Two seismic events that shook the La Grange area Nov. 4 were only seven seconds apart, but a geologist says more data is needed to connect the two occurrences.
“With the information that’s available now, it’s difficult to say one way or another,” said Bob Bauer, an engineer and geologist with the Illinois State Geological Survey, based in Champaign.
Bauer said seismic recording equipment for the U.S. Geological Survey is based at two stations, one being 30 miles southwest of Hanson Materials Co. in McCook, where workers detonated a surface blast at about 12:39 p.m. Nov. 4. The other station is about 30 miles northwest of McCook.
The quarry blast hardly would be noticed on monitoring equipment, compared to the second event, he noted. The quarry blast measured 0.35 inches per second for vibrations at the nearest protected structure, well below the regulatory limit of 1 inch per second, according to the Illinois Department of Natural Resources.
“The second event measuring a 3.2 on the Richter scale is a good shake,” Bauer said. “It’s not at a level for cosmetic damage, like a brick being knocked out of a chimney.”
Data so far shows the second event as more than a mile west of the quarry and plus or minus 2 kilometers, or 1.25 miles, below the surface, Bauer said.
“Our natural earthquakes recorded in northern Illinois are about 3 to 5 miles down,” Bauer said.
“But the main question is did the blasting initiate some seismic event,” he said. “That’s difficult from several standpoints.”
More information is needed on the precise epicenter of the event, the direction of and depth of any fault in the surrounding rock and the type of rock so its properties can be assessed, Bauer said.
“Blasting can’t necessarily change the cracks in rock,” he said. “If a fault is in a certain direction and ready to go, which means the friction on that plane is at the level of the compression forces, a number of things possibly could happen. It’s pure speculation.”
At least 15 earthquakes measuring a magnitude of 2 or higher have naturally occurred in the Chicago region since the days of Fort Dearborn, including a 3.8 event in Kane County in 2010 felt in eight states.
“There are large cracks throughout the bedrock below the region, and it’s just a normal condition,” Bauer said. “If we have a high horizontal stress compressing east to west and it’s close to a vertical one on an angle to that, one side wants to move past the other.”
Information expected to be shared between the quarry operators, Department of Natural Resources and the U.S. Geological Survey at an upcoming meeting may shed some light on the situation, Bauer said.