CAMDEN — For some residents who were awake, the loud boom around 1:30 a.m. Sunday was unmistakable. For others, who were asleep, the sound jarred them awake.

Following the single loud noise, nothing came across the police scanners indicating that first responders or police were investigating or responding to an emergency. A call to Knox County Regional Communications Center Sunday morning revealed that nobody had called in to report an issue, or hearing a loud boom sound.

But a post on Facebook brought out reports from others who heard it.

It’s not clear where the boom originated, but there have been reports on Facebook of it being heard on lower Chestnut Street, Sea Street, Trim Street and Free Street. I know I heard it on Chestnut Street, near Highland Avenue, but the sound was in the distance as it came in from an open west-facing window.

One possible explanation this morning comes from fellow editor Lynda Clancy, who shared a link to the state Geological Survey page on cryoseisms, or frost quakes, in Maine. Frost quakes are caused by the sudden freezing and expanding of moisture in the ground.

According to the state agency, a cryoseism (don’t ask me to pronounce it) is a “natural phenomenon that produces ground shaking and noises similar to an earthquake, but is caused by sudden deep freezing of the ground. They typically occur in the first cold snap of the year when temperatures drop from above freezing to below zero, particularly if there is no snow cover to insulate the ground. ”

The National Weather Service Saturday evening issued frost warnings and watches, the later for coastal areas, so it’s quite possible that a cryoseism is what many of us heard early Sunday morning.

A frost quake is very localized, compared to an earthquake that can be felt by people far away from the epicenter, and according to the Geological Survey page, in some cases people in houses a few hundred yards away do not feel anything

“The reason that the vibrations do not travel very far is that cryoseisms don't release much energy compared with a true earthquake caused by dislocation of rock within the earth. On the other hand, since cryoseisms occur at the ground surface they can cause significant effects right at the site, enough to jar people awake,” said that agency.

No surprise, cryoseisms typically occur between midnight and dawn, during the coldest part of the night. If conditions are right, they may occur in a series of booms and shakes over a few hours or even on successive nights.

But that’s apparently not what happened this time, as those who were either awake or wakened by the sound reported hearing only that single one. And the site of this morning’s cryoseism has not been located, since all of those commenting on Facebook only reported hearing it, not feeling it. And the overnight frost watch for the coast, did not pan out, since the lowest temperature here on Chestnut Street was 41 degrees Fahrenheit at 3:40 a.m., according to Weather Underground and the 7 Cedars weather station we keep an eye on (for our honeybees).

For more information, click here and here.

Editorial Director Holly S. Edwards can be reached at hollyedwards@penbaypilot.com or 706-6655.