The 3.2 magnitude earthquake occurred within seconds after a routine explosion at a quarry in McCook, and most likely in a nearby—if not the same, location—according to USGS.
The USGS National Earthquake Information Center initially reported the event was caused by the quarry blast. But after consultation with experts, quarry operators and technical consultants, they concluded that "the quarry blast itself was too small to account for the signals recorded by the regional seismographic network," according to USGS.
"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," USGS reports.
The Illinois Department of Natural Resources (IDNR) conducted a two-day inspection following the ground-shaking seismic event and reported the blasts at Hanson Material Service quarry in McCook were within statutory limits.
Read more about the Nov. 4 earthquake:
- No Blasting Violations Took Place Monday at Hanson Quarry, DNR Says
- Nov. 4 Quarry Blast 'Demands Additional Scrutiny,' Rep. Lipinski Says
- Quarry Operations Suspended; Hanson, Village Officials to Discuss Mining Practices With Quarry
- Hanson Quarry to Review Monday Blast, Denies Responsibility for Tremor
- Twitter, Facebook React to Monday's Groundshaking Tremor
- So What the Heck Caused Monday's Tremor?