Editor’s note: Previous versions of this story referred to the USGS as the United States Geographical Survey. This version correctly refers to the USGS as the United States Geological Survey.

Two organizations that track seismic activity have confirmed that the region experienced an earthquake on Friday, answering questions raised about the mysterious booms heard by residents from Montville to Mystic.

The U.S. Geological Survey recorded a 2.1 magnitude earthquake at 9:05 a.m. Friday that was centered about 2 miles east of Conning Towers-Nautilus Park, or 41.383°N 72.028°W in Groton.

The Weston Observatory, an earthquake research arm of Boston College, reported two earthquakes, one at 9:05 a.m. and another at 9:42 a.m., said Justin Starr, a research assistant.

The quakes, Starr said, were recorded somewhere in the area of the Thames River, close to Ledyard. He said seismometers recorded both quakes as magnitude 1.8. He said there are several ways to measure the earth's movement and the discrepancy in the magnitude recorded by the U.S.G.S. is not uncommon.

The noises produced by the earthquakes led to numerous calls to local police and fire departments, which searched the area looking for signs of an explosion.

After she said her home in Old Mystic shook, Nancy Peta had guessed it was either an earthquake or a plane crash.

"To hear it was an earthquake, I'm sure a lot of people will be relieved insomuch that now they know what it was," she said Monday.

Starr said Friday's quakes were small and close to the surface. The release of seismic energy causes the earth's surface to act like a speaker and emit a sound like an explosion.

Starr said similar quakes occurred in Massachusetts and New Hampshire in October and they are not as uncommon as many would think.

"They do happen every now and again," Starr said.

The Moodus section of East Haddam is famous for small quakes, which to native Americans would have sounded like loud drums, he said. The high school sports teams there are named the Noises.

Why they happen is a little more technical. Starr said the general consensus is that in New England earthquakes are a reactivation of ancient fault lines that are being compressed from east to west. Stress builds up until the pent-up energy is released.

g.smith@theday.com