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North Fork Valley Seismic Network - Fourth Quarter 2008 <br />with known locations. Station elevation corrections (Table 1) were incorporated into the location <br />program based on the elevation differences between the station and the velocity model datum <br />(7938 ft [2420m]) and using a weighted average velocity over the layers between the datum and <br />the station elevation. <br />Magnitude Estimation <br />Magnitudes in terms of ML were estimated for all mining-related events of ML 1.7 and greater <br />based on. peak amplitudes as recorded on the Mesa State Seismic Network (MSSN) operated by <br />David Wolny. The standard Richter magnitude formula was used to calculate the ML values. <br />Magnitude uncertainties are typically 0.1 to 0.2 magnitude units based on comparisons with <br />magnitudes of larger events as estimated by the USGS National Earthquake Information Center <br />(NEIC) in Golden, Colorado (D. Wolny, personal communication, 2007). Coda duration <br />magnitudes (Mc) are also automatically calculated by Earthworm based on a USGS northern <br />California formula. Mc values are approximately the same as ML. <br />RESULTS <br />Seismic Event Locations <br />u <br />Table 3 lists the events recorded and relocated in the fourth quarter (ML >_ 1.7). The date and <br />origin time (hours, minute, and second) of the event, location (latitude, longitude, and general <br />area), magnitudes, and largest PGA values are listed. <br />Figure 2 shows the resulting locations for the 258 seismic events of approximate ML 1.7 and <br />larger. The majority of mining-related seismic events cluster in two areas: one cluster probably <br />associated with the active longwall operations at Elk Creek Mine and a second cluster located <br />just within the Bowie Permit area probably associated with the Bowie longwall operations. <br />There were 233 events located within or very near the Bowie Mine Permit area representing 90% <br />of the total activity in this quarter (Figure 2). No mining-related seismic events were located in <br />the vicinity of Bruce Park Dam and the adjacent slump area (Figure 2). <br />The largest events to occur during the reporting period were two ML 2.6 events at the Bowie <br />Mine area on 7 October and 22 November. <br />During the fourth quarter, one small probable tectonic earthquake, ML 1.6, was recorded on 17 <br />October by NFVSN and located at Bowie Mine (Figure 2 and bolded in Table 3). This event <br />exhibited compressional first motions ("up") at some of the NFVSN stations indicating a double- <br />couple mechanism. Such earthquakes are typical of the pattern of seismicity in western <br />Colorado. <br />Seismic Event Magnitudes <br />All of the 258 events relocated were assigned magnitudes (Table 3). ML values were assigned to <br />123 events by Dave Wolny (written communication, 2008) as determined from the MSSN. MC • <br />values assigned by Earthworm are also listed in Table 3. <br />UM WAx wcfs\PROJECTS\North Fork-Bow e\Quarterly reports\2008 Q4\Q4_2008_BRL.doc\0AK 4