2007-08-10_PERMIT FILE - C1980007 (3)

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relationship developed by Joyner and Boore (1998) was used. The computed peak ground acceleration for an M=6.5 earthquake at 18 km was O.15g. 7.2 Mine-Induced Seismicity Longwall coal mining can cause or induce seismic events as the result of roof collapses, roof caving, or rock bursts. The mine-induced seismic events, resulting from Longwall mining, tend to have shorter durations (2 to 3 seconds) and higher frequencies (greater than 10 Hz) than natural seismic events. As a result, mine-induced seismic events have lower potential to be damaging to structures. Most of the data available on mine-induced seismicity in the United States is from the Wasatch Plateau-Book Cliffs area of Utah and the northwest corner of the Colorado Plateau. This is because those areas have a consistently high incident ofmine-induced seismic activity and this has been the focus of several studies. 7.2.1 Historic Mine-Induced Earthquakes A study in the Trail Mountain area recorded and evaluated a dataset of 1,913 mine-induced seismic events. The magnitude of these events ranged from zero to 2.2. One of the largest events associated with Longwall mining recorded in the Book Cliffs area was an M 4.2 seismic event at a depth of 2,100 feet in the Willow Creek Coal Mine in March 2000 (Ellenberger and Heasley, 2000). The available literature indicates that mine-induced earthquakes are typically magnitude 2.5 or lower, and magnitude 1.2 appears to be the lowest limit (threshold) of reliable measurements, which reflects ' practical detection settings for typical recording instruments. The larger mine-induced earthquakes (M>2) are generally associated with overburden depths greater than about 1,500 feet. A National Institute for Occupational Safety and Health (NIOSH) study by Ellenberger and Heasley (2000) indicates a greater likelihood of mine-induced seismicity if the following conditions are present: • Overburden depths greater than 1,000 feet • Strong geologic formations in the roof or floor of the mine seam ' • An unusually massive rock unit in the main roof ' These conditions for mine-induced seismicity are associated with higher confining stresses at greater depths and a geologic setting that would undergo a more brittle type failure. Ellenberger and Heasley's study found very few detectable mining-induced seismic events with less than 1,000 feet of overburden depth. They also found that most of the activity occurs at overburden depths greater than 1,250 to 1,500 feet. ' A search of the NEIC USGS Earthquake catalog, reported by GEI (2006), found 30 earthquakes identified as likely mine-related by the USGS based on a variety of criteria including: (1) location r P:\Mpls\06 CO\26\0626067\WorkFi]es\DesignReport\FINAL\DesignReportFINAL.doc 14