A biting polar vortex has settled over the Midwest, causing frigid temperatures that have snapped power lines, grounded thousands of flights and led to the deaths of at least eight people. The deep freeze may have also been responsible for the mysterious booms and bangs that gave some Chicago residents a fright in the early hours of Wednesday morning—a phenomenon known as “frost quakes,” according to CNN affiliate WGN9.
The outlet says that after posting about the possible quakes on social media, it received “tons” of responses from people who had heard strange booming noises during the night.
“I was scared and thought it was the furnace,” one Facebook user wrote. “I kept walking through the house. I had everyone’s jackets on the table in case we had to run out of here.”
Frost quakes, also known as “cryoseisms,” happen when underground water freezes and expands (as frozen water is wont to do) quickly. This rapid expansion pushes against soil and rock, causing them to crack, which in turn creates loud booms. According to Live Science’s Rafi Letzter, frost quakes are relatively rare events that require three conditions to occur: rain or melting snow that saturates the ground, a sudden fall in temperature that causes the earth to freeze, and ground that is free of snow, which can insulate the soil from rapid temperature drops.
At least one recent study suggests that these subzero shake-ups could become more common, possibly due to factors related to climate change—in Canada, at least, Letzter writes. With a predicted frequency of warm, wet winter air masses, the ground will remain damp and snow-free more often, so frost quakes will accompany extreme cold snaps when they do occur.
Conditions may have been right for frost quakes to rattle Chicago; according to Melissa Griffin of ABC News, parts of the Midwest were covered with melting snow before being walloped by temperatures that plummeted well below zero. But it will be difficult to confirm if the quakes actually happened; the booms they create may sound powerful, but frost quakes are actually “very small compared to even a small earthquake,” John Bellini, a geophysicist with the U.S. Geological Survey, tells Alicia Fabbre of the Naperville Sun.
“You’d have to have a seismologer right next to where it occurs,” Bellini adds.
Ben Duebelbeiss, a meteorologist with the National Weather Service, tells Fabbre that the cracks heard this week could be attributed to factors other than frost quakes, like falling branches or houses creaking in strong gusts of wind. Whatever the case may be, it’s best for those affected by the polar vortex to heed officials’ warning and stay indoors until the bitter cold subside.