McCOOK – How could a 3.2 magnitude tremor felt as far as south suburban Milwaukee have nothing to do with a quarry blast 7 seconds earlier at almost the same location in an area practically immune to earthquakes?
That was the biggest question after a seismic event Nov. 4 shook buildings throughout the western suburbs and had some residents thinking they were experiencing an earthquake.
"It's unusual to have that many people feel it," said Paul Caruso, a seismologist at the U.S. Geological Survey's (USGS) National Earthquake Information Center. "Sometimes people do feel quarry blasts. It was a large number of people who went to our website and reported this event."
To be exact, 864 people reported it – by far the most received in response to any of the 36 seismic events the USGS recorded between Oct. 31 and Nov. 6.
Seven seconds before the tremor, at about 12:35 p.m., a blast occurred at the Federal Quarry in McCook. Hanson Material Service Corporation, one of two companies that operates out of the quarry, said it had conducted a scheduled blast and that there was no reason to think it was connected to the seismic event that followed.
"At this point we still cannot find an obvious connection to the [tremor] other than they were 7 seconds apart," said Jeff Sieg, spokesperson for Hanson parent company Lehigh Hanson.
The USGS initially classified the event as a 3.7 magnitude earthquake, but quickly changed it to a 3.2 magnitude quarry blast.
"We know what an explosion looks like on a seismic record, and these look like an explosion," Caruso said.
Experts differ on cause of event
Suzan van der Lee, a seismologist and associate professor at Northwestern University, isn't so sure.
"To me, the seismographs look like they could be an earthquake," said van der Lee, who examined records of the tremor from a Northwestern-owned seismograph at Ryerson Woods Forest Preserve in Lake County. "The blast would have had to set off much more than 10 times as much as explosive as typical [to register at 3.2 on the Richter Scale]. It seems unlikely that setting off such a large, erroneous amount of explosive would happen by accident, or on purpose."
van der Lee said she trusts the USGS classification, but wants to look at records from the quarry to test her hypothesis that what happened Nov. 4 might actually have been an earthquake.
"If you had an earthquake at shallow depths, it could look like an explosion at reasonable distances," said Ken Smith, associate director of the Seismology Lab at the University of Nevada, Reno.
Earthquake or not, it would seem that the tremor was somehow related to the quarry blast based on their shared timing and location.
"Given the rarity of earthquakes in Illinois, it is extremely unlikely that an earthquake would randomly take place in or near a quarry, within seconds from a blast," van der Lee said.
Not the first 'blast' to be felt in area
But a similar event has happened before, and recently. On Aug. 31, 2010, a 2.7 magnitude tremor felt across the area was traced back to the quarry.
"It was not as intense," said Andrianna Peterson, assistant village manager of La Grange. "But it was something that was felt on a widespread regional basis as well."
Afterward, Hanson vowed to be a "silent neighbor" to surrounding municipalities and said it would consider alternative ways to blast in the area active that day.
During the 2010 incident, Hanson was still using its own employees and equipment to conduct blasting, though almost all quarries outsource blasting to more specialized companies, Sieg said. About a year or two later, Hanson began outsourcing blasting at its quarries, but Sieg doesn't know if the change had anything to do with the McCook incident.
State testing holds Hansen blameless
Meanwhile, after a two-day investigation at the quarry, the Illinois Department of Natural Resources said Nov. 6 that Hanson operated within statutory limits for blasting and quarry operation. The velocity of the blast reported by Hanson registered at .35 inches per second, well below the 1 inch-per-second limit set by state regulators. The measurement was taken properly, according to the DNR investigation.
"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," said Mike Falter, head of the Blasting and Explosives Unit for the DNR Office of Mines and Minerals, in a press release. "What this secondary event was, or what caused it, is outside our regulatory expertise."
U.S. Rep Dan Lipinski is calling for additional federal testing.
Geologist Donald Mikulic of the Illinois State Geological Survey has worked with quarries in the Chicago area, including the one in McCook.
"Quarries in Illinois have been blasting rocks since the 1830s, and I cant say I've come across any connection between seismic events and blasting," Mikulic said. "Unless it's happening regularly … it's difficult to say the quarry is at fault."
Getting to the bottom of this quandary, therefore, could be as difficult as throwing a rock out of a quarry.
"Could and were are two different things," said Timothy Larson, senior geophysicist at the Illinois State Geological Survey, about whether the blast and tremor were connected. "It could [be], but was it? That'll take some major amount of effort to determine if it was in fact related. I don't know at the moment, and I think there's going to be a lot of investigation to figure out if in fact it was. That's a big leap to go from could to even the probability of it being [connected] … It's hard to say."