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Geatechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey & Whitney, LLP <br />Section 4 -Seismic Loading <br />4.1 Natural Seismicity <br />Seismic events caused by natural tectonic movements, which occur due to a slip of Earth's <br />crust along a fault, typically produce shaking that may last for seconds to minutes with a <br />predominant frequency content between 0.5 and 5 Hz (predominant period between 0.2 to <br />2 seconds). The estimated maximum considered earthquake (MCE) for this area of Colorado <br />is in the range of M 5.5 to 6.5 (Kirkham and Rogers, 1981). <br />The approximate duration of strong shaking associated with Richter local magnitude (ML) 5 <br />is 2 seconds, and for ML 6 is 12 seconds. Strong shaking is not generally considered for ML <br />less than 5 (Day, 2002). <br />4.2 Mine-Induced Seismicity <br />Seismic events induced by longwall coal mining can typically be caused by roof collapses, <br />rockbursts, and the predicted caving of the roof associated with longwall mining. Most <br />• mine-induced seismic events are less than magnitude (ML) 2, which classify asmicro-seismic <br />events. Mine-induced seismic events generally associated with longwall mining tend to have <br />shorter durations (typically only 2 to 3 seconds) and higher frequencies (typically greater <br />than 10 Hz (periods less than 0.1 second)) than natural seismicity, which reduces their <br />potential to be damaging to structures. <br />Published information and seismic records associated with mine-induced seismicity <br />comprises a relatively small set of data. Mos[ of the data available onmine-induced <br />seismicity in the United States is from the Wasatch Plateau-Book Cliffs area of Utah and the <br />northwest comer of the Colorado Plateau, because this is an area that consistently has a high <br />incidence ofmine-induced seismic activity and has been the focus of several studies. A case <br />study in the Trail Mountain area, Emery County, Utah (Arabasz et. al, 2005) recorded and <br />evaluated a data set of 1,913 mine-induces seismicity events, which ranged in magnitude <br />from less than zero to 2.2. The median magnitude in the Trail Mountain data set was 0.8 and <br />the 15x'- and 85`x-percentile magnitude values were approximately 0.4 and 1.2, respectively. <br />One of the largest events related to longwall mining recorded in the Book Cliffs area was a <br />ML 4.2 seismic event at a depth of 2,100 feet in Willow Creek Coal Mine in March 2000 <br />(Ellenberger, et al., 2001). <br /> <br />The available literature indicates that mine-induced earthquakes are typically magnitude 2.5 <br />or lower, and magnitude 1.2 appears to be the lowest limit (threshold) of reliable <br />GEI <br />12 <br />06035D OS03-16 Fbnwnmt Darn Gw,cA,tical Evd <br />March 2006 <br />