2008-10-10_REPORT - C1996083 - Laserfiche WebLink

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• North Fork VaIIeV Seismic Network - Second Quarter 2008 Seismic Event Magnitudes All of the 143 events relocated were assigned magnitudes (Table 3). ML values were assigned to 112 events by Dave Wolny (written communication, 2008) 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 0.01 g at station MFB for a ML 2.1 event at a distance of approximately 1.0 km [0.6 mi]. This event occurred on 15 May 2008 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 hypocentral distance for events during the third and fourth quarter of 2007 and first and second quarters of 2008 (Figure 3). To calculate hypocentral distance, we used an average event depth of 0.56 km for the events at Bowie and Oxbow mines and 0.73 km for those at the West Elk Mine. These values correspond to the approximate depths just above the currently mined coal seams. (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 hypocentral distances of less than 10 km from the recording stations. The measurements are shown color coded in two magnitude bins: ML 1.5 to 2.5 and ML 2.5 to 3.5. Only a few events occurred that were large enough to be in the last category. PGA's recorded for the four Mt Lamborn events from the 2008 first quarter and two from the 2007 fourth quarter (ML 1.8 to 2.4) recorded at multiple stations are shown in green (Figure 3) some of which were recorded at distances greater than 10 km. Also shown in Figure 3 is the attenuation relationship of McGarr and Fletcher (2005) for ML 2.0 and 3.0. This attenuation relationship was developed from coal mining-induced seismicity in the Trail Mountain Area, Emery County, Utah for use in ground motion prediction and hazard assessment of a nearby dam. The range in hypocentral distances used was 500 in (1,640 ft) to 10 km (6.3 mi), very similar to event distances recorded at Bowie No. 2, West Elk and Oxbow Mines by the NFVSN. Ground motions from mining-induced events with magnitudes of up to moment magnitude (M) 2.2, augmented with ground motion data from a nearby ML 4.2 event, were used to develop the relationship using a two-stage regression analysis. They developed prediction equations for PGA, peak velocity, and pseudo-velocity response spectra at 5% damping for periods 0.1, 0.2, 0.5, 1.0 and 2.0 sec. For those events that we located but did not have an ML assigned, we converted the coda magnitudes (Table 3) assigned by the automatic location program to ML, using a relationship we developed for seismic events, which had both Mc and ML. • As can be noted in Figure 3, the typical scatter seen in ground motion data is exhibited by the NFVSN data. The McGarr and Fletcher (2005) curves lie above the central tendency of the data UM WAx_wcfslPROJECTSWorth Fork-Bowie\Quarterly reports12008 Q2\Q2_2008_8RL.d0c10AK 5