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North Fork Valley Seismic Network - Third Quarter 2008 <br />SUMMARY <br />During the three-month period from 1 October to 31 December 2008, 258 mining-related seismic <br />events of approximate Richter local magnitudes (ML) 1.7 and larger were relocated using data <br />recorded on the North Fork Valley Seismic Network. Most of the seismic events were located in <br />two clusters, at Elk Creek Mine and at the Bowie Mine. Only about 5% of the seismic events <br />(14) occurred within the West Elk Mine permit area. The largest events during this quarter were <br />two ML 2.6 events, which were located at the Bowie Mine. The largest events at the West Elk <br />Mine were two ML 2.1 events on 5 and 19 December. The highest peak horizontal ground <br />acceleration (PGA) observed during the fourth quarter was 0.04 g recorded at station MFB in the <br />Bowie Mine area resulting from a ML 2.1 event at a distance of approximately 1.12 km (0.70 <br />mi). The largest PGA recorded at the West Elk Mine was 0.01 g at station WFW resulting from <br />a 14 December ML 1.7 event located approximately 0.09 km (0.06 mi) from the WFW. <br />INTRODUCTION <br />This report has been prepared in partial fulfillment of a commitment by Mountain Coal Company <br />(MCC) to monitor seismicity (including the microseismic activity near the longwall mining <br />activity) in the North Fork Valley just east of Paonia, Colorado. This commitment is to the <br />Colorado Department of Natural Resources, Division of Reclamation, Mining and Safety. This <br />report contains analyses of the data collected for the fourth quarter of 2008 (1 October through <br />31 December). <br />The area included in this study encompasses the West Elk Mine, operated by MCC and Bowie <br />No. 2 Mine, property of Bowie Resources, LLC (BRL) (Figure 1). Seismicity from Elk Creek <br />Mine, owned by Oxbow Mining, Inc. is also recorded on the North Fork Valley Seismic Network <br />(NFVSN). <br />The 10-station NFVSN was established to (1) monitor the mining-related seismicity occurring in <br />the vicinity of the West Elk and Bowie No. 2 mines at a detection threshold magnitude of ML <br />1.0, (2) provide event locations and magnitudes and estimates of ground motions, and (3) <br />determine focal mechanisms when feasible. <br />NETWORK OPERATIONS <br />The NFVSN consists of 10 permanent stations, which were installed as a cooperative effort <br />between MCC, BRL and the Spokane Research Laboratory of the National Institute of <br />Occupational Safety and Health/Centers for Disease Control and Prevention (NIOSH/CDC). <br />The following factors were used to determine the station locations: access, permission of land <br />owners, network geometry, radio line-of-sight to the common recording site at Paonia airport or <br />a repeater station, and uniform coverage of the target area (Figure 1). Each station consists of <br />three-component accelerometers. The locations of seismographic stations operating during the <br />2008 third quarter are presented in Table 1 and shown on Figure 1. <br />UM W:\X_WCFS\PROJECTS\WEST ELK-MTN COAL\200804\04_2008_ MCC.DOC\17-FEB-09\\OAK I