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2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />• about 400 feet west of the confluence of the Horse Gulch drainage with the reservoir. <br />The cracks, which Ron Hanna had been observing during the past two years at this <br />location, occurred prior to any longwall mining in the western longwall mining panels <br />(panels 12, 13, and 13A) of the Apache Rocks mining azea. No mine-induced seismic <br />effects could have affected this renewed slide activity. <br />10.2 Mine-induced Seismic Activity <br />It is important to note that not tremors were felt while traversing the area above longwall mining <br />panels 12, 13, and 13A, during the annual subsidence field observations. No earth tremors <br />(seismic activity) were felt in the Apache Rocks mining azea in 1999, for example, during the <br />annual traverse to an observation point above longwall panel 13, when mining was occurring <br />directly below it at a depth of 1,200 feet. Longwall mining, at least in relatively uniform <br />overburden (like the Apache Rocks and South of Divide mining aeeas), basically involves 1) the <br />cutting of a coal face, 2) the caving of the roof behind the face, and 3) the recompression of the <br />• caved material behind the support system. This system, beneath the overburden depth range <br />present in the Apache Rocks and South of Divide mining areas, appazently causes only a minimal <br />amount of very low-magnitude seismic activity below the threshold of feeling and hearing. <br />Based on the above mentioned historical observations and evidence, the landslide areas located <br />north and south of Minnesota Reservoir aze not expected to be affected by longwall mining. In <br />years of nonnal precipitarion neither tilt nor strain caused by longwall mining or mine-induced <br />seismic activity is expected to affect these landslides. Should longwall panel E9 be mined during <br />a very wet period, renewed landslide activity could occur. However, based on the conclusion <br />that very little, if any, mining effects were observed on landslides on Jumbo Mountain during the <br />mining of longwall panels 8 and 9, any renewed landslide activity would be primarily caused by <br />high ground saturation, rather than by any mine-induced seismic activity. <br />n <br />U <br />631-032.621 Wright Water Engineers, Inc. Page 42 <br />November 2004 <br />