ST. LOUIS – Chuck Herron heard the loud thud, then another and another. It sounded like someone was dropping big snowballs on the roof of his home.
The house is more than 100 years old and creaks, Herron said, but he had “never heard anything like that before.”
As his neighbors in tiny Paris, Mo., watched the Super Bowl on Sunday, many were startled by similar strange noises. Some even saw flashes of light and called 911.
Scientists say the community experienced a rare natural phenomenon known as a “frost quake,” which happens when moisture in the ground suddenly freezes and expands. If conditions are just right, the soil or bedrock breaks, generating mysterious noises that range from an earthquake-like rumble to sharp cracking sounds sometimes mistaken for falling trees.
This winter has been ripe for frost quakes, known technically as cryoseism. The weather has been frigid, but occasional warm-ups have allowed for thawing. And the temperature swings have sometimes been abrupt.
That was the case last weekend in Missouri, where temperatures in the 40s on Saturday gave way to single-digit readings by Sunday night.
Some people compared the noise to a sonic boom that rattles windows, said Michael Hall, executive director of the 911 center that covers the Hannibal area. Others described it as sounding like “somebody banging on their house.”
Frost quakes were reported last month in Canada and in Indiana, Michigan and Ohio.
In DeKalb, Ill., Lisa Kammes and her family were getting ready for bed earlier this winter when the loud popping noises began.
“The louder ones sounded like somebody was throwing snowballs at the house,” Kammes said.
“It wasn’t the regular noise you hear when your house is creaking, blowing in the wind or ice is breaking,” she said.
The light flashes reported by some people are believed to come from electrical changes that occur when the freezing compresses rocks.
Robert Herrmann of the St. Louis University Earthquake Center said frost quakes are far different from real earthquakes. Tremors typically occur a mile or two underground. Frost quakes are near the surface and do not show up on seismographs.
“But the process is the same,” he said. “There is something that is causing rock to move, and as the rock moves, it generates sound waves and the ground motion.”
Experts say damage is rare but homeowners who experience a frost quake should check for foundation cracks.