Scientists of the U.S. Geological Survey say it's likely it was an earthquake that shook the western suburbs after a blast at a quarry.
USGS scientist Jim Dewey says research indicates quarry blasts don't cause larger follow-up tremors. However, a quarry blast in McCook was followed by a related tremor seven seconds later.
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Dewey says USGS scientists now believe the blast triggered a release of tectonic strain at a shallow depth, not miles deep in the earth's crust where earthquakes usually originate.
The USGS and Hanson Materials, owner of the quarry, will install a seismograph in the quarry which will make hourly readings of the earth's behavior.
The Nov. 4 tremor shook several communities southwest of Chicago. There were no reports of damage or injuries.