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North Fork Va11eV Seismic Network - Third 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 third 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 347 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 El Creek Mine and a second cluster located just <br />within the Bowie Permit area probably associated with the Bowie longwall operations. There <br />were 191 events located within or very near the Bowie Mine Permit area representing 55% of the <br />total activity in this quarter (Figure 2). No mining-related seismic events were located in the <br />vicinity of Bruce Park Dam and the adjacent slump area (Figure 2). <br />The largest event to occur during the reporting period was a ML 2.8 event at the Elk Creek Mine <br />area on 6 September. The largest events at Bowie Mine were both ML 2.5 that occurred on 30 <br />August and 5 September. <br />During the third quarter, one small probable tectonic earthquake, ML 2.1, was recorded on 15 <br />September by NFVSN and located at Elk Creek Mine (Figure 2 and bolded in Table 3). This <br />event exhibited compressional first motions ("up") at some of the NFVSN stations indicating a <br />double-couple mechanism. Such earthquakes are typical of the pattern of seismicity in western <br />Colorado. <br />• <br />UM WAx_wdsTROJECTSWorth Fork-BoWelQuarterly reports\2008 Q3\Q3_2008_BRL.doc\OAK 4