2008-11-24_PERMIT FILE - C1996083 (17)

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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 m (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 of 0.1, 0.2, 0.5, 1.0, and 2.0 sec. Figures 1 and 2 show 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. In particular, the curves are overly conservative at distances less than 1 km. The PGAs at the North Fork Valley mines do not appear to be as high as the McGarr and Fletcher (2005) model would predict at least for ML 1.5 to 2.5 (Figure 1). A simple exponential fit was performed to the data in the bins described above (Figure 2). As can be seen, the exponential relationships predict PGAs less than 0.01 g for ML 2.5 to 2.9 and less than 0.1 g for ML 3.0 to 3.4 although the sparcity of data at distances less than 1 km makes such estimation less well constrained. The planned closest approach of the mining will be at a horizontal distance of 610 m (2000 ft) and at a depth of 610 m (2000 ft) resulting in a shortest distance of 863 m (2828 ft). Based on Figure 2, the median PGA predicted at Bruce Park Dam from a mine tremor of ML 3.4 (the upper end of that bin) at a distance of 863 m would be significantly less than 0.10 g. It is difficult to extrapolate to larger magnitudes but a ML 4.2 event would also probably yield a median PGA of less than 0.10 g. It is not the PGA value of the mine tremor that is significant in terms of damage potential, although PGA does provide an overall estimate of the severity of the ground shaking. What is most pertinent to damage is the frequency content of the ground motions at periods of 0.1 to 1.0 sec. Numerous analyses have shown that mining-induced seismic events and natural tectonic earthquakes share more similar characteristics than they do differences. In fact, Art McGarr of the U.S. Geological Survey, the foremost authority on mine seismicity in the U.S., stated in 1984 that "to date, no systematic physical differences have been discovered between the two phenomena" referring to mine tremors and natural earthquakes (McGarr, 1984). However, Wong et al. (1989) and Wong and McGarr (1990) concluded that implosional-shear failure was the mechanism accounting for the vast majority of mine tremors in the Book Cliffs and eastern Wasatch Plateau coal mines of Utah. This type of failure has now been reported in other coal mines as well as in other types of mines worldwide (Wong and McGarr, 1990). Observations to date indicate the implosional-shear failure is also the source mechanism of the mine tremors in the North Fork Valley coal mines. Despite the implosional component in the failure mechanism in these events, the characteristics of the time history (Figure 3) and frequency content are very similar to natural -2- 05/08 PR-10 ? PPRov?l?