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<br />North Fork Valley Seismic Network - First 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 />Table 3 lists the events recorded and relocated in the first 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 143 seismic events of approximate ML 1.7 and <br />larger. The majority of mining-related seismic events cluster in four areas generally indicating <br />where the mining activity occurred during this quarter. Approximately 84 events can be <br />associated with the Bowie mining operations. The majority of these events are occuring is one <br />large cluster at the Bowie No. 2 Mine, centered over the west-central portion of the mine in an <br />area where active longwall mining was occurring (Figure 2). No mining-related seismic events <br />were located in the vicinity of Bruce Park Dam and the adjacent slump area (Figure 2). <br />The largest events to occur during the reporting period were four ML 2.3 events, two of which <br />occurred at the Bowie No. 2 Mine and two at the Elk Creek Mine. The two ML 2.3 events at <br />Bowie No. 2 Mine occurred on 11 January and 19 February 2008. <br />During the first quarter, five small tectonic earthquakes were located south of the NFVSN; four <br />of which were located near Mt. Lamborn (Figure 2) and one near Black Canyon of the Gunnison. <br />The two largest events of the five were of ML 2.4 (Table 3), one near Mt Lamborn and the other <br />near Black Canyon. <br />Seismic Event Magnitudes <br />All of the 143 events relocated were assigned magnitudes (Table 3). ML values were assigned to <br />112 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 />WAx_WCfs\PROJECTS\North Fork-Bowie\Quarterly reports\2008 Q1\Q1_2008_BRL_flnal.d0C\0AK 4