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Subsidence Evaluation for the <br />Exhibit 80E South of Divide and Dry Fork Mining Areas Page 40 <br />14.0 POTENTIAL IMPACTS FROM LOCAL SEISMIC ACTIVITY <br />Earth tremors have been recorded or felt by local residents in the Somerset area since the early <br />1960s. The tremors commonly are the result of coal mine bumps and rock bursts, which are <br />spontaneous releases of strain energy in highly stressed mined/caved coal and rock. In the <br />Somerset Mine area before closure, the bumps and rock bursts were common in room- and -pillar <br />mining areas where stresses concentrated within isolated pillars and blocks of coal (called bump <br />blocks). Earth tremors have continued sporadically in the Somerset Mine area since the mine <br />was closed in the 1980s. <br />Tremors generated by bumps and rock bursts in the Somerset Mine area attain magnitudes that <br />have shaken structures in the West Elk Mine area and have sometimes been felt by West Elk <br />Mine personnel. These local tremors may affect, to a minor degree, underground workings, <br />landslide or potential rockfall areas, particularly during prolonged periods of increased <br />precipitation. It is noteworthy, however, that the Rulison nuclear shot in 1969, which produced a <br />tremor with a Richter magnitude of 5.2 (the magnitude of energy released was many times <br />greater than the magnitudes of any recorded bump or rock burst), did not affect the Somerset <br />Mine. To our knowledge the Rulison nuclear shot did not trigger any landslides or rockfalls, nor <br />did it impact reservoirs, ponds, or streams in the South of Divide or Dry Fork mining areas. <br />r <br />LJ <br />• <br />831- 032.810 Wright Water Engineers, Inc. <br />