A ‘swarm’ of earthquakes has Peabody residents rattled. So the city called in an expert

Peabody City HallCity of Peabody

Officials in Peabody called in an expert to discuss the earthquakes that have shaken the city and rattled residents’ nerves in recent weeks.

John E. Ebel, a professor in the Department of Earth and Environmental Sciences at Boston College and senior research scientist at the Weston Observatory, spoke to concerned residents at a public meeting on Wednesday and said what they’ve been experiencing is known as an “earthquake swarm.”

“It isn’t unique to Peabody, it’s happened in other parts of New England,” said Ebel, who’s also the author of the book “New England Earthquakes: The Surprising History of Seismic Activity in the Northeast.”

Ebel said seismic instruments detected three earthquakes that occurred in Peabody over the past month: a 1.4 magnitude earthquake on July 25, a 1.2 magnitude earthquake on Aug. 4, and a 1.3 magnitude earthquake on Aug. 18.

The earthquakes in Peabody have been low in magnitude. A magnitude 4 earthquake might cause an unstable knickknack to fall off a shelf, and a magnitude 5 earthquake could cause walls to crack, he said.

When the quakes have occurred, residents have described hearing what sounds like a loud boom.

“It’s a very quick boom, an explosion,” Ebel said. “If it’s wintertime, you might think you heard the furnace blew up in the basement. In the summertime, people often compare it to an explosion. It’s just a boom, and then it’s gone, but it’s noticeable to people who are right near the epicenter.”

Ebel said the quakes in Peabody appear to be occurring a mile or less below the surface of the earth.

Peabody Mayor Edward A. Bettencourt, Jr. said he first learned of reports of loud booms and explosions being heard by residents back in February.

“These are very real,” Bettencourt said at the meeting on Wednesday. “The earthquakes that are happening, what’s been taking place in Peabody, is very real and we need to address it and find out what’s taking place here.”

At the meeting Ebel gave a presentation and answered questions from concerned residents.

He also spoke about what could happen in the future.

“Earthquake swarms that I’ve studied have typically lasted weeks to months,” Ebel said. “I don’t know when this swarm began. People are reporting events back in February.”

It’s unclear why so many earthquakes have occurred in Peabody lately. “Earthquakes can pretty much happen anywhere,” he said.

Ebel said there could be no more events, or more small earthquakes could occur. He also couldn’t rule out the possibility of a larger earthquake. He estimated that there’s about a 5 percent chance of a larger (but not damaging) earthquake occurring, and less than a 1 percent chance of a larger, damaging (magnitude 5 to 6) earthquake.

“We don’t really know what the chances are of that earthquake being centered here,” he said. “It’s certainly much, much less than 1 percent. There is a chance, but the chances are small.”

If a strong earthquake were to occur, Ebel said people are better off staying inside and taking cover under a piece of heavy furniture, such as a table or desk.

“People are hurt and killed in earthquakes by things falling on them,” he said.

Ebel said the chances of a damaging earthquake happening here is quite low.

“I couldn’t rule out a bigger earthquake happening, but the probability is very, very low,” he said.