Peabody In Midst Of 'Earthquake Swarm': BC Professor

PEABODY, MA — A recent series of Peabody-centered earthquakes — amid reports of rattling and loud noises going back more than six months — is likely part of what Boston College professor Dr. John Ebel called an "earthquake swarm" at Wednesday night's public forum on the quakes at City Hall.

Ebel, formerly of the Weston Observatory, compared what has happened in Peabody in recent months to similar earthquake swarms observed in Plainfield, Connecticut in 2015, Searsport, Maine in 2011 and Bar Harbor, Maine in 2006 when a series of 100 or more earthquakes — most too small to be detected without advanced equipment — were felt over a short period before the activity suddenly stopped.

"What you are experiencing here in the town of Peabody is not an everyday occurrence," he told the forum that drew dozens of residents and local officials to Wiggin Auditorium. "But it's not that unusual."

The U.S. Geological Survey defines an earthquake swarm as a "sequence of mostly small earthquakes with no identifiable mainshock. Swarms are usually short-lived, but they can continue for days, weeks or sometimes even months. They often occur at the same location."

Ebel said in 2015 a swarm began around a quarry in Plainfield, Connecticut where they had been no prior seismic activity with more than 180 events detected over a span of six months. He added that there may be frequent "microearthquakes" going on in Peabody each week that are too small to be felt.

Ebel said the smaller earthquakes are often reported as gunshots, shotgun blasts and explosions — matching the descriptions from Peabody residents dating back to two reported "explosions" in February. He added that sound waves from smaller quakes mean that they can often be heard but not felt.

While Ebel said earthquakes are unpredictable he added that based on data available the chances that a larger earthquake could ensue amid a swarm is less than 5 percent, while the chances that a major, damaging earthquake might follow is "much, much less than 1 percent."

Mayor Ted Bettencourt called for the earthquake forum after the third earthquake of larger than 1.0 was confirmed in Peabody in the past month.

"These are very real," Bettencourt said Wednesday night. "We needed to address it and find out what's happening here.

"It's very unsettling. It's very concerning."

Bettencourt said the city was preparing a pamphlet of earthquake facts to distribute to residents and will make a list of emergency contacts and information available to residents in the event some larger event should ever occur.

He added that the fire department was in the process of purchasing seismic equipment that could better pinpoint any more earthquakes and help deliver details to Ebel and the USGS.

"I would love if there were a (predictive) pattern," Ebel said. "There just is no pattern."

Ebel is the author of "New England Earthquakes: The Surprising History of Seismic Activity in the Northeast" and a BC professor of earth and environmental studies.

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(Scott Souza is a Patch field editor covering Beverly, Danvers, Marblehead, Peabody, Salem and Swampscott. He can be reached at Scott.Souza@Patch.com. Twitter: @Scott_Souza.)

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