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<br />earthquakes. This has been shown to be true in the Utah mines (Smith et al., 1974;
<br />Wong et al., 1989; Williams and Arabasz, 1989).
<br />Figure 4 shows the acceleration response spectrum of an event, M 1.9, recorded on
<br />15 September 2007. This event has the largest PGA observed to date (0.10 g) since
<br />NVFSN seismic monitoring began. Figure 4 also shows the average spectrum for an
<br />earthquake of M 4.5 at 1 km as predicted by an empirical attenuation model for
<br />earthquakes. As observed for small magnitude mine tremors at short distances, they
<br />are generally deficient at moderate and long periods (> 0.1 sec) but are rich at short-
<br />periods (< 0.1 sec) or high frequencies. Ground motions damaging to engineered
<br />structures commonly have periods of 0.1 to 1 sec. Given that mine tremors are
<br />generally deficient in this period range as recorded at the ground surface, they rarely
<br />will be damaging to structures at the surface.
<br />The use of the time history of the 1940 M
<br />analysis of Bruce Park Dam, scaled down
<br />conservative because it is much more rich
<br />mine tremor would be.
<br />7.1 El Centro earthquake by GEI in the
<br />to match a ML 4.2 mine tremor, is very
<br />in the bandwidth of 0.1 to 1 sec than a
<br />BRL will adopt a "threshold" event where the associated PGA value is 0.05 g or
<br />higher as recorded at station TCR adjacent to the dam. No damage is expected at
<br />this level of ground motion but inspection of the dam and analysis of the event will be
<br />performed. Ground motions (including time histories) with average maximum
<br />accelerations of over 0.16 g (GEI Consultants 2002) were used to evaluate the
<br />Bruce Park Dam. The dam was judged to be safe under that input of seismic
<br />loading.
<br />References
<br />Arabasz, W.J., Nava, S.J., McCarter, M.K., Paukow, K.L., Pechmann, J.C., Ake, J.,
<br />and McGarr, A., 2005, Coal-mining seismicity and ground shaking hazard: A case
<br />study in the Trail Mountain area, Emery County, Utah, Bulletin of the Seismological
<br />Society of America, v. 95, p. 1-17.
<br />McGarr, A.,1984, Some applications of seismic source mechanism studies to
<br />assessing underground hazard, in Rockbursts and Seismicity in Mines (eds. Gay,
<br />N.C. and Wainwright, E.H.) South African Institute of Mining and Metallurgy
<br />Symposium Series No. 6,148-152.
<br />McGarr, A. and Fletcher, J.B., 2005, Development of ground motion prediction
<br />equations relevant to shallow mining-induced seismicity in the Trail Mountain area,
<br />Emery County, Utah, Bulletin of the Seismological Society of America, 95, 31-47.
<br />Smith, R.B., Winkler, P.L., Anderson, J.G., and Scholz, C.H., 1974, Source
<br />mechanisms of microearthquakes associated with underground mines in eastern
<br />Utah, Bulletin of the Seismological Society of America, 64, p 1295 - 1317.
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