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

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North Fork Valley Seismic Network- Fourth Quarter 2001 The largest events to occur during the reporting period were two ML 3.2 events and one ML 3.4 event which all occurred at the West Elk Mine on 2 November, 2 December and 11 November, respectively (shown as red symbols on Figure 3). The largest event, ML 3.4, was located within a few hundred meters of the actual collapse event as reported by MCC (Wendell Koontz, written communication, 2007). Minor damage was reported at the mine such as spalling of ribs and possible floor heave but no equipment was damaged. The largest event to occur at Bowie No. 2 Mine was an ML 2.9 event that occurred on 3 November. As previously mentioned, significant bumps occurred in the 1 East Mains CM development section of the Bowie No. 2 Mine on 2 October, 20 October and 3 November 2007. Each event resulted in 0.3 to 0.6 in (1 to 2 feet) of floor heave in the entries and crosscuts. The event of 3 November 2007 resulted in complete disruption of the ventilation structures in the mining section over a distance of about 488 in (1,600 feet). As a result, the Mine Safety Administration issued a "k-order" and all mining activity was stopped. Further mining was prohibited. BRL was allowed to make repairs to the damaged ventilation structures and to recover mining equipment out of the section. Mining was resumed in the 1 Northeast Mains (Figure 2). Figure 4 outlines the extent of floor heave resulting from each of the bump events. During the fourth uarter, three small tectonic earthquakes were located south of the NFVSN near Mt. Lamborn (Figure 3). The largest event of the three was a ML 2.1. Seismic Event Magnitudes Of the 227 events relocated, 223 were assigned magnitudes (Table 3). ML values were assigned to 202 events by Dave Wolny (written communication, 2007) as determined from the MSSN. Mc 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 0.04 g at station MFB for a ML 2.9 event at a distance of approximately 0.1 km [0.1 mi]. This event occurred on 3 November 2007 at the Bowie No. 2 Mine. Note distances may be accurate to within only a few hundred meters due to uncertainties in the velocity model used in the relocations. 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 events during the third and fourth quarter of 2007 (Figure 5). 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 epicenter, which is the location of the seismic event on the earth's surface. Hypocentral distance is the distance to the event's hypocenter or the location where the event was initiated at depth.) The majority of PGAs were measured at epicentral distances of less than 10 km from the UM W:\x_wcfs\PROJECTS\North Fork-Bowie\Quarterly reports\2007 Q4\Q4 Final Report 030308\Q4_2007_BRL_Final_030308.doC\OAK 5