The U.S. Geological Survey (USGS) on Thursday revised its stance on the seismic event that occurred in the southwest suburbs shortly after 12:30 p.m. on Nov. 4, stating that the 3.2 magnitude tremor — felt as far away as southern Wisconsin — was, indeed, an earthquake.

Initially, the USGS attributed the tremor to blasting that took place about 12:35 p.m. at the Hanson Material Service quarry, located at 47th Street and East Avenue in McCook.

But after a meeting with quarry officials on Wednesday and analyzing readings taken by seismographs at the quarry on the day of the tremor, USGS officials determined that their original theory was incorrect.

According to the USGS website, which was updated Thursday, “The regionally recorded seismic event that was felt in the Chicago area corresponds to a release of tectonic strain at shallow depth, at or very near to the quarry, that happened about seven seconds after the quarry blast.”

Dr. James Dewey, a research scientist emeritus for the USGS and seismologist who has worked for the agency for 42 years, confirmed the change in opinion.

“Initially we called it a blast, like many coal mine explosions we’ve seen in Wyoming and West Virginia,” said Dewey.

But when the USGS reviewed the data from seismographs used at the quarry, they realized the blasting done on Nov. 4 could not have produced a tremor of that magnitude.

“The blast was 100 times smaller than the Wyoming blast,” said Dewey. “That explains why we only saw one event. [The quarry blast] was too small to be seen.”

There is still some uncertainty about how closely the blasting at the quarry and the earthquake seven seconds later are related, but Dewey indicated that the USGS believes the two may be linked. However, a quake triggered by such a small blast would be extremely unusual.

“I haven’t before found where a small explosion triggered that effect,” said Dewey.

He said one theory now under consideration is whether the blasting removed a layer of rock, changing the stress in the rock underneath it.

“In the central U.S. you see that stress is nearly horizontal. When you remove the load, you’re taking away the source of the load that kept the rock from rupturing. In this quake, it appears to have generated a very shallow earthquake close to the quarry.”

Dewey also said the USGS would be changing the designation of an Aug. 31, 2010 seismic event, centered near the quarry, from a blast to an earthquake. That tremor was smaller, at 2.7, but the two events appear to be similar, Dewey said. In both instances the blasting and the stronger tremor happened nearly simultaneously.

But because the data received by the USGS in 2010 gave that tremor the appearance of a quarry blast, it was designated as such.

“We were able to get data from that event,” Dewey said. “We overlaid the wave forms and they were very similar and virtually in the same location as far as we can resolve it.”

He said the existence of the 2010 event has caused some concern.

“It suggests that these things may happen every few hundred blasts,” said Dewey, who added that Hanson has blasted 41 times at the quarry in 2013. “We’re hoping to develop a strategy where we don’t get this effect.”

Both Hanson Material Service and the USGS want to dig a little deeper into the potential cause of the two quakes. The two have agreed tentatively to place a USGS seismograph at the quarry site that would operate 24 hours and day, seven days a week. The quarry’s current seismographs are used only when blasting occurs. The closest USGS seismograph is 25 miles south of Chicago, said Dewey.

“We’d like to know if there are tiny micro-quakes happening that didn’t trigger the [existing USGS] instruments,” Dewey said. “We want to install more instruments and get very precise locations.”

Jeff Sieg, the director of corporate communications for Lehigh Hanson Inc., the Texas-based parent company of the McCook quarry, said blasting has been suspended at the quarry pending further study by the company and the USGS. The quarry continues to ship materials from the site.

“We’ve suspended blasting, but we’ve found no reason why we couldn’t resume,” said Sieg. “But there’s been no decision when we will do that. We’d like to resume in the near future. With the cold weather coming, we only have a little more time left this year.”

In the meantime, Hanson Material Service has fielded complaints of building damage caused by the tremor and its inspectors are following up on those claims, Sieg said.

Brookfield Village Manager Riccardo Ginex said his office has not received any complaints about building damage. However, the village’s Department of Public Works reported that some windows in its building, which is located just blocks from the quarry, were cracked in the wake of the tremor.

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