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I~ Possible Subsidence Consequences Predicted subsidence impacts for the Apache Rocks and the Box Canyon mining azeas have been described in detail above. The greatest surface impacts aze expected to occur along the precipitous slopes and cliffs that flank West Flatiron and in the steep canyon slopes of the Box Canyon mining area (Box Canyon, Sylvester Gulch tributaries to the east, and the unnamed drainage to the west of Box Canyon). Though unlikely, the worst possible consequences foreseen are that: Cracks as much as 50 to 100 feet deep may develop above ithe chain pillazs common to the first three northern panels and may divert intermittent flow iii Box Canyon and the unnamed canyon to the west of Box Canyon to the mine through local fractures or to permeable rocks in the overburden (see Map 14 for details). Flow, in this unlikely event, would likely be through lineaz zones of structural weakness (i.e., fractures,) joints) which may have caused the canyon to form initially. It is believed, however, that fractures and joints will terminate in the soft, ductile shale and claystone units. 2. Cracks could locally form and be as much as 75 to 200 feet deep above chain pillars and barrier pillazs on the precipitous slopes, ridges, and/or cliffs that flank West Flatiron and may accelerate the naturally-occurring rock fall and landslide process. • The worst possible, but again very unlikely, underground impacts in the Apache Rocks and the Box Canyon mining areas may be that: 1. Interconnected fractures in the fractured zone may form in the Marine Sandstone that underlies the D-Seam and impact isolated groundwater that may be present. Any water 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 metliane 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. 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 pillars and blocks of coal Earth tremors have continued sporadically in the Somerset Mine azea since the mine was closed. , i . Tremors generated by bumps and rock bursts in the Somerset Mine azea attain magnitudes that have shaken structures in the West Elk Mine azea and have been felt sometimes by West Elk Mine 2.65-132 March 1005PR// ~A 7 -1