In a sense, it was with some relief that the U.S. Geological Survey (USGS) switched lanes last week and concluded that the tremor that shook the southwest suburbs on Nov. 4 was, indeed, an earthquake.
Until USGS officials actually looked at seismograph information from Hanson Material Service, which had done blasting around the same time the earth moved, the agency stood by its conclusion that the blasting was the source of the tremor.
All along, the quarry maintained that the blasting and a tremor that occurred just seconds later were two separate, perhaps unrelated, events. Readings from their seismographs showed that, indeed, they were separate events.
As to whether they are unrelated … well the jury is still out, but it appears to be pointing toward a connection between the two. And that is a bit of a concern.
Back in 2010, on a day when the quarry was blasting — a practice that takes place dozens of times per year — there was a magnitude 2.7 tremor. That event, too, was characterized by the USGS as a quarry blast, not an earthquake.
Now the USGS is not so sure. The wave patterns from the 2010 and 2013 events are almost identical, according to a USGS geophysicist the Landmark spoke with last week. The circumstances also appear to be very similar.
What scientists are now trying to determine is whether these two incidents were random, isolated occurrences or whether blasting at the quarry is actively contributing to triggering the tremors.
That’s going to be a thorny problem because linking the two could impact the future operation of the quarry. If such quakes are going to be triggered every few hundred blasts, how is that going to play with other nearby business owners in McCook and residents who live next door in LaGrange, Brookfield and Countryside?
A 3.2 magnitude earthquake is a pretty small event, all things considered. The last one was 2.7. But especially near the epicenter, the tremor was very noticeable and, according to several reports, caused some minor damage to structures. It sure as heck scared a lot of people.
Will these events get bigger, or is it completely unpredictable?
For now, the quarry has decided to suspend blasting operations. We think that’s the right call.
The USGS soon hopes to have one of its seismographs, which will run 24 hours a day, seven days a week, installed at the quarry to closely monitor the situation directly underneath the quarry. Is there seismic activity occurring that we don’t know about? It would be nice to know that possibility is being checked.
What’s unsettling about this particular situation is, if you ask the folks at the USGS, such small quarry blasts triggering seismic events is incredibly rare, perhaps unprecedented.
And yet, here we appear to have two such events in the past three years. Small events, but tremors just the same. If these things are not supposed to be happening, why do they appear to be happening?
We need a lot more answers before anyone in the neighborhood will feel comfortable with more blasting at the quarry in the future.