2011-06-01_REPORT - C1980007 (3)

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North Fork Valley Seismic Network — Second Quarter 2010 Depth to Top of layer (km) [mi] Velocity (km /sec) [ft /sec] 3.50 [2.17] 6.00 [19,686] 6.00 [3.73] 6.5 [21,327] Magnitude Estimation M magnitudes are normally provided for the larger events recorded and located by the National Earthquake Information Center (NEIC). However, no events within North Fork Valley were recorded and located by NEIC during this quarter. M values are automatically calculated by Earthworm, based on a USGS northern California formula. Mc values are approximately equivalent to M L . RESULTS Seismic Event Locations Table 3 lists the 35 events recorded and relocated (Mc >_ 1.7) in the second quarter. The date and origin time (hours, minute, and second) of the event, location (latitude, longitude, and general area), magnitude, and largest PGA values (in g's; lg = 980 cm /sec are listed. Figure 2 shows the resulting locations for the mining - related seismic events of approximate M 1.7 and larger. Note event locations are better constrained when they occur within the perimeter of the seismic network (greater azimuthal coverage). The events are located in a rather diffuse pattern in the mining areas with the majority of events concentrated in the Bowie Mine area with five events in the Elk Creek Mine Permit Areas. The largest event to occur during the reporting period was a Mc 2.5 event on 29 May (Figure 2). The event occurred at 08:54 GMT within the Bowie Mine Permit Area. Only 5 events occurred within the West Elk Permit Area (Figure 2). The largest event to occur in the West Elk Mine was a Mc 2.2 event on 26 April 2010. Seismic Event Magnitudes Of the 35 mining- related events, all were assigned magnitudes (Table 3). Coda duration magnitudes (M values assigned by Earthworm are also listed in Table 3. Recorded Peak Ground Accelerations PGA values are generally 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 second quarter was 0.002 g at station BWK for a M 1.9 event at a distance of approximately 0.94 kin (0.58 mi). This event occurred on 18 April 2010 08:58 GMT at the Bowie No. 2 Mine. The largest PGA recorded at the West Elk Mine was 0.0004 g at station MCD resulting from the 18 April 12:04 GMT Mc 1.9 seismic event that occurred at a distance of approximately 4.68 km (2.91 mi). Note distances are I= W:\X_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q2 \Q2_2010_ MCC. DOC\31 -AUG-1 0\\OAK 4