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North Fork Valley Seismic Network - First Quarter 2010 <br />Magnitude Estimation <br />ML magnitudes are provided for events recorded and located by the National Earthquake <br />Information Center (NEIC). Mc values are also automatically calculated by Earthworm, based <br />on a USGS northern California formula. Mc values are approximately equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the 103 events recorded, four of them possibly of tectonic origin, and relocated (ML <br />>_ 1.7) in the first quarter. The date and origin time (hours, minute, and second) of the event, <br />location (latitude, longitude, and general area), magnitude, and largest PGA values (in g's; 1 g = <br />980 cm/sec2) 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 are located in a rather diffuse <br />pattern in the mining areas with the majority of events concentrated in the Bowie Mine area with <br />some events in the Elk Creek Mine Permit Areas. The largest event to occur during the reporting <br />period was a ML 3.0 event on 13 February (Figure 2). The event occurred at 04:22 GMT within <br />the Bowie Mine Permit Area. <br />One possible tectonic event occurred beneath Bowie Mine on 8 January and three events on 5, 6, <br />and 11 March within the Elk Creek Permit Area (Figure 2). These Mc 1.7 to 2.0 events had <br />shallow focal depths of about 1.3 km but exhibited some compressional first motions ("up") at <br />the NFVSN stations. The mining-related seismic events that have been observed to date by the <br />NFVSN generally exhibit all dilatational ("down") first motions, typical of coal mine seismicity <br />in Utah (e.g., Wong and McGarr, 1990). <br />Only 4 events occurred within the West Elk Permit Area (Figure 2). The largest event to occur <br />in the West Elk Mine was a Mc 2.5 event on 12 March 2010. <br />Seismic Event Magnitudes <br />Of the 103 mining-related events, all were assigned magnitudes (Table 3). ML values were <br />assigned to 5 events by NEIC. Coda duration magnitudes (Mc) values assigned by Earthworm <br />are also listed in Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values are generally recorded for all triggered events on the three-component <br />accelerometers. We have included the largest PGA recorded for each relocated seismic event in <br />Table 3 when available, along with the station that recorded the ground motion and the event-to- <br />station distance. The largest PGA recorded during the quarterly period was 0.009 g at station <br />FGH for a Mc; 1.8 event at a distance of approximately 0.38 km (0.24 mi). This event occurred <br />um WAX WCFS\PROJECTS\WEST ELK-MTN COAL\2010 Q1\Q1_2010_MCC.DOC\31-AUG-10\\OAK 4