2009-05-01_REPORT - C1980007 (3)

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North Fork Valiev Seismic Network - Fourth Quarter 2001 in Golden, Colorado (D. Wolny, personal communication, 2007). Coda duration magnitudes (Mc) are also automatically calculated by Earthworm, based on a USGS northern California formula. Mc values are approximately the same as ML RESULTS Seismic Event Locations Table 3 lists the events recorded and relocated in the third quarter (ML >_ 1.7). The date and origin time (hours, minute, and second) of the event, location (latitude, longitude, and general area), magnitudes, and largest PGA values (in g's; 1 g = 980cm/sec2) are listed. Figure 2 shows the resulting locations for the 227 events of approximate ML 1.7 and larger and 4 earthquakes that occurred near Mt. Lamborn. The mining-related seismic events cluster in four areas, two at Bowie No. 2 Mine, one at Elk Creek mine, and one at West Elk Mine, indicating where the mining activity occurred during this quarter. The largest events to occur during the reporting period were two ML 3.2 events and one ML 3.4 event which all occurred at the West Elk Mine on 2 November, 2 December and 11 November, respectively (shown as red symbols on Figure 2). Approximately 33 events can be associated with the West Elk mining operations. The activity at West Elk Mine corresponds reasonably well with the location of mining activity during the quarter (Figure 2) although the locations are slightly west of the mining activity. The largest event, ML 3.4, was located within a few hundred meters of the actual collapse event as reported by MCC (Wendell Koontz, written communication, 2007). Minor damage was reported at the mine such as spalling of ribs and possible floor heave but no equipment was damaged. Events are located reasonably close to the active mining despite the fact that they are outside of the NFVSN station configuration. Although event locations are better constrained when they occur within the perimeter of the seismic network (greater azimuthal coverage), the seismic activity in the West Elk area is still relatively well located because it occurs close to the network perimeter. Seismic Event Magnitudes Of the 227 mining-related events, 223 were assigned magnitudes (Table 3). ML values were assigned to 202 events by Dave Wolny (written communications, 2008) as determined from the MSSN. Coda duration magnitudes (Mc) assigned by Earthworm are also listed in Table 3. Recorded Peak Ground Accelerations PGA values (in g's; lg = 980cm/sec2) were also recorded for all triggered events on the three- component accelerometers. We have included the largest PGA recorded for each relocated seismic event in Table 3 when available, along with the station that recorded the ground motion and the event to station distance. The largest PGA recorded during the quarterly period was 0.04g at station MFB for a ML 2.9 event at a distance of approximately 0.2 km [0.1 mi]. This event occurred on 3 November 2007 at the Bowie No. 2 Mine. The largest PGA recorded at the West Elk Mine was 0.01 g at station WFW resulting from a ML 3.2 seismic event that occurred on 2 December 2007 at a distance of approximately 2.1 km [1.3 mi]. Note distances are M W:\X WCFS\PROJECTS\WEST ELK-MTN COAL\2007 Q4\Q4 2007 MCC FINAL.DOC\22-FEB-08\\OAK 4