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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 26 • present in these lenticulaz sand units of the Marine Sandstone might therefore be diverted to the mine workings. 2. Though unlikely, fractures filled with water and methane that were reported in the abandoned Oliver No. 2 Mine (Dunrud 1976, p. 30-34) might be encountered or intersected by the B- Seam fractured zone that may divert the water and methane to the B-Seam workings. 12.0 POTENTIAL IMPACTS FROM LOCAL SEISMIC ACTIVITY Earth tremors have been recorded or felt by local residents in the Somerset area since the eazly 1960s. The tremors commonly are the result of coal mine bumps and rock bursts, which are spontaneous releases of strain energy in highly stressed coal and rock. In the Somerset Mine azea before closure, the bumps and rock bursts were common in room-and-pillar mining areas where stresses concentrated within isolated pillazs and blocks of coal. Earth tremors have continued sporadically in the Somerset Mine area since the mine was closed. Tremors generated by bumps and rock bursts in the Somerset Mine area attain magnitudes that have shaken structures in the West Elk Mine area and have been felt sometimes by West Elk Mine personnel. These local tremors may affect, to a minor degree, underground workings, landslide or potential rock fall areas, particularly during prolonged periods of increased precipitation. It is noteworthy, however, that the Rulison nuclear shot in 1969, which produced a tremor with a Richter magnitude of 5.2 (the magnitude of energy released was many times greater energy than the magnitudes of any recorded bump or rock burst), did not affect the Somerset Mine, and did not trigger any known landslides or rock falls. 13.0 SUBSIDENCE CONTROL PLAN Longwall mining is currently planned for extraction of the B-Seam and E-Seam (in Sections 29 and 30) in the proposed Apache Rocks and the Box Canyon mining areas. Although longwall mining may initially induce more caving and fracturing of the roof rocks as compared to the room-and-pillaz method due to the complete removal of coal in the panel, it offers the advantages of maximizing resource recovery. The longwall method also causes more complete subsidence (full extraction of panel) and causes equilibrium conditions to be reached in a shorter period of time (i.e., there is no additional, lingering pillar crushing in panels). As such, to the extent possible, the longwall panels should be located so that barriers or boundaries, particularly in low cover areas, are not under or within the angle of draw or influence to sensitive renewable resource lands or existing occupied residential dwellings. 13.1 Anticipated Effects Long-term impacts on the surface are predicted to be minimal above the longwall panels. The • few surface cracks over the mining panels that may occur are expected to close once the longwall 831-032.181 Wright Water Engineers, Inc.