Friday, August 10, 2007

How do they know it wasn't caused by an earthquake?

Much to the chagrin of Robert Murray, CEO of the company that owns the Utah mine where 6 workers are trapped; scientists have confirmed that the collapse was not caused by an earthquake. The magnitude 3.9 event was the collapse itself.

Murray insisted there was no way the collapse was not caused by an earthquake - "It was a natural disaster and I'll prove it to you" - even though a federal geologist said Tuesday evening the collapse was absolutely not caused by an earthquake

Murray spent as much time attacking union workers, environmentalists and global warming theory as he did addressing the concerns of the miner's families so he's hardly a sympathetic figure.

Still, how do seismologists really know that there was no earthquake?

By looking at the data from seismometers, scientists can determine how the energy emerged from the source. In simple terms, if all the energy is expanding outward you've got an explosion, all the energy moving inwards identifies an implosion or collapse. Earthquakes slip along faults and some of the energy moves outwards, some inwards in a characteristic pattern called a double-couple.

Scientists at the Berkeley Seismology Lab looked at the data from 16 nearby instruments and the identified the source as a collapse. In the figure above earthquakes fall in the blue region, explosions in the red, and collapses in the green. The Utah event, indicated by the red star, falls well outside the earthquake zone.

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