every year, but we hardly notice them. The magnitude ranges from very low to average and often they aren’t recorded in the press with only half a dozen or so felt by anyone.

The threshold for damage is around 5 on the Richter Scale. Anything bigger could do harm to structures. Damage varies and depends on multiple factors including distance to the epicenter of the earthquake. As seismic waves travel from the epicenter, they die off, just like ripples from a skipping rock.

Soil conditions also matter because softer soils such as those near a riverbank or marsh cannot tolerate horizontal shaking. If a structure is built on softer soil, this shaking is amplified. Rocky areas and hillsides are stronger and able to sustain horizontal shaking.

John E. Ebel, Professor of Geophysics at Boston College and Senior Research Scientist at the Weston Observatory of Boston College, has been studying earthquakes in New England for 40 years. He says it’s possible that a city which is closer to the epicenter with harder soils, may have less damage than a town further way but with soft soils.

The city of Portland was built close to the ocean and several buildings likely sit on softer soils. The city is at risk of damage if an earthquake over 5 hits the state of Maine, Ebel says. Possible damage would most likely include collapsing chimneys and cracked foundations.

The good news is, Maine does have earthquake protocols and building codes to help prevent extreme and significant damage.

Seismologists can calculate the magnitude of an earthquake that happens hundreds of years prior with mathematical formulas. Using historical records, the scientists can figure out how far the earthquake was felt and how much damage was caused.

It is because of this formula that we know that an earthquake over 6 in magnitude hit Cape Anne, just outside of Boston, in 1755. The epicenter was probably 30 miles offshore. It caused damage as far north as Portland. Wooden framed houses can tolerate horizontal shaking so damage to these early colony homesteads was not horribly extensive. There are historical reports of this earthquake being felt from Lake Champlain to North Carolina.

The last big damaging earthquake to strike Maine was in 1905. The epicenter was located near Eastport and the northern tip of Penobscot Bay. The magnitude of that quake was calculated at 5.6 on the Richter Scale.

The last nondamaging yet significant earthquake occurred in 2006. It measured 4.6 on the Richter Scale and the epicenter was around Bar Harbor. Although there was no structural damage as a result, there were reports of large rock falls in Acadia National Park and many trail closures.

On March 30, 2013, the U.S. Geological Survey recorded a 1.9 magnitude earthquake near North Windham. Its epicenter was 7 kilometers, which is about 4.3 miles from North Windham. In September, a 2.3 magnitude quake was recorded in Greenwood in Oxford County.

Perhaps the most significant earthquake to be felt in New England was one that struck south of Quebec in 1663. It is estimated that it was a 7.5 in magnitude. It was felt throughout Maine and even caused damage in Massachusetts which was 400 miles away from the epicenter.

Ebel says that the damage was so bad, there were landslides along the Lawrence River and six months later the river was still impassable because so many trees had fallen into the water. It took years for the streams to fully clear.

Earthquakes have no set patterns, which drives seismologists crazy, says Ebel. Sometimes there is a large seismic event followed by smaller aftershocks. It can also happen that the before shocks are small and a major earthquake occurs afterward.

In some cases, several small earthquakes occur over the course of a month. The earthquakes that hit Jonesboro in August are a good example of this. Many of these small earthquakes were less than 3 in magnitude with the largest registering at about 3.1.

“Maine is half the spatial area of New England and New England gets 20 earthquakes every year,” Ebel said. “One could assume that because Maine is so big it is probably seeing more earthquakes than other New England states.”

Seismologists know that earthquakes with magnitudes 2 and 3 happen several times a year in Maine and only occasionally do we get one that is over 4. Only once in 450 years would we experience an earthquake with magnitude 6 or over, and every several thousand years, we would potentially get an earthquake over 7.

This is a tricky event to predict though, Ebel says, because scientists don’t have the data. It’s possible that an earthquake did strike here thousands of years ago, and we don’t have human records that describe it.

If that’s the case, we may be due for another large seismic event at some point. However, it’s very difficult to tell.

“Average is calculated but is not predictive,” says Ebel. “Records of earthquakes don’t always tell us when we are due or when we are not due.”

Ebel works for the Department of Earth and Environmental Sciences at Boston College and is the author of “New England Earthquakes: The Surprising History of Seismic Activity in the Northeast.” <

No comments:

Post a Comment