2011-06-01_REPORT - C1980007 (2)

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North Fork Valley Seismic Network — Third Quarter 2010 Magnitude Estimation M magnitudes are norinally 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. M values are approximately equivalent to M L . RESULTS Seismic Event Locations Table 3 lists the 32 events recorded and relocated (M >_ 1.7) in the third 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 and Oxbow Mine area with four events in the West Elk Mine Permit Area. The largest event to occur during the reporting period was a M 2.6 event on 4 September (Figure 2). The event occurred at 22:41 GMT within the West Elk Mine Permit Area. Three possible tectonic events occurred within the Bowie Mine or just north of the Oxbow Mine on 23, 27, and 30 September with magnitudes of Mc 1.8, 2.6, and 2.3, respectively (Figure 2). Two of the events occurred at depths greater than 3 km and the third event was shallow (0.5 km) (Table 3). The events exhibited compressional first motions ( "up ") at most of the NFVSN stations. The mining - related seismic events that have been observed to date by the NFVSN generally exhibit all dilatational ( "down ") first motions, typical of coal mine seismicity in Utah (e.g., Wong and McGarr, 1990). Only four events occurred within the West Elk Permit Area (Figure 2). The largest event to occur in the West Elk Mine was a M 2.5 event on 18 September 2010 at 10:09 GMT. Seismic Event Magnitudes Of the 29 mining - related events, all were assigned magnitudes (Table 3). Coda duration magnitudes (M values assigned by Earthworm are also listed in Table 3. According to the National Earthquake Information Center (NEIC), no events of M >_ 3.0 occurred within the bounds of Figure 2. 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 UM WAX_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q3 \Q3_2010_MCC.DOC\11- JAN -11\ \OAK 4