WASHINGTON – The mining technique used in the Crandall Canyon Mine in Huntington, Utah, where six miners are trapped, involved collapsing the roof of the mine – a method that dislodges such a 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 ormer 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, said spokeswoman Amy Louviere. The mining agency will investigate whether the mine was complying with the agreed-upon procedures.

Robert Murray, the president and chief executive of the company that 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 said wave patterns from the shock were consistent with the type “induced by underground coal mining,” but they said the possibility of a quake having occurred along nearby fault lines could not be ruled out.

In the Utah coal belt, university scientists said, mine tremors account for about 25 percent of the seismic activity; the remaining 75 percent is produced by the activity of tectonic plates.

But 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, although the opposite has occurred.

Initial studies suggest that particular mining areas are creating most of the tremors. “We’ve seen events in the vicinity of this mine in the past,” Burlacu said. Most of the seismic activity, he added, is caused by mining at deeper levels. The Crandall Canyon miners are 1,500 feet underground.

Quakes have been linked to both “longwall” and “room-and-pillar” mining. Each method involves allowing the top of the mine to fall in. In longwall mining – more common in Utah, according to the state mining association – miners move artificial roof supports as they work, allowing the top of the mine where they have finished to cave in behind them.

All coal mining is risky, but the last phase of room-and-pillar mining is disproportionately dangerous, the National Institute for Occupational Safety and Health has found. In a 2003 study, the institute reported that retreat mining accounted for 10 percent of U.S. coal production but 27 percent of mining deaths.

However, Luke Popovich, a spokesman for the National Mining Association, noted that the institute’s sample was small – 100 fatalities. “I’m not sure what we can say statistically about the relative risk of this technique,” he said.