Reviewing seismograph records for evidence of very small tremors down to a level equivalent to one-tenth of the energy released by a hand grenade the scientists found "strong statistical evidence" of 569 aftershocks to go along with 759 seismic events that preceded the collapse, which registered as a 3.9 magnitude earthquake.
When the collapse occurred at 2:48 a.m. on Aug. 6, 2007, walls of the Huntington Canyon mine blew in on a six-man crew excavating pillars of coal holding up the roof in a section mined out earlier.
Rescuers dug through debris-filled tunnels for 10 days, hoping to find the miners alive. But on Aug. 16, another implosion of the mine walls killed three rescuers and injured six others, ending the underground effort to rescue the trapped six. Their bodies remain sealed in the mine.
Bob Murray, founder of Murray Energy Co., the mine's co-owner and operator, has contended from the start that an earthquake triggered the collapse. A company statement Friday said the new study "confirms what [Murray Energy] has always said, which is that an earthquake, related to the massive geologic faulting in the area, caused the event. … Nothing that occurred inside the Crandall Canyon mine caused the seismic event of August 2007."
But U. spokesman Lee Siegel said the study doesn't support the view of Murray Energy.
"[Murray Energy's] statement is not based on science and is divorced from reality. From the day of the Crandall Canyon mine collapse, the University of Utah Seismograph Stations pointed out the magnitude-3.9 seismic event recorded was not a natural earthquake, but in fact was the seismic signal of the mine collapse itself. The new study and a 2008 university study both reinforce that view, contrary to [Murray Energy's] claim," Siegel said.
The 2008 study by U. seismologist Jim Pechmann counted 55 seismic events of magnitude 1.6 or higher around the Aug. 6 collapse and calculated the area that blew in covered 50 square acres, four times more than originally thought.
This new research shows that foreshocks and aftershocks were clustered in three areas: where the buried miners were working, where the rescuers were blasted, and in the mine's western depths.
The cluster at the western end indicates the collapse spread into that area, which had not been included as a blow-in zone on the 2008 study.
"We have three clusters to look at and try to come up with an explanation of why there were three," said McCarter, adding that uncertainty remains about why some seismic activity was recorded "in an area where no mining occurred." Murray Energy's statement said this finding verified its position about what occurred.
Tex Kubacki, a U. graduate student and the study's lead author, disagreed with that interpretation by the company.
"The vast majority of seismicity we discovered is in the area where mining took place," he said, adding that most foreshocks were caused by mining. Further research is needed to see if these "small jolts" might have warned of a collapse.
So far, McCarter added, "nothing measured would have said, 'Here's a [mine collapse] that's ready to happen.' "
In 2012, Murray Energy paid nearly $1.5 million to resolve criminal charges and mine-safety citations. The company earlier agreed to an undisclosed settlement with disaster victims.
University of Utah authors of the study presented Friday were Tex Kubacki and Kim McCarter from the mining engineering department and Keith Koper and Kris Pankow from the seismograph stations.