Geotimes Videocast Tuesday, July 8, 2008
Forensic seismologists: Utah mine collapse source of "earthquake"
It is a chicken-or-egg question: On Aug. 6, 2007, Utah's Crandall Canyon coal mine collapsed, killing six miners. At about the same time, seismic stations in Utah recorded a seismic event of magnitude 3.9. The question is which came first: Did a minor earthquake cause the mine to collapse? Or was the collapse itself the seismic event?
Immediately following the tragedy, the co-owner of the Crandall Canyon mine, Robert Murray of the Murray Energy Corporation, asserted that an earthquake had triggered the collapse. But seismologists have maintained from the start that the early data do not support that interpretation. And after nearly a year of investigation, a team of seismologists has produced a detailed report of the findings. Their conclusion: The mine collapse produced the seismic waves.
"We're as certain as we can be," one of the paper's co-authors, seismologist Walter Arabasz at the University of Utah in Salt Lake City, said in a press release accompanying the report, which was submitted both to Mine Safety and Health Administration (MSHA) investigators and to the journal Seismological Research Letters in early June.
In the early days of the mine, from 1939 to 1955, miners in Crandall Canyon used a "room and pillar" method to excavate the mine, creating "rooms" by removing most of the coal but leaving behind pillars of coal to support the ceiling. From 1997-2003, miners used another technique called longwall mining, extracting the coal in long slices while leaving behind new pillars of coal. According to a mining plan approved by MSHA in 2007, the most recent miners were conducting "retreat mining" — removing the pillars that remained while retreating toward the entrance of the mine.
Based on information from a variety of sources, including their own research, analyses of seismic data by a team of researchers at the University of California at Berkeley (UCB) and Lawrence Livermore National Laboratory (LLNL) and borehole data from the MSHA accident investigation team, the researchers pinned down where the event began to a place within the mine's boundaries, and made new estimates of the collapse's size and duration.
Very early estimates of that location placed it near but outside the western edge of the mine, where the miners were working. That lent support to the notion that an earthquake had triggered the collapse. However, seismologists noted that although the estimated epicenter location was based on seismic data from 33 stations, the closest was 19 kilometers away, which made it difficult to be more precise.
The seismic stations did provide some of the strongest early lines of evidence that the mine collapse caused the seismic event, however. The recorded directions of the first seismic waves that arrived at the stations were an important clue: An analysis of the data from the stations suggested that the motions of those first waves were consistently downward, rather than a mix of upward and downward motions, as might be expected during an earthquake.
Another important source of information was a second tragedy, says Jim Pechmann, also a seismologist at the University of Utah and the study's lead author. The seismologists installed five seismometers above and near the mine following the initial collapse. On Aug.16, three rescuers involved in the search for the miners were killed in a subsequent collapse, which registered as a magnitude-1.6 seismic event. That event, Pechmann says, was a small-scale analogue for the earlier collapse — and this time, they had a more close-up view of the event. "In this case, we knew the precise location of one of the aftershocks," he says. "That allowed us to convert the relative locations into absolute locations.'
On Aug. 16, as the rescuers searched underground, "the coal just exploded from one of the mine walls and filled up a lot of the mine tunnel where these miners were working," Pechmann says. "It wasn't so much that the roof fell on them — it was that the pillars just broke apart suddenly and violently."
All of the additional data provided a way to determine new estimates of the size of the collapse area: about 920 meters by 220 meters across, which is four times as large as previous estimates. From there, the team, which also included University of Utah seismologists Kris Pankow and Relu Burlacu and mining engineer Michael McCarter, proposed a model for the collapse that fits the data. Based on the new location of the epicenter (which is actually the surface expression of the true origin point of a seismic event, called the hypocenter), "we think [the collapse] started at the western edge and then propagated to the east," Pechmann says. Although this collapse was responsible for most of the recorded first waves, the UCB-LLNL team found that a small percentage of the seismic data suggested shearing motions — characteristic of slip along a fault line — as well. "We think the most likely explanation is slip along a crack zone that developed in the mine's roof during the course of the collapse," Pechmann says.
The team also provided an estimate of the duration of the collapse that countered an earlier estimate that Pechmann says was based on a misinterpretation of seismological data (and ended up in congressional testimony). That testimony gave the impression that the collapse lasted four minutes, offering a harrowing picture of the miners' extended suffering. However, the report suggests that the event took only a few seconds at most.
Seismic stations also recorded about 37 "aftershocks" that August, which tended to cluster around the eastern and western edges of the collapse area. Those aftershocks, the researchers say, likely represent additional pillar failures or adjustments to the roof of the mine.
The authors have not yet received any comments on their report from the mine owners, Pechmann says. But he hopes the report will be useful to MSHA as well as provide some answers to the miners' families. "We're hoping our results will help the accident investigation team figure out what caused the collapse to occur," he says, "and thereby help prevent future such collapses."