WASHINGTON — The mining technique used in Crandall Canyon Mine in Utah, where six miners are trapped, involved collapsing the roof of the mine -- a method that dislodges such tremendous volume of earth with such force that it causes quake activity.
"It's the most dangerous type of mining that there is," said Tony Oppegard, a mining lawyer and former federal mine safety official.
In June, U.S. regulators approved a roof control plan for the "room and pillar" technique, also known as retreat mining, at Crandall Canyon.
It's a delicate endeavor: Columns of coal are left in place to hold up the roof of the mine while the vein is tapped. Once the reserves have been extracted, the miners harvest the last of the coal on the way out, cutting carefully into the pillars and scrambling out of the way as the roof caves in.
The final column to be slashed is known among miners as the "suicide pillar."
The Mine Safety and Health Administration, part of the Department of Labor, signed off on Crandall Canyon's pillar safety plan, spokeswoman Amy Louviere said. The mining agency will investigate whether the mine was complying with the agreed-upon procedures.
Robert E. Murray, president of Murray Energy Corp., which operates the mine, said Tuesday that there was no retreat mining in the immediate vicinity of the miners and that the collapse was caused by an earthquake.
But Don Blakeman, an analyst at the National Earthquake Information Center in Golden, Colo., said Tuesday that Monday's tremor near the mine -- registering 3.9 on the Richter scale -- "just doesn't look like a natural event."
Seismographers at the University of Utah also said that wave patterns from the shock were consistent with the type "induced by underground coal mining" but that a quake along nearby fault lines could not be ruled out because those also occur in central Utah. In 1988, a magnitude-5.2 earthquake was recorded within 40 miles of the mine.
In the Utah coal belt, university scientists said, mine tremors account for about 25% of seismic activity; the other 75% are produced by the activity of tectonic plates.
If Murray is right, and a natural earthquake caused the collapse, that would be a first in Utah.
"We haven't seen an earthquake followed by a mine collapse," said Relu Burlacu, manager of the university's seismograph stations. But the opposite has certainly been the case.
