2008-08-26_REPORT - c1980007 (5)

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North Fork Valley Seismic Network - Second Quarter 2008 SUMMARY During the three-month period, 1 April to 30 June 2008, 81 mining-related seismic events were relocated with approximate Richter local magnitudes (ML) 1.7 and larger using data recorded on the North Fork Valley Seismic Network. Most of the seismic events were located in two clusters: one at Elk Creek Mine and one at the West Elk Mine. About 40% of the seismic events (34) occurred at the West Elk Mine. The largest event during this quarter was a ML 2.7 event, which was located at the West Elk Mine. Other large events that occurred at the West Elk Mine were a ML 2.5 event on 9 May and a ML 2.4 event on 8 April. The highest peak horizontal ground acceleration (PGA) recorded during the second quarter was 0.01 g at station MFB in the Bowie Mine area which resulted from a ML 2.1 event at a distance of approximately 1.0 km (0.64 mi). The largest PGA recorded at the West Elk Mine was 0.002 g at station WFW resulting from a ML 2.7 event that occurred on 9 May at a distance of approximately 3.7 km (2.3 mi). INTRODUCTION This report has been prepared in partial fulfillment of a commitment by Mountain Coal Company (MCC) to monitor seismicity (including the microseismic activity near the longwall mining activity) in the North Fork Valley just east of Paonia, Colorado. This commitment is to the Colorado Department of Natural Resources, Division of Reclamation, Mining and Safety. This report contains analyses of the data collected for the second quarter of 2008 (1 April through 30 June). The area included in this study encompasses the West Elk Mine, operated by MCC and Bowie No. 2 Mine, property of Bowie Resources, LLC (BRL) (Figure 1). Seismicity from Elk Creek Mine, owned by Oxbow Mining, Inc. is also recorded on the North Fork Valley Seismic Network (NFVSN). The 10-station NFVSN was established to monitor the mining-related seismicity occurring in the vicinity of the West Elk and Bowie No. 2 mines at a detection threshold magnitude of ML 1.0, provide event locations and magnitudes and estimates of ground motions, and to determine focal mechanisms when feasible. NETWORK OPERATIONS The NFVSN consists of 10 permanent stations, which were installed as a cooperative effort between MCC, BRL and the Spokane Research Laboratory of the National Institute of Occupational Safety and Health/Centers for Disease Control and Prevention (NIOSH/CDC). The following factors were used to determine the station locations: access, permission of land owners, network geometry, radio line-of-sight to the common recording site at Paonia airport or a repeater station, and uniform coverage of the target area (Figure 1). Each station consists of three-component accelerometers. The locations of seismographic stations operating during the 2008 first quarter are presented in Table 1 and shown on Figure 1. Elm W:\X_WCFS\PROJECTS\WEST ELK-MTN COAL\2008 Q2\Q2_2008_MCC.DOC\25-AUG-0800AK 1