2008-11-24_PERMIT FILE - C1996083 (5)

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North Fork Valley Seismic Network -Third Quarter 2007T Magnitude uncertainties are typically 0.1 to 0.2 magnitude units based on comparisons with magnitudes of larger events as estimated by the USGS National Earthquake Information Center (NEIC) in Golden, Colorado (D. Wolny, personal communication, 2007). Coda duration magnitudes (M~) are also automatically calculated by Earthworm based on a USGS northern California formula. MC values are approximately the same as ML. RESULTS Seismic Event Locations Table 3 lists the events recorded and relocated in the third quarter (ML >_ 1.5). The date and origin time (hours, minute, and second) of the event, location (latitude, longitude, and general area), magnitudes, and largest PGA values are listed. Figure 2 shows the resulting locations for the 187 seismic events of approximate ML 1.5 and larger. The seismic events cluster in four distinct areas indicating where the mining activity occurred during this quarter. No mining-related seismic events were located in the vicinity of Bruce Park Dam or the landslide area (Figure 2). The largest events to occur during the reporting period were two ML 2.6 events both of which occurred at the West Elk Mine on 9 and 15 August (shown in red on Figure 2). The largest earthquake to occur at Bowie No. 2 Mine was an ML 2.1 event that occurred on 21 July. Approximately 54 events can be associated with the Bowie mining operations. The activity at Bowie No. 2 Mine corresponds reasonably well with the location of mining activity during the quarter (Figure 2). There are two clusters of events at the Bowie No. 2 Mine. One of these clusters is located about 1.5 km (0.9 mi) northwest of Bowie No. 2 and the other is about 2.5 km (1.6 mi) north-northeast of the mine area (Figure 2). Seismic Event Magnitudes Of the 187 events relocated, 184 were assigned magnitudes (Table 3). ML values were assigned to 117 events by-Dave Wolny (written communication, 2007) as determined from the MSSN. M~ values assigned by Earthworm are also listed in Table 3. Recorded Peak Ground Accelerations PGA values (in g's: 1 g = 980 cm/sec2) were also 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, the station that recorded the ground motion and the event-to-station distance. The largest PGA recorded during the quarterly period was 0.09 g at station FGH for a ML 1.9 event at a distance of 0.7 km (0.4 mi). This event occurred on 15 September 2007 at the Bowie No. 2 Mine. To determine potential ground motions at the nearby Bruce Park Dam and landslide area and within the mine areas, we have plotted the recorded PGA values as a function of epicentral distance for earthquakes during the third quarter of 2007 (Figure 3). We used epicentral distances for plotting the ground motions because the mining-related events are very shallow and so epicentral and hypocentral distances are similar. (Epicentral distance is the distance to the ~~ W:U(_WCFS\PROJECTS\NORTHFORK-BOWIE\QUARTERLYREPORTS\200703\03_2007_BRL_FINAL.DOCN2-FEB-08\\OAK 4