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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 (ML >_ 1.7) in the fourth quarter. 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; lg = 980 cm/sec 2) are listed. <br />Figure 2 shows the resulting locations for the mining-related seismic events of approximate ML <br />1.7 and larger. Note event locations are better constrained when they occur within the perimeter <br />of the seismic network (greater azimuthal coverage). The events cluster in two areas, indicating <br />where the active mining occurred during this quarter. The majority of events in the fourth <br />quarter were concentrated at the Bowie Mine. A small number of seismic events are <br />concentrated in the Elk Creek Mine. Only 14 events occurred within the West Elk Permit Area <br />and several of them are located away from the areas of active mining (Figure 2). The largest <br />event to occur during the reporting period were two ML 2.6 events, which occurred at the Bowie <br />Mine on 7 October and 22 November (Figure 2). <br />During the fourth quarter, one small possible tectonic earthquake, ML 1.6, was recorded on 17 <br />November by NFVSN and located at Bowie Mine (Figure 2 and bolded in Table 3). Although <br />this event had a shallow depth of about 1.7 km, this event exhibited several compressional first <br />motions ("up") at some of the NFVSN stations indicating a double-couple mechanism. Such <br />earthquakes are typical of seismicity in western Colorado. <br />Seismic Event Magnitudes <br />Of the 258 mining-related events, all were assigned magnitudes (Table 3). ML values were <br />assigned to 123 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 were also recorded for all triggered events on the three-component accelerometers. <br />We have included the largest PGA recorded for each relocated seismic event in Table 3 when <br />available, along with the station that recorded the ground motion and the event-to-station <br />distance. The largest PGA recorded during the quarterly period was 0.04 g at station MFB for a <br />ML 2.1 event at a distance of approximately 1.12 km (0.70 mi). This event occurred on 28 <br />October 2008 at the Bowie No. 2 Mine. The largest PGA recorded at the West Elk Mine was <br />0.01 g at station WFW resulting from the 14 December ML 1.7 seismic event that occurred at a <br />distance of approximately 0.09 km (0.06 mi). Note distances are approximate given <br />UM W:\X_WCFS\PROJECTS\WEST ELK-MTN COAL\2008 Q4\Q4_2008_MCC.DOC\17-FEB-09\\OAK 4