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The mystery of an unexplained loud boom county residents heard Saturday still hasn't been solved, but area scientists have a few theories about what might have caused it.

Calls: From around 5 p.m. to 6:30 p.m. on Saturday, York County 911 fielded about a dozen calls reporting a loud explosion, a 911 official said. Calls came from the area south of York City; 911 reported many from the Windsor and York New Salem areas.

Emergency personnel searched for anything that could have caused the boom, like a fire or remnants of an explosion, but they came up empty. On Sunday afternoon, a 911 official said, the cause of the sound had not been identified.

Some emergency personnel tried to get in touch with BAE Systems' factory in the New Salem area; Jeff Cardbaugh, an employee in the security office of the plant, told The York Dispatch he'd heard the bang, but it didn't come from BAE.

He described it as "a big boom" that he heard even though he was inside a building.

Not an earthquake: It wasn't an earthquake, said Charles Scharnberger, professor emeritus at Millersville University.

Scharnberger, who monitors the university's seismograph, said that when he was asked about the boom he checked Saturday's seismic record and saw no evidence of a quake or an explosion in the ground.

In addition to earthquakes, the instrument picks up vibrations caused by things like workers blasting rocks in a quarry or mine, he said.

"There was absolutely nothing: no kind of signal," he said.

Theories: On The York Dispatch's Facebook page, many readers offered ideas as to the origin of the boom. They ranged from earthquakes to aliens to the presence of Sasquatch.

One commenter, hailing from Windsor, noted that in the past few weeks he'd seen military jets flying overhead in "groups and spread patterns," and raised the possibility that one of the jets broke the sound barrier, producing a sonic boom.

Scharnberger said he'd been exchanging emails with a seismologist at Columbia University who raised the same possibility.

In the '60s and '70s, he said, before the military established a policy barring pilots from flying faster than the speed of sound in a populated area, sonic booms were a common nuisance, startling civilians and rattling windows with regularity.

Attempts to contact the press offices at the Air National Guard, the nearest Air Force bases and the Air Force national headquarters were not immediately successful.

And the distribution of the 911 calls over time may point to an entirely different explanation.

Frost quakes: "I'm pretty sure it was frost quakes," said local geologist Jeri Jones.

Jones said he talked with several emergency responders about the boom, or booms, and received many reports of it himself via social media.

Frost quakes happen, he said, when the weather turns from warm to cold.

"When the soil is wet and the water in the soil freezes, it expands and then explodes," he said.

This phenomenon creates little booms, he said.

Multiple frost quakes across the area Saturday evening might explain why not everyone called at once. And the fact that frost quakes take place in the soil and not the rock would explain the lack of activity on the seismograph.

"Frost quakes are very localized," Jones said, "but they come in bunches."

— Reach Julia Scheib at jscheib@yorkdispatch.com.

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