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U of U Seismograph Stations Research:

ROCK BURSTS AND MINING TREMORS

Earthquakes? in Mining Regions




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ROCK BURSTS AND MINING TREMORS

One important aspect of research at the University of Utah Seismograph Stations is related to mining in earthquake country. Specifically, researchers are working to answer two questions, "Do earthquakes trigger mine collapses?" and "Does mining trigger earthquakes?"

This research has resulted from several important issues related to mine collapse accidents:

  • distinguishing between real earthquakes and mining activity
  • accountability and liability after a mine collapse
  • mine safety and engineering

The research done at the University of Utah Seismograph Stations has been conducted in Utah and Wyoming. In Utah, earthquakes related to mining (mining seismicity) occur in an arcuate pattern around Price, Utah. The mines in the Wasatch Plateau and the Book Cliffs are coal mines which employ long-wall mining techniques. This unusual pattern to earthquake activity is at least in part due to to coal mining. Some of these seismic events are the result of "rock bursts", where rocks in the wall of the mine suddenly burst into the open mine tunnel.

Normally, rocks in the Earth's crust are constantly subjected to pressures from all sides. This is called a confining pressure. However, the mining of coal leaves a low pressure zone (empty space). After mining, pressures are still being applied to the rock on all other sides, except at the wall of the tunnel. If the rock is not strong enough to withstand this stress, the wall of the mine tunnel gives way. These "bursts" are well named because there is often no warning and large rocks can be thrown horizontally or vertically, as well as drop from the ceiling during these events. In addition to these relatively small rock bursts, more catastrophic "implosional" failures can also occur on a much larger scale. Sometimes large sections of mines can collapse.

"Mine tremors", rock bursts or mine collapse events, are recorded on seismograph instruments in exactly the same manner that earthquakes are recorded. If the instruments are sensitive enough and there are enough instruments set up in the mining district, it is possible to distinguish between real earthquakes due to natural causes and "mine tremors" caused by mining activity. The key is to determine the exact location of the rock movement, within the mine or below it.

Recent studies in Utah's coal fields near Price have focused on determining which earthquakes are mining related and whether or not it can be determined which mines are creating them. In the Utah coal belt, most mine tremors are less than magnitude 4 and they account for about one-fourth of all the seismic events in the area. The other three-fourths are natural earthquakes. Initial studies also suggest that particular mines are creating most of the mine tremors and that mining to deeper levels induces mine tremor activity. Data from individual mines is proprietary and is not available.

Other recent studies on mining seismicity have been directed at distinguishing between a mine collapse and a natural earthquake in the Trona Mining District of southwestern Wyoming. On February 3, 1995, a magnitude 5.4 seismic event occurred in a region 29 km west of Green River, Wyoming and in the vicinity of five active underground trona mines. Trona is processed into soda ash and used to manufacture glass, paper, and sodium-based chemicals such as sodium-bicarbonate. During this event, the Solvay Mine suffered a partial collapse in an inactive area of the mine.

Several lines of evidence suggest that the seismic event in the trona mine was caused by the collapse of unstable ground and not induced by shaking during a natural earthquake. Although not conclusive, detailed examination of the recordings of the event did not have many characteristics of rock movement normally seen in natural earthquakes. There are many different energy waves recorded in seismic events. Some types of energy waves are characteristic of natural earthquakes and these were not recorded, suggesting there may not have been a natural earthquake. Modeling of these energy waves also shows that the shape of the waves better fits with a shallow implosion or collapse of rock, rather than rock sliding along a fault during a deeper natural earthquake. In addition, calculations based on the amount of rock which collapsed and the distance the rock moved, show that the amount of energy release that was recorded was well accounted for by only the collapse itself. There is no need to call upon the existence of an earthquake to account for the amount of seismic energy released during the event.

The cause of the seismic event was investigated by University of Utah Seismograph Stations researchers for the purpose of determining if the event was a natural earthquake or if it was the result of a mine collapse. Answers to these types of questions have important implications for insurance companies and mining companies when liability is an issue, particularly if workers are injured and equipment is damaged.

 



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