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Results-General <br />December was the most seismically active month of the quarter. BRL was moving their longwall <br />from the B3 panel to the B4 panel to the northeast during the beginning of December and seismic <br />activity near their mining was low during that month. <br />Results-Magnitudes <br />Forty-three events with sufficient arrivals were located during the fourth quarter. Magnitude <br />assignments from the MSS network were available for only a portion of the events at the date of <br />this report. No events in this general area were located by the National Earthquake Information <br />Center during this quarter. Preliminary analysis of the ground motion recordings do not indicate <br />that any events reached a magnitude threshold of 3.0 during the quarter. <br />Results-Accelerations <br />Events with intensities from 50 to 150,000 digital counts have been recorded. For the events <br />located, the maximum horizontal component of ground motion recorded was about 0.45% (less <br />than 1/2 of 1 percent) of g. These results are based on the preliminary analysis of a sample of the <br />data. <br />Ground motions (including time histories) with average maximum accelerations of over 16% of g <br />(GEI Consultants, 2005) were used to evaluate the Monument Dam. The dam was judged to be <br />safe under that input of seismic loading. The maximum horizontal component recorded at MCD, <br />the closest station to Monument Dam was 0.07% of g. <br />Ground motions (including time histories) with average maximum accelerations of over 16% of g <br />(GEI Consultants, 2002) were used to evaluate the Bruce Park Dam. The dam was judged to be <br />safe under that input of seismic loading. The maximum horizontal component recorded at TCR, <br />the nearest station to the Bruce Park Dam was 0.03% of g. <br />Results-Locations <br />For a selected set of the larger events preliminary locations were calculated. These locations are <br />shown on Figure 2. Though the locations are preliminary and the sample is not exhaustive, three <br />distinct areas of activity are present, one SW of station FGH, one ENE of station FGH and a <br />cluster of events north of station WFW. <br />The cluster of events located north of the fourth-quarter mining activity by MCC are located based <br />on the preliminary velocity model, without station delays, and are outside the Mountain Coal <br />subarray. Systematic offsets of calculated event locations from their actual positions are often <br />caused by 1) Differences between the layered velocity model and the actual geologic conditions in <br />the ground or 2) Station timing differences caused by elevation differences and other station- <br />specific time delays or advances 3) the particular array geometry use to record the events. Such <br />errors are amplified when the events originate outside of the areal bounds of the seismic stations <br />and are reduced when the stations surround the source of seismic activity (Johnson and Butler, <br />1975).