Scientists: Collapse caused tremor
Seismologists now definitively say the collapse of the Crandall Canyon mine caused a seismic event, and no earthquake occurred, despite owner Robert Murray's insistence Tuesday that a quake caused the mine collapse that trapped six miners.
Rafael Abreu, a geologist with the U.S. Geological Survey National Earthquake Information Center in Colorado, said Tuesday that a 3.9 magnitude event recorded at 2:48 a.m. Monday didn't have wave forms characteristic of a naturally occurring earthquake.
"What we are seeing is a mine collapse," Abreu said.
Earlier in the day, seismologists disagreed with Murray's assessment, leaning toward a hypothesis that the mine collapse caused the seismic event instead of an earthquake causing the collapse.
In a lengthy and defensive statement Tuesday morning, Murray said he had proof the seismic event was an earthquake and not the collapse.
He said he had information from the University of Utah and the U.S. Geological Survey giving definitive numbers on the depth and location of the seismic event, both of which were too far south and too deep to be in the mine.
However, U. and USGS officials said they released no such information because it wasn't available early Tuesday.
The seismic event ''could be at the mine level,'' said Jim Dewey, a research geophysicist with the USGS earthquake information center in Colorado. He added that it is impossible with existing readouts to detect the exact location and depth cited by Murray.
"It's not possible to get that resolution. Our instruments just don't permit it," he said.
Walter Arabasz, director of the University of Utah Seismograph Stations, said he, too, lacked definitive information about the depth or location of the seismic event.
"Our seismic network is not designed to locate mining-related seismic activity with near precision," Arabasz said. "The closest seismic station we have is 19 kilometers from the mine, and in terms of its absolute accuracy, we could be off by a mile or more."
He added it's even more difficult to pinpoint the actual depth of the event.
More information will have to be collected from other seismograph stations and from the mine to understand the sequence of events, he said.
U. spokesman Lee Siegel said in Utah's recorded history there has never been a case of a naturally occurring earthquake triggering a mine collapse or cave in.
"However, there have been numerous cases where the collapse was recorded as an earthquake," he said.
Murray, nonetheless, argued Tuesday morning that the duration of the seismic event proves it was an earthquake and not a mine collapse.
"The bumps that we incur in mining are instantaneous and don't last 4.3 minutes. It was an earthquake," he said.
Dewey countered that the actual release of energy lasted no more than five seconds.
Murray also said small seismic events that began occurring about 2 1/2 hours after the first event proved the first event was an earthquake followed by aftershocks.
The U.'s Arabasz said there are two hypotheses regarding the 10 events that occurred after the initial event. If the first event was a naturally occurring earthquake, then the events afterward were naturally occurring aftershocks. But if the first event was the collapse of the mine, the events afterward could have been the rock mass resettling around the collapse.
The largest after-event occurred at 1:12 a.m. Tuesday and registered a 2.2.
"The event has the same signature, that of an implosion, as the first event," the U.'s Siegel said.
Regardless of what caused the collapse, ''at this point, the appropriate focus is on the mine rescue,'' Arabasz said. "Realistically, it's going to take one or more weeks to piece together the information."