2011-06-01_REPORT - C1980007 (4)

Home Browse Search

North Fork Valley Seismic Network — First Quarter 2010 Magnitude Estimation M magnitudes are provided for events recorded and located by the National Earthquake Information Center (NEIC). M values are also automatically calculated by Earthworm, based on a USGS northern California formula. Mc values are approximately equivalent to M RESULTS Seismic Event Locations Table 3 lists the 103 events recorded, four of them possibly of tectonic origin, and relocated (ML >_ 1.7) in the first 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 some events in the Elk Creek Mine Permit Areas. The largest event to occur during the reporting period was a M 3.0 event on 13 February (Figure 2). The event occurred at 04:22 GMT within the Bowie Mine Permit Area. One possible tectonic event occurred beneath Bowie Mine on 8 January and three events on 5, 6, and 11 March within the Elk Creek Permit Area (Figure 2). These M 1.7 to 2.0 events had shallow focal depths of about 1.3 kin but exhibited some compressional first motions ( "up ") at 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 4 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 12 March 2010. Seismic Event Magnitudes Of the 103 mining- related events, all were assigned magnitudes (Table 3). ML values were assigned to 5 events by NEIC. 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 quarterly period was 0.009 g at station FGH for a M 1.8 event at a distance of approximately 0.38 km (0.24 mi). This event occurred W:\X_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q1 \Q1_2010_ MCC. DOC \31- AUG -10\ \OAK 4