2008-08-26_REPORT - c1980007 (5)

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North Fork Valley Seismic Network - Second Quarter 2008 magnitudes of larger events as estimated by the USGS National Earthquake Information Center 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 origin time (hours, minute, and second) of the area), magnitudes, and largest PGA values (in g in the second quarter (ML >_ 1.7). The date and event, location (latitude, longitude, and general s; 1 g = 980cm/sec2) are listed. Figure 2 shows the resulting locations for the 81 mining-related seismic events of approximate ML 1.7 and larger. These events cluster in two areas: Elk Creek Mine and West Elk Mine, indicating where the mining activity occurred during this quarter. The largest event to occur during the reporting period was a ML 2.7 event, which occurred at the West Elk Mine on 9 May (Figure 2). Approximately 34 events can be associated with the West Elk mining operations. The second largest event, ML 2.5 occurred on 8 April. Events are located close to the active mining despite the fact that they are occurring outside of the NFVSN station configuration (Figure 2). Event locations are better constrained when they occur within the perimeter of the seismic network (greater azimuthal coverage). During the second quarter, two small probable tectonic earthquakes, ML 1.9 and 2.0, were recorded by NFVSN and located (Figure 2 and Table 3). These events occurred at greater depths (> 5 km) than the mining-related seismic events and they also exhibited compressional first motions ("up") at some of the NFVSN stations indicating a double-couple mechanism. Such earthquakes are typical of the pattern of seismicity in western Colorado. Seismic Event Magnitudes Of the 81 mining-related events, all were assigned magnitudes (Table 3). ML values were assigned to 61 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 0.01g at station MFB for a ML 2.1 event at a distance of approximately 1.0 km (0.64 mi). This event occurred on 15 May 2008 at the Bowie No. 2 Mine. The largest PGA recorded at the West Elk Mine was 0.002 g at station WFW resulting from the 9 May ML 2.7 seismic event that occurred at a distance of approximately 3.7 km (2.3 mi). Note distances are approximate given UM WAX_WCFS\PROJECTSRNEST ELK-MTN COAL\2008 Q2\Q2_2008_MCC.DOC\25-AUG-0B\\OAK 4