Muskego police on Sunday received reports of “loud booming noises” in the city but were unable to determine the source of the sounds, according to the department.
Meteorologists think residents could have been hearing “frost quakes.”
The sounds were reported just after noon and rattled homes’ windows, according to a post on the Muskego Police Department’s Facebook page.
“Investigating officers were unable to determine a source of the sounds or any evidence that they originated in Muskego,” the department said in the post. “As of this time, we have not received any reports of damage or injuries from citizens.”
The department noted that social media posts indicated the sounds were heard outside the city, too, including in Waukesha, Big Bend, Wind Lake and Vernon.
WISN-TV Chief Meteorologist Mark Baden tweeted that the likely culprit was frost quakes.
“We went from pretty cold to warmer air pretty quickly this morning,” he tweeted.
National Weather Service Meteorologist Chris Stumpf, too, told the Milwaukee Journal Sentinel Sunday that frost quakes are a reasonable explanation for the sounds.
“We can’t confirm anything because they happen pretty randomly, but it’s not out of the realm of possibility to occur here,” he said.
In this case, he said, there’s enough moisture in the ground that it freezes the top layer or so, forming a very tough surface. As temperatures start to warm up, there is expansion in some areas that remain frozen, causing a frost quake.
The phenomenon is not something the National Weather Service tracks, he said.