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Seismologists stand by hypothesis that there was no earthquake

Published August 7, 2007 12:23 pm
This is an archived article that was published on sltrib.com in 2007, and information in the article may be outdated. It is provided only for personal research purposes and may not be reprinted.

Posted: 12:24 PM- Though Crandall Canyon mine owner Robert Murray today said an earthquake definitively caused the mine collapse that trapped six miners, seismologists say they have no proof to support his conclusion.

"The current front-running hypothesis is that the seismic event actually represents the collapse of the mine, but we're still waiting for more evidence to make a definitive statement on how this seismic event is related to the collapse of the mine," said Jim Dewey, a research geophysicist for the Colorado-based National Earthquake Information Center run by the U.S. Geological Survey.

In a lengthy and defensive statement this morning, Murray said he had proof the seismic event was an earthquake and not the collapse.

He said he had information from both the University of Utah and the U.S. Geological Survey giving definitive numbers on the depth and location of the earthquake, both of which were too far south and too deep to be the mine.

However, U. of U. and USGS officials said that information is currently unavailable and therefore cannot be released.

"[The seismic event] could be at the mine level," Dewey said, adding it's impossible with existing read outs 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 doesn't have definitive information about the depth or location of the seismic event, either.

"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 this 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- hours after the first event proved the first event was an earthquake followed by aftershocks.

The U.'s Arabasz said there are two hypotheses behind the 10 after events that occurred after the initial event. If the first event was a natural event, then the after events would be naturally occurring aftershocks. However, if the first event were the collapse of the mine, the after events could be the rock mass resettling around the collapse.

The largest after event occurred at 1:12 a.m. today and registered a 2.2 on the Richter scale.

"The event has the same signature, that of an implosion, as the first event," the U.'s Siegel said.

Regardless of what caused the mine to 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."

-- Sheena McFarland can be contacted at smcfarland@sltrib.com or 801-257-8619.




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