Sonic boom mistaken for frostquake

It was the boom heard across part of a county in southeast Wisconsin. But its cause has been debated.

Was it a sonic boom from a plane or was it a natural phenomenon called a frostquake? Several media outlets reported it was the latter.

Tuesday night, Dodge County Sheriff Dale Schmidt was in the Beaver Dam town hall when the building began to rattle and shake.

"I wasn't sure what it was at the time," Schmidt said Thursday.

Community members called the sheriff's office to say the noise was a frostquake, a phenomenon which happens when water seeps into the ground, freezes, expands and causes the ground to fracture and to emit a booming sound. But Dodge County dispatch had a hunch and called the National Guard.

Dispatch learned that the National Guard was conducting night flights and one of its jets, a 115th Fighter Wing F-16 Fighting Falcon,  had broken the speed of sound, Schmidt said. It was possible the sound was coming from a sonic boom, not a frostquake.

Ben Herzog, meteorologist at the National Weather Service in Milwaukee, investigated the nighttime boom, as well.

As calls came in from southeast Wisconsin about the noise Jan. 12, the weather service spotted pieces of airplane debris on the weather radar, Herzog said. This is consistent with a sonic boom. The weather service also confirmed that the Air Force was conducting military exercises in the area.

Odds are it was not a frostquake, Herzog said.

A frostquake, also called a cyroseismic boom, is uncommon but has happened in Wisconsin and other frigid states at this time of year.

In 2014, a frostquake rattled residents in Waupun in Fond du Lac County, north of Dodge County. It created a crack in resident Dennis Olsen's driveway 10 inches deep.