It was a quarry blast similar to this one-- a routine occurrence in this McCook quarry. The state says it was within regulatory limits, but the tremor that followed seven seconds later was eight times more powerful, and no one knows yet why that happened.
"The most important thing is we do everything we can so this doesn't happen again," said Congressman Dan Lipinski (D-Chicago).
The congressman whose house shook the day of the tremor headlined a meeting Tuesday of parties searching for a geologic answer. The meeting itself was closed to public and press, but everyone in it says they don't want this to be left as a mystery tremor.
"Right now we aren't eliminating anything. We just don't know what it could be. If it is something - it's going to have to be unusual and that's something we're looking to the USGS for guidance on," said Mike Stanczak, Hanson Aggregates, quarry owner.
The quarry owners will meet Wednesday with the US Geological Survey to compare facts and perhaps theories. Routine quarry blasts don't typically trigger larger earth tremors, but the timing of what happened a week ago Monday raises questions.
"It's impossible at this point without more data, it's impossible to say whether the previous event set off this blast several seconds later, but there is very likely an indirect relationship between the two," said Professor Suzan van der Lee, seismologist, Northwestern University.
The quarry blast wasn't picked up on local seismographs, but the big shake seven seconds later most certainly was. Theories abound. Might it have been a natural gas pocket disrupted, or tightly packed limestone layers under stress that shifted violently. Going forward, van der Lee suggests more sophisticated seismology gear may be needed.
"And then see if there's anything happening that's not directly related to the blast, and then try to figure out what's happening," said van der Lee.
Congressman Lipinski has called for federal mines and mineral inspectors to dig into the mystery. Whether they or anyone else can come up with a clear-cut geologic explanation of what shook the earth November 4 is the question.
In the meantime, Hanson, the owner, has suspended blasting at the McCook quarry.