U.S. Geological Survey officials Wednesday were standing by their belief that suburban residents felt a quarry blast earlier this week, despite word from state officials that equipment "picked up vibrations from a second and separate event."
The Monday tremor, which measured magnitude 3.2, initially was reported to be an earthquake. It was felt in Elmhurst, Oak Brook, Hinsdale and other West suburban areas.
Officials said the blast happened at a limestone quarry in McCook. On Wednesday, the Illinois Department of Natural Resources announced that a state inspection of the quarry found no violations.
The two-day inspection by the IDNR's Office of Mines and Minerals Blasting and Explosives Unit found the quarry "operated within the statutory limits for blasting and quarry operation," according to a news release.
"The Office of Mines and Minerals takes the proper handling and use of explosives very seriously," OMM Blasting and Explosives Unit chief Mike Falter said in a written statement. "In this instance, our inspection found no violations of state law."
State officials also said measurement instruments at the quarry detected "a secondary event" seconds after the blast.
"Our review of quarry records does show measurement equipment picked up vibrations from a second and separate event approximately seven seconds after the initial blast," Falter said. "What this secondary event was, or what caused it, is outside our regulatory expertise."
But U.S. Geological Survey officials say they have no record of a second event.
"We didn't record any second event at all," said John Bellini, a geophysicist with the agency.
Bellini said the agency is continuing to classify what happened as a quarry blast.
"That's what our current information is showing," he said, adding the agency hasn't reviewed the data from the quarry.