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Results-General <br />May was the most seismically active month of the quarter. Magnitudes were assigned to 351 <br />events during the quarter. Ground motions were tabulated for 109 of these 351 events and for an <br />additional 35 events for which the ground motions at TCR, the station closest to the Terror Creek <br />Reservoir, or MCD, the station closest to the Minnesota Creek Dam were analyzed. <br />Results-Magnitudes <br />One hundred and nine events with sufficient arrivals were located during the fourth quarter. <br />Magnitude assignments from the MSSN network were available for the entire quarter at the date of <br />this report. Five events in this general area were located by the National Earthquake Information <br />Center during this quarter; three of the NEIC events were located using local data. <br />Results-Threshold Monitoring Program and Threshold Limits <br />The maximum horizontal component recorded at TCR, the closest station to Terror Creek <br />Reservoir, was 0.0017g, far less than the safe threshold limits for the dam and the landslide. <br />Events with intensities from 45 to 498,000 digital counts have been recorded. For the events <br />located, the maximum horizontal component of ground motion recorded was about 0.023g. These <br />results are based on the analysis of this sample of the data. <br />Ground motions (including time histories) with average maximum accelerations of 0.16g (GEI <br />Consultants, 2005) were used to evaluate the Bruce Park Dam. The dam was judged to be safe <br />under that input of seismic loading. <br />Ground motions (including time histories) with average maximum accelerations of 0.015g (Yeh <br />and Associates, 2006) were used to evaluate the landslide existent adjacent to the Bruce Park <br />Dam. The landslide was judged to be stable under that input of seismic loading. <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, two areas of activity are present, one <br />west of station FGH and one north and east of FGH. The concentration of events to the west of <br />FGH correlates well with the BRL mining during the quarter and the events to the north and east of <br />FGH are near the location of Oxbow Mining activity. <br />Systematic offsets of calculated event locations from their actual positions are often caused by 1) <br />Differences between the layered velocity model and the actual geologic conditions in the ground or <br />2) Station timing differences caused by elevation differences and other station-specific time delays <br />or advances 3) the particular array geometry used to record the events. Such errors are amplified <br />when the events originate outside of the areal bounds of the seismic stations and are reduced <br />when the stations surround the source of seismic activity (Johnson and Butler, 1975). Changes in <br />array parameters (such as station delays and the velocity model) should be based on good <br />coverage of the area of interest with events and on a large number of events. <br />i <br />