The owner of the Crandall Canyon coal mine initially blamed the collapse, which killed six miners and three rescue workers, on an earthquake but University of Utah researchers say analysis of the recordings of the tremor and hundreds of small aftershocks suggests they were a result of mining activity and the subsequent collapse.
"We can see now that, prior to the collapse, the seismicity was occurring where the mining was taking place and that, after the collapse, the seismicity migrated to both ends of the collapse zone," said Tex Kubacki, a graduate student in mining engineering.
Mapping the locations of the aftershocks "helps us better delineate the extent of the collapse at Crandall Canyon," he said.
A previous University of Utah study, based on far fewer aftershocks, said the epicenter of the collapse was near where the miners were working and the aftershocks showed the collapse area covered 50 acres.
The new study, based on data of hundreds of additional aftershocks, has extended the area of the collapse to the full extent of the western end of the mine, Kubacki said.
"It's gotten bigger," he said.
Most of the seismic activity before the collapse was due to mining, the researchers said, although they are investigating whether any of those small jolts might have been signs of the impending collapse.
So far, however, "there is nothing measured that would have said, 'Here's an event [mine collapse] that's ready to happen,'" said Michael "Kim" McCarter, a mining engineering professor and the study's co-author.