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r? <br />North Fork Valley Seismic Network - Fourth Quarter 2007 <br />recording stations. The measurements are shown color coded in two magnitude bins: ML 1.5 to <br />2.5 and ML 2.5 to 3.5. Only a few events occurred that were large enough to be in the last <br />category. PGA's recorded for two of the Mt Lamborn events (ML 1.8 and 2.1) are shown in <br />green (Figure 5) some of which were recorded at greater than 10 km distance. <br />Also shown in Figure 5 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 />As can be noted in Figure 5, the typical scatter seen in ground motion data is exhibited by the <br />NFVSN data. The McGarr and Fletcher (2005) curves lie slightly above the central tendency of <br />the data. In particular, the curves are conservative at distances less than 1 km. The PGAs at the <br />North Fork Valley mines do not appear to be as high as the McGarr and Fletcher (2005) model; <br />lie at least for ML 1.5 to 2.5. As further data is collected, we will develop a model that fits the North <br />Fork Valley data particularly at the higher magnitudes. We believe the McGarr and Fletcher <br />(2005) relationship can be used on an interim basis to estimate conservative ground motions at <br />the nearby Bruce Park Dam. <br />ACKNOWLEDGMENTS <br /> <br />The NFVSN data analysis and evaluation are jointly supported by MCC and BRL. Our thanks to <br />Collin Stewart, BRL Project Manager and Wendell Koontz, MCC Project Manager for their <br />support. Our appreciation to Pete Swanson (NIOSH/CDS) and Dave Wolny (Mesa State <br />College) for their assistance in the operation and maintenance of the network. Our thanks to <br />Melinda Lee for her assistance in the preparation of this report. <br />U,S WAx_wcfs\PROJECTS\North Fork-Bowie\Quarterly reports\2007 Q4\Q4 Final Report 030308\Q4_2007_BRL_Final_030308.doc\OAK 6