The Crandall Canyon Mine collapse began near where workers were excavating coal and caused the seismic event that led to the deaths of six miners, according to a report released Monday by University of Utah scientists.
"As seismologists, we're as certain as we can be that the seismic event registered as a magnitude-3.9 shock was due to the collapse of the mine and not a naturally occurring earthquake," said Walter Arabasz, director of the University of Utah Seismograph Stations and a co-author of the report.
The researchers estimated that the size of the collapse was about four times larger than was initially thought following the fatal Aug. 6, 2007, disaster that resulted in the deaths of the six miners. Their bodies were never recovered.
"The collapse happened really quickly and probably only lasted a few seconds, at most," said Jim Pechmann, University of Utah seismologist and report co-author. "There was certainly no time for people to have gotten out of the way."
Investigators with the Mine Safety and Health Administration believe that the miners were struck by chunks of coal debris flying off the mine walls during the collapse.
The MSHA investigators had initially estimated the collapse extended about 13 acres, but the cave-in actually covered about 50 acres, according to the U. scientists' report.
Ten days after the collapse, three men died after another cave-in during a failed rescue attempt. The mine has since been permanently shuttered.
"We think that the Aug. 16 seismic event that killed three rescuers was basically a small-scale version of the larger collapse," Pechmann said.
The full 53-page report has been submitted to the journal Seismological Research Letters and to MSHA investigators. The study was funded by the state of Utah and the U.S. Geological Survey.
The seismologists recalculated the epicenter of the initial mine collapse and found it was "within the mine boundary and very close to where the miners were working," Pechmann said. The epicenter is the point on the ground surface above a seismic event's hypocenter, which is the underground point where the event begins.
Arabasz said the seismologists were not granted full access to information gathered by the MSHA investigators. "We had to develop our interpretations, to a significant extent, independent of key information in the mine," he said.
Scientists usually don't release studies until they are published in journals. But due to numerous requests for the information, as a matter of public interest, the researchers decided to release the report. They delayed the release by a couple of weeks at MSHA's request, to give the agency time to inform the disaster victims' families.
MSHA spokesman Matthew Faraci said Monday that the agency's investigation is in its final stages, although no date has been set for releasing a report of the findings.
"MSHA's Crandall Canyon accident investigation team is analyzing the seismology reports, and they will take it into consideration as they prepare their final report," he said.
The U. researchers noted that from Jan. 1, 2007, until the Aug. 6 collapse, there were 28 seismic events large enough to be detected and located at Crandall Canyon and within 1.9 miles of it. The events occurred primarily in areas that had concurrent or recent mining activity.In studying the initial Crandall Canyon cave-in, "we didn't see any indication of accelerating seismic activity in the hours before the collapse," Pechmann said. "We specifically looked for that."