November 21, 2013
Officials with the U.S. Geological Survey (USGS) said on Friday that a blast at a quarry in McCook, Illinois, likely caused a 3.2-magnitude tremor earlier this month, according to a report from the Chicago Tribune.
USGS officials added that the tremor was, in fact, an earthquake. The officials initially determined it wasn’t.
Jim Dewey, a research geophysicist with USGS, told the Tribune on Friday that the blast likely triggered the tremor, which occurred about seven seconds after the blast.
“The current working hypothesis … is that the tremor was the result of tectonic stress release induced by the quarrying process,” Dewey said. “Its timing appears to have been influenced by the small explosion.”
After meeting with officials from quarry owner Lehigh Hanson and comparing data, USGS officials changed the classification of the tremor to an earthquake.
Officials continue to point to the blast as the cause of the earthquake, comparing it to a “rockburst,” which a sudden shift that occurs after mining causes an uneven amount of pressure between rock plates.
Lehigh Hanson spokesman Jeff Sieg told the Tribune that company officials were happy to have come to an agreement with USGS on calling the event an earthquake. However, Lehigh Hanson does not believe its blasting caused the quake.
Blasting at the the quarry remains suspended indefinitely. Other portions of the site remain in operation.