Idaho Mine Collapse
9:19 am
Tue December 20, 2011

Idaho mining accident registers as 2.2 earthquake

The rockburst that injured seven miners in north Idaho last week registered as a 2.2 seismic event according to the U.S. Geological Survey. That contradicts earlier reports about what could have caused the collapse. The mine remains closed while federal officials investigate the incident.

It's a matter of cause and effect. Did an earthquake trigger the rockburst at the Lucky Friday Mine ... or was the shaking caused by the collapse a mile underground?

The mine's owner, Hecla Mining, had attributed a rockburst last month to seismic activity. That led to speculation that a quake caused this accident. Now even the company says the latest burst of rock was in fact the event that seismologists saw.

Mike Stickney of the Montana Bureau of Mines and Geology says rockbursts are the earth's response to voids in the ground -– namely the mine.

“Stress has been concentrated and is released suddenly, and that sudden release of that energy generates seismic waves, just as an earthquake would,” Stickney explains.

Stickney says the hard, yet brittle quartzite rock in north Idaho mines can store a great deal of stress, making them more prone to sudden failures like the one last week.

Hecla says all the injured miners have been released from local hospitals. It’s the third major accident at the Lucky Friday this year.