The earth shook. But how? Why?
By Chicago Tribune
When nature called just before 1 p.m. Monday, security guard John Harbacek left his post at a shipping yard in McCook and headed over to a porta-potty.
His trip to the facilities turned out to be a little more exciting than normal.
First came a concussive blast from Federal Quarry next door to the yard. Seconds later, things got crazy.
"The whole thing was shaking," the 61-year-old Lyons resident said of the porta-potty. "I've been in there when people shake it as a joke, but this thing was lifting two, three inches off the ground."
The tremor, which had a 3.2 magnitude, shook homes and businesses — and likely a few more porta-potties — as far away as Wisconsin.
It also has sparked outrage from residents who live near the quarry and have complained about the blasting for years, including U.S. Rep. Daniel Lipinski, who asked the U.S. Department of Labor and the Bureau of Alcohol, Tobacco, Firearms and Explosives to investigate.
Officials from the U.S. Geological Survey, the Illinois Department of Natural Resources and Lehigh Hanson, the company that owns the quarry, plan to meet soon to compare data. For now, no one knows for sure what caused the tremor that followed the blast.
It wasn't an earthquake, said geophysicists at the USGS, who noted that the shock wave signature recorded by their instruments was typical of a quarry blast.
It wasn't the blast, said officials from the Illinois Department of Natural Resources, which found that data at the site showed the blast and subsequent tremor were separate events.
The conundrum has scientists like Tim Larson scratching their heads.
"This is kind of weird — the two pieces of information contradict each," said Larson, the senior geophysicist for the Illinois State Geological Survey.
Larson said that unlike earthquakes, which are recorded as large spikes when massive plates miles below the surface suddenly shift, blasts from quarries tend to look like little ripples. What made this tremor unique is that it had the force of an earthquake, but the vibrations were consistent with a quarry blast.
But the seven-second delay between blast and tremor remains a mystery.
"The question is whether you can have an earthquake that is triggered by a blast that also shows similar characteristics to a blast," Larson said.
USGS officials remain skeptical the tremor was caused by anything other than the blast but said they haven't seen the raw recordings taken by the quarry owner's equipment and are leaving open the possibility that there is another explanation.
"The evidence ... indicates the event was very shallow, which in our way of speaking means it was in the upper mile or two of the earth's crust," said Jim Dewey, research geophysicist emeritus with the agency. "The typical natural earthquake in the area would occur at a depth of more like five or 10 miles."
It is rare for quarry operations to cause earthquakes, Dewey said. He recalled once in New York where researchers concluded a series of small tremors were the result of a small fault that was able to shift as a quarry operation above it removed rock that had previously pinned the fault in place.
He said massive quakes can cause small tremors in quake-prone areas. But it's unlikely the force of Monday's blast could trigger a larger tremor.
While the USGS measures magnitude by the amount of energy released at the epicenter of a seismic event, seismographs at the quarry used a different measurement called Peak Particle Velocity (PPV) to record the velocity of the ground vibration.
According to Lehigh Hanson officials and the state Department of Natural Resources, the blast caused a PPV of .35 inches per second, well below the state-mandated limit of 1 inch per second. The subsequent tremor posted a PPV of 2.93 inches per second, more than eight times that of the initial explosion.
"In fact, right next to us is the Vulcan (Materials Co.) quarry," said Lehigh Hanson spokesman Jeff Sieg. "Their seismographs didn't pick up our blast, but they did pick up the second event. That's why we've been so adamant that the tremor wasn't caused by our blast."
Dewey hypothesized that perhaps some combination of unknown factors made the small blast feel much larger than normal.
Sieg said the quarry is still operating, but blasting at the site was suspended until at least this week, when officials plan to meet with the Lyons Township Quarry Council.
Meanwhile, the tremor continues to reverberate in the minds of local residents .
The village of La Grange fielded hundreds of calls last week. Officials told residents to document any damage and file claims with Lehigh Hanson and the village.
La Grange resident Karen McDonnell said the blast was unusually strong, but the shock that followed shook chairs and knocked a piece of china from a cabinet to the floor.
"We get a lot of tremors from the blasts here, but it was obviously much more significant than that," she said. "Maybe the people at the quarry are able to learn from it so it doesn't happen again."
Harbacek believes the blast was the source of the tremor.
But at least the porta-potty didn't tip over.
"Then I would have really been mad," he said.
Contribute Volunteer Lawn Sign Get Updates