— -- Police dispatchers have been fielding calls from frantic residents in Tennessee who believe they're hearing gunfire or exploding gas lines. But it turns out they're hearing a winter phenomenon that's uncommon to the region.
"It sounded like an explosion underneath. It moved the floor," one resident told the 911 call center in Spring Hill, Tennessee.
This cryoseismic activity or booms are sometimes referred to as frost quakes or ice quakes. Water expands when it freezes and stress builds up until it is released.
People often hear this noise more acutely at night because ambient noise during the day tends to drown it out.
"Whenever we get really cold weather, with temperatures around zero, ice or snow on the ground or previous rain event, you'll usually hear this at night," said Bobby Boyd, meteorologist for the National Weather Service in Nashville. "It's just a sudden cracking in the frozen soil or rock that's saturated with ice."
During his 49 years of experience, Boyd said he's been hearing more of this activity this winter, which he attributes it to the record-breaking cold. This winter has been the first time there have been sub-zero degrees Fahrenheit temperatures in Nashville since February 1996.
Boyd, 68, said he is surprised that callers have likened the sound to a loud boom or believed it was an airplane crash.
"It's like an explosion but a muffled boom sound. It's not really loud, at least the ones I heard," Boyd said.
But it was loud enough to wake Boyd up at 4:30 a.m. two nights ago, and clearly audible at 7 p.m. last night.
Boyd said he hasn't heard of a frost quake causing property damage, as it often happens in a relatively shallow part of the ground, but he doesn't rule it out.
"Any movement in the ground where you have frost and freeze, even without the explosion, can cause cracks in sidewalks or heaving in the soil. Winter time is hard on roads and paved areas," he said.