USGS decides that Nov. 4 tremor was an earthquake
Rare tremor that rocked Chicago's western suburbs may take years to determine exact cause
McCOOK – It was an earthquake.
The tremor that shook the western suburbs Nov. 4 seven seconds after a routine explosion at McCook’s Federal Quarry was an earthquake, not a quarry blast, the U.S. Geological Survey determined Wednesday after examining data from Hanson Material Service Corporation, the company that conducted the blast.
Hanson suspended blasting at the quarry after the incident and has yet to resume. Hanson spokesperson Jeff Sieg said the company would likely decide next week if and when to restart blasting, which it typically conducts every few days.
“We don’t have any reason to believe that we won’t resume blasting soon," Sieg said Thursday. "We just haven’t made the decision to do that yet.”
A meeting Wednesday in Colorado started with USGS scientists at odds with Hanson’s stance that the tremor was not caused by its blast alone. But after examining more precise data taken by Hanson’s instruments at the quarry, the USGS determined that the blast and tremor were separate events, the second a 3.2-magnitude earthquake.
Case closed, right? The short answer: not even close.
Scientists still think the blast and earthquake were related, considering the rarity of seismic events in the Chicago area.
“The working hypothesis is this shallow seismic event is somehow associated with the quarrying process and that it was somehow triggered by this very small explosion,” said Jim Dewey, geophysicist at the USGS’ National Earthquake information Center in Colorado. “It’s very likely [the events were connected] just because it would be such an extraordinary coincidence otherwise.”
On first look, what happened Nov. 4 in McCook appears to be so unique, Dewey said, that he estimates it will take several years to figure out what exactly happened, in addition to what should be done about it.
Dewey said he could think of only five or six cases since the mid-1970s in the world where quarry activity was linked to an earthquake.
“We’re sort of in the awkward position of being in effect a doctor diagnosing some weird illness that we don’t really know how best to treat,” he said.
The USGS is considering installing its own seismographs at the quarry to help determine whether “micro earthquakes” are occurring there regularly, Dewey said. If so, seismographs would allow scientists to locate the quakes' origins to within 30 meters, information that could allow quarry operators to adjust operations to prevent triggering shocks, Dewey said.
“The question is what’s causing this and can it be mitigated?” he said.
Representatives from La Grange, Countryside and McCook met with with Hanson officials Tuesday regarding the event. Also present was U.S. Rep. Dan Lipinski, who has requested a federal investigation of the incident.
As of Wednesday, about 50 La Grange residents had notified the village of damage reports filed with Hanson as a result of the earthquake.
Should residents be on their toes for another quake? Given the similar incident that occurred Aug. 31, 2010 – which the USGS is now re-classifying as an earthquake, not a quarry blast, after Wednesday’s discovery – maybe.
“I would not rule out the possibility of something similar to what happened Nov. 4 to happen in the future,” Dewey said.
Shutting down quarry could cause more problems
Even if more research finds the quarry is a hotbed for seismic activity, shutting it down won’t necessarily be the right response.
“Some of the cases we’ve had, the quarries have been shut down for several years. In some cases, the shutting down of the quarry may have exacerbated the problem,” Dewey said.
In one example, a quarry that had been shut down (for reasons unrelated to earthquakes) was filled with water. Scientists hypothesized that the water lubricated a fracture underneath the earth’s surface and triggered shocks.
“[Closing the quarry is] obviously something that people have to think about,” Dewey said. “The problem at present is that might actually make the situation worse.”