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• <br />North Fork Valley Seismic Network — Fourth Q uarter 2009 <br />generally exhibit all dilatational ( "down ") first motions, typical of coal mine seismicity in Utah <br />(e.g., Wong and McGarr, 1990). Only 7 events occurred within the West Elk Permit Area <br />(Figure 2). The largest event to occur in the West Elk Mine was a M 2.0 event on 15 November <br />2009. <br />Seismic Event Magnitudes <br />All of the 46 mining - related events relocated were assigned magnitudes (Table 3). ML values <br />were assigned to 15 events by Dave Wolny (written communication, 2008) as determined from <br />the MSSN. MC values assigned by Earthworm are also listed in Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values are generally recorded for all triggered events on the three - component <br />accelerometers. We have included the largest PGA recorded for each relocated seismic event in <br />Table 3 when available, the station that recorded the ground motion and the event -to- station <br />distance. The largest PGA recorded during the quarterly period was 0.016 g at station FGH for a <br />MC 1.8 event, at 10:09 GMT, at a distance of approximately 1.08 km [0.66 mi]. This event <br />occurred on 11 November 2009 at the Bowie No. 2 Mine. Note distances may be accurate to <br />within only a few hundred meters due to in part, uncertainties in the velocity model used in the <br />• relocations. <br />To estimate potential ground motions at the nearby Bruce Park Dam and landslide area and <br />within the mine areas, we have plotted the recorded PGA values as a function of hypocentral <br />distance for events during the third and fourth quarter of 2007 and all four quarters of 2008 and <br />2009 (Figure 3). To calculate hypocentral distance, we used an average event depth of 0.56 km <br />(1837 ft) for the events at Bowie and Oxbow Mines and 0.73 km (2394 ft) for those at the West <br />Elk Mine. These values correspond to the approximate depths just above the currently mined <br />coal seams. ( Hypocentral distance is the distance to the event's hypocenter or the location where <br />the event was initiated at depth.) The majority of PGAs were measured at hypocentral distances <br />of less than 10 km (6.3 mi) from the recording stations. The measurements are shown color <br />coded in two magnitude bins: ML 1.5 to 2.5 and ML 2.5 to 3.5. Only a few events occurred that <br />were large enough to be in the last category. PGA's recorded for four Mt. Lamborn events from <br />the first quarter of 2008 and two from the 2007 fourth quarter (M 1.8 to 2.4) recorded at <br />multiple stations are shown in green (Figure 3) some of which were recorded at distances greater <br />than 10 km. <br />Also shown in Figure 3 is the attenuation relationship of McGarr and Fletcher (2005) for M 2.0 <br />and 3.0. This attenuation relationship was developed from coal mining- induced seismicity in the <br />Trail Mountain Area, Emery County, Utah for use in ground motion prediction and hazard <br />assessment of a nearby dam. The range in hypocentral distances used was 500 m (1,640 ft) to 10 <br />km (6.3 mi), very similar to event distances recorded at Bowie No. 2, West Elk and Oxbow <br />Mines by the NFVSN. Ground motions from mining- induced events with magnitudes of up to <br />moment magnitude (M) 2.2, augmented with ground motion data from a nearby ML 4.2 event, <br />• were used to develop the relationship using a two -stage regression analysis. They developed <br />prediction equations for PGA, peak ground velocity, and pseudo - velocity response spectra at 5% <br />damping for periods 0.1, 0.2, 0.5, 1.0 and 2.0 sec. For those events that we located but did not <br />UM WAx_wcfs \PROJECTS \North Fork- Bowie\Quarterly reports\2009 Q4 \Q4_2009_BRL.d0C \0AK 5 <br />