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• <br />North Fork V8IIeV Seismic Network - Fourth Quarter 2008 <br />Recorded Peak Ground Accelerations <br />PGA values (in g's: 1 g = 980 cm/sec2) were also recorded for all triggered events on the three- <br />component accelerometers. We have included the largest PGA recorded for each relocated <br />seismic event in Table 3 when available, the station that recorded the ground motion and the <br />event-to-station distance. The largest PGA recorded during the quarterly period was 0.04 g at <br />station MFB for a ML 2.1 event at a distance of approximately 1.12 km [0.7 mi]. This event <br />occurred on 28 October 2008 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 />PGA values were not initially available due to network difficulties from 1 to 20 October 2008. <br />Ordinarily, the strong motion data is automatically picked and recorded in log files, however, <br />this feature was not available during this time. We obtained the original strong motion records <br />for the three closest stations to the 46 earthquakes that were missing measurements. We hand- <br />picked PGAs from the three closest stations to obtain the largest value in Table 3 with the <br />exception of two events on 3 and 7 October 2008. <br />To determine 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 <br />(Figure 3). To calculate hypocentral distance, we used an average event depth of 0.56 km (1837 <br />ft) for the events at Bowie and Oxbow mines and 0.73 km (2394 ft) for those at the West Elk <br />Mine. These values correspond to the approximate depths just above the currently mined coal <br />seams. (Hypocentral distance is the distance to the event's hypocenter or the location where the <br />event was initiated at depth.) The majority of PGAs were measured at hypocentral distances of <br />less than 10 km (6.3 mi) from the recording stations. The measurements are shown color coded <br />in two magnitude bins: ML 1.5 to 2.5 and ML 2.5 to 3.5. Only a few events occurred that were <br />large enough to be in the last category. PGA's recorded for four Mt. Lamborn events from the <br />first quarter of 2008 and two from the 2007 fourth quarter (ML 1.8 to 2.4) recorded at multiple <br />stations are shown in green (Figure 3) some of which were recorded at distances greater than 10 <br />km. <br />Also shown in Figure 3 is the attenuation relationship of McGarr and Fletcher (2005) for ML 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 in (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 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 />have an ML assigned, we converted the coda magnitudes (Table 3) assigned by the automatic <br />location program to ML, using a relationship we developed for seismic events, which had both <br />• Mc and ML. <br />um WAx_wcfs\PROJECTS\North Fork-Bowie\Quarterly reports\2008 Q4\Q4_2008_BRL.d0C\0AK 5