McCook quarry blast likely an earthquake, scientists say

Earthquake experts believe that the routine quarry blasts in McCook that were initially unnoticeable, were followed seconds later by an earthquake felt miles away. Because the events were seconds apart in the same area, the US Geological Survey now believes the two events are related.
November 14, 2013 4:23:31 PM PST
Scientists say it's likely that an earthquake shook the western suburbs after a blast at a quarry.

The U.S. Geological Survey, the earthquake experts, now agree that two things happened November 4th, a routine quarry blast that was not very noticeable, followed seconds later by an earthquake very noticeably felt by thousands for miles and miles.

Because they happened seconds apart in the same area, they now think they are related.

The research suggests that planned quarry blasts do not cause larger follow-up tremors, but the U.S. Geological Survey now believes that it is more probable than not that the quarry blast and what followed seven seconds later are related.

In other words, the blast likely triggered what the USGS now believes was a release of tectonic strain.

''To release the strain basically means that the strain caused fracturing of the earth's crust,'' said USGS scientist Jim Dewey.

That fracturing occurred at a shallow depth, not miles deep into the earth's crust where earthquakes typically originate.

The USGS and Hanson Materials, which owns the quarry plan on putting a seismograph in the quarry itself which would read the earths behavior 24/7.

The quarry seismographs in use now are activated only for planned blasts.

''We are trying to develop more knowledge as far as what the earth's surface are, especially the shallow layers,'' said Mike Stanczak of Hason Aggregates.

They are also revisiting a similar tremor that occurred three years ago and was explained at the time as being related to atmospherics.

''We have done a comparison of the signals recorded in 2010 with this event and they're extremely similar in such a way that implies that the 2010 event occurred at almost the same location,'' Dewey said.

Two significant tremors in three years, same general location raises questions about the fragility of the earth's surface where quarry operations have been underway for more than a half century.

''If we thought there was danger to the public, we would cease blasting forever but there is no direct correlation as far as what the cause and effect and we will continue to determine when would be a safe time, an appropriate time to commence operations there," Dewey said.

Hanson has temporarily suspended blasting at its McCook quarry. A new continuously-operating seismograph should be able to determine if there are any microquakes and to give a better read on whether mother earth might be inclined to hiccup again. The USGS has reclassified this event.


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