2009-05-01_REPORT - C1980007 (3)

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North Fork Valley Seismic Network - Fourth Quarter 2007 SUMMARY During the three-month period, 1 October to December 2007, 227 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 four clusters: two at the Bowie No. 2 Mine, one at the Elk Creek Mine, and one at the West Elk Mine. About 15% of the seismic events (33) occurred at the West Elk Mine. The largest event, a ML 3.4, was located at the West Elk Mine and occurred on 11 November. Minor damage was reported at the West Elk Mine, mostly rib spalling and possible floor heave. Two ML 3.2 events also occurred at the West Elk Mine on 2 November and 2 December. The highest peak horizontal ground acceleration (PGA) recorded during the fourth quarter was 0.04 g at station MFB in the Bowie Mine area which resulted from a ML 2.9 event at a distance of approximately 0.2 km [0.1 mi]. The largest PGA recorded at the West Elk Mine was 0.01 g at station WFW resulting from a ML 3.2 event that occurred on 2 December at a distance of approximately 2.1 km [1.3 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 preliminary analyses of the data collected for the fourth quarter of 2007 (1 October through 31 December). 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). Seismicity from Elk Creek Mine, owned by Oxbow Mining, Inc. is also recorded on the North Fork Network. The 10-station North Fork Valley Seismic Network (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 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 2007 fourth quarter are presented in Table 1 and shown on Figure 1. Viii WAX WCFS\PROJECTS\WEST ELK-MTN COAL\2007 04\04 2007 MCC FINAL.DOC\22-FEB-08\\OAK I