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North Fork Valley Seismic Network - Third Quarter 2008 <br />magnitudes of larger events as estimated by the USGS National Earthquake Information Center <br />in Golden, Colorado (D. Wolny, personal communication, 2007). Coda duration magnitudes <br />(Mc) are also automatically calculated by Earthworm, based on a USGS northern California <br />formula. Mc values are approximately the same as ML <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated in the third quarter (ML >_ 1.7). The date and <br />origin time (hours, minute, and second) of the event, location (latitude, longitude, and general <br />area), magnitudes, and largest PGA values (in g's; 1g = 980 cm/sec 2) are listed. <br />Figure 2 shows the resulting locations for the 347 mining-related seismic events of approximate <br />ML 1.7 and larger. These events cluster in two areas: Elk Creek Mine and Bowie Mine, <br />indicating where the mining activity occurred during this quarter. The largest event to occur <br />during the reporting period was a ML 2.8 event, which occurred at the Elk Creek Mine on 6 <br />September (Figure 2). The second largest event, ML 2.5 occurred on 28 September at Elk Creek <br />Mine. Only 16 events appear to be associated with the West Elk mining operations. Events are <br />located close to the active mining despite the fact that they are occurring outside of the NFVSN <br />station configuration (Figure 2). Event locations are better constrained when they occur within <br />the perimeter of the seismic network (greater azimuthal coverage). <br />During the third quarter, one small probable tectonic earthquake, ML 2.1, was recorded on 15 <br />September by NFVSN and located at Elk Creek Mine (Figure 2 and bolded in Table 3). This <br />event exhibited compressional first motions ("up") at some of the NFVSN stations indicating a <br />double-couple mechanism. Such earthquakes are typical of seismicity in western Colorado. <br />Seismic Event Magnitudes <br />Of the 347 mining-related events, all were assigned magnitudes (Table 3). ML values were <br />assigned to 196 events by Dave Wolny (written communications, 2008) as determined from the <br />MSSN. Coda duration magnitudes (Mc) assigned by Earthworm are also listed in Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values (in g's; lg = 980cm/sec2) were also recorded for all triggered events on the three- <br />component accelerometers. We have included the largest PGA recorded for each relocated <br />seismic event in Table 3 when available, along with the station that recorded the ground motion <br />and the event-to-station distance. The largest PGA recorded during the quarterly period was 0.12 <br />g at station FGH for a ML 1.8 event at a distance of approximately 0.3 km (0.21 mi). This event <br />occurred on 2 August 2008 at the Bowie No. 2 Mine. The largest PGA recorded at the West Elk <br />Mine was 0.02 g at station WFW resulting from the 28 August ML 1.9 seismic event that <br />occurred at a distance of approximately 0.85 km (0.53 mi). Note distances are approximate <br />given uncertainties in the event locations due to uncertainties in the velocity model used in the <br />relocations. <br />UM WAX_WCFS\PROJECTS\WEST ELK-MTN COAL\2008 Q3\Q3_2008_MCC.D0C\18-DEC-08\\OAK 4