This winter has been ripe for frost quakes, known technically as cryoseism. Temperatures have 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.
In Mark Twain's hometown of Hannibal, Mo., 100 miles north of St. Louis, police and emergency dispatches received several calls within about two hours. Facebook feeds were filled with worries.
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."
Missouri isn't alone. Frost quakes were reported last month in Canada and in several other states — Indiana, Michigan, Ohio, Wisconsin.
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.
Neighbors heard noises too, and several contacted police.
"It wasn't the regular noise you hear when your house is creaking, blowing in the wind or ice is breaking," Kammes 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 Saint 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.
"People nearby will probably notice them a lot because they are shallow, but because they do not penetrate deep within the earth, the magnitude will not be large," Herrmann said. "But the process is the same. There is something that is causing rock to move, and as the rock moves, it generates sound waves and the ground motion."
AccuWeather meteorologist Alex Sosnowski said frost quakes are most common in the northern Plains.
"It has to get suddenly colder," Sosnowski said. "If you have a mild winter, you're probably not going to have a lot of this going on."