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magnitude determinations, the magnitude scale can be extrapolated to the smaller, more <br />numerous events. <br />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 an <br />additional 35 events derived from consideration of the largest arrivals on TCR, the station closest <br />to the Terror Creek Reservoir, or MCD, the station closest to the Minnesota Creek Dam. <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. <br />Preliminary analysis of the ground motion recordings and NEIC records indicate that the largest <br />event was a magnitude 3.3 event on May 19. However, canvassing of mine and surface workers <br />in the vicinity of Bowie Mine did not turn up any felt reports or damage in the BRL area(Collin <br />Stewart, personal communication). Additional information about this event is contained in the BRL <br />report for this time period. <br />Results-Accelerations <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 0.0238. These <br />results are based on the preliminary analysis of this sample of the data. <br />Ground motions (including time histories) with average maximum accelerations of over 0.16 g <br />(GEI Consultants, 2005) were used to evaluate the Minnesota Creek Dam. The dam was judged <br />to be safe under that input of seismic loading. The maximum horizontal component recorded at <br />MCD, the closest station to Minnesota Creek Dam, was 0.00054 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, 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 />Only a scattering of events on the east side of the North Fork Valley were located. The event <br />locations do not correlate with MCC mining activity during the quarter. <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 use 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).