Fatal mine collapse covered 50 acres

"The mostly likely explanation is a vertical, north-south crack in the roof of the mine that developed along the western edge of the collapse," with the ground on the east side of the crack dropping downward, Pechmann says.

He emphasizes that seismic records show the shearing motion "did not occur at the start of collapse. It cannot be interpreted as an earthquake that triggered the collapse."

Size of the Collapse

MSHA initially estimated the collapse extended 680 meters (2,231 feet) east to west – or from crosscut 137 to crosscut 120 – and at least 80 meters (262 feet) north to south, the study says, quoting an MSHA official. That is about 13 acres.

Pechmann says his team's "model," which shows a 50-acre collapse beginning at crosscut 143, is "based on seismological data, available underground observations, and constraints on how much collapse could occur given the amount of coal left in the pillars.

The Wasatch Plateau-Book Cliffs coal-mining area of central-eastern Utah is shown in this map. Epicenters of "seismic events " during almost 30 years are shown as red circles and yellow stars.... Click here for more information.

"It's not the only possible scenario to describe the collapsed area, but it fits all the available data we have," he adds. "The epicenter was near the western end of our proposed collapse area, suggesting the collapse started at the western end, and propagated mostly eastward" toward the mine entrance.

How did the calculated 1-foot roof collapse have such deadly consequences?

Pechmann says that within the collapsed area, only 37 percent of coal had been removed, and the rest was left behind in support pillars. "If those pillars shatter and covert to rubble – and if the coal increases 40 percent in volume when it shatters – then the closure you can get between the roof and the floor averages 0.3 meters [1 foot]." The roof can only collapse that far "because it gets stopped by the rubble," he adds.

Collapse Began Near Where Miners Worked

The seismology report notes that the MSHA-approved 2007 amended mining plan called for removing coal from east-west tunnels called "entries" and from north-south tunnels, or crosscuts, leaving behind pillars about 110 feet long and 60 feet wide.

"The next phase of the plan was to mine coal in some of these pillars, working from west to east and allowing the roof around these pillars to collapse," the report says.

The study shows the collapse hypocenter "was right at the edge of where miners were removing pillars in July and early August," Pechmann says. The last known working location of the six miners was just east of where those pillars were removed.