U.S. Geological Survey officials say they believe a quarry blast caused a tremor Monday with a preliminary magnitude of 3.2 in northern Illinois.
The agency said the event, which some initially mistook for an earthquake, happened about 12:35 p.m. northwest of Countryside in Cook County.
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"It's not an earthquake," said Don Blakeman, a geophysicist with the U.S. Geological Survey. "We're pretty sure it's a large quarry blast."
Blakeman said experts believe a blast caused the tremor because there's a limestone quarry in nearby McCook.
Elmhurst police said they received several phone calls from residents who reported feeling ground tremors on the south side of town. DuComm, which handles the emergency 911 calls for Elmhurst, also reported receiving calls, officials said.
"It's freaky," said Michael Maladonado, 50, who was playing video games in his Chicago bungalow when his couch and TV started to shake. "I felt it for maybe four or five seconds. It was like somebody was bouncing on the floor."
As of 2:15 p.m., more than 600 people reported to federal officials that they felt the tremor. Social media users also took to Twitter and Facebook with accounts from Oak Brook, Hinsdale and other West suburban areas.
"We felt it come as a wave," said Jenylle Rys, an office manager at Advocate Good Samaritan Hospital's Health and Wellness Center in Downers Grove.
Rys said she was on the second floor and packing up for lunch when the walls started creaking and shaking.
"You could definitely feel it on the floor," she said. "I actually thought somebody was in the closet next door doing construction."
There were no immediate reports of damage or injuries. Blakeman said he doesn't expect any.
"Even though lots of people felt this," he said, "a 3.2 is not usually big enough to cause any damage. It's not strong enough to do any structural damage to houses or anything like that."
Earthquakes between 2.5 magnitude and 3.0 are typically some of the smallest felt by humans.