This earthquake occurred within seconds after a routine explosion at a quarry;  the location determined with seismographic data is not significantly different from the location of the quarry.  The event was initially interpreted by the USGS National Earthquake Information Center as the seismic signature of the quarry blast.   Following consultation with additional seismological experts, quarry operators and technical consultants, and on the basis of examination of records obtained at the quarry, the USGS National Earthquake Information Center has concluded that the quarry blast itself was too small to account for the signals recorded by the regional seismographic network. 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.