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North Fork Valley Seismic Network - Second Quarter 2010 <br />Depth to Top of layer (km) [mil Velocity (km/sec) [ft/sec] <br />3.50 [2.17] 6.00 [19,686] <br />6.00 [3.73] 6.5 [21,327] <br />Magnitude Estimation <br />ML magnitudes are normally provided for the larger events recorded and located by the National <br />Earthquake Information Center (NEIC). However, no events within North Fork Valley were <br />recorded and located by NEIC during this quarter. Mc values are automatically calculated by <br />Earthworm, based on a USGS northern California formula. Mc values are approximately <br />equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the 35 events recorded and relocated (Mc >_ 1.7) in the second quarter. The date and <br />origin time (hours, minute, and second) of the event, location (latitude, longitude, and general <br />area), magnitude, 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 Mc <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 />five events in the Elk Creek Mine Permit Areas. The largest event to occur during the reporting <br />period was a Mc 2.5 event on 29 May (Figure 2). The event occurred at 08:54 GMT within the <br />Bowie Mine Permit Area. <br />Only 5 events occurred within the West Elk Permit Area (Figure 2). The largest event to occur <br />in the West Elk Mine was a Mc 2.2 event on 26 April 2010. <br />Seismic Event Magnitudes <br />Of the 35 mining-related events, all were assigned magnitudes (Table 3). Coda duration <br />magnitudes (Nip) values assigned by Earthworm 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 second quarter was 0.002 g at station <br />BWK for a Mr 1.9 event at a distance of approximately 0.94 km (0.58 mi). This event occurred <br />on 18 April 2010 08:58 GMT at the Bowie No. 2 Mine. The largest PGA recorded at the West <br />Elk Mine was 0.0004 g at station MCD resulting from the 18 April 12:04 GMT Mc 1.9 seismic <br />event that occurred at a distance of approximately 4.68 km (2.91 mi). Note distances are <br />um W:\X_WCFS\PROJECTS\WEST ELK-MTN COAL\2010 02\02_2010_MCC.DOC\31-AUG-10\\OAK 4