Once all the data were shaken out, it turns out that rumble in Chicago's western suburbs last week WAS an earthquake.  The U.S. Geological Survey had said the seismologic profile of the November 4 tremor looked more like a blast from a quarry in McCook than a quake.  

Quarry operators denied exceeding blasting limits, and after another look at the readings, the USGS says the shaker that caused houses to quiver but did no serious damage was what they called a "rockburst."  That's defined as a release of tectonic strain at a shallow depth, in this case near the quarry.


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