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North Fork Valley Seismic Network - Third Quarter 2009 <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 generally estimated for all mining-related events of ML 1.7 and <br />greater based on peak amplitudes as recorded on the Mesa State Seismic Network (MSSN) <br />operated by David Wolny. For some events, their recordings were too noisy to calculate a ML. <br />The standard Richter magnitude formula was used to calculate the ML values. Magnitude <br />uncertainties are typically 0.1 to 0.2 magnitude units based on comparisons with magnitudes of <br />larger events as estimated by the USGS National Earthquake Information Center (NEIC) in <br />Golden, Colorado (D. Wolny, personal communication, 2007). Mc are also automatically <br />calculated "by Earthworm based on a USGS northern California formula. Mc values are <br />approximately the equivalent to ML. <br />RESULTS <br />0 <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated in the third quarter (ML >_ 1.7). A total of only 22 <br />events were located in this quarter, the lowest count since URS started reporting for Bowie LLC. <br />The date and origin time (hours, minute, and second) of the event, location (latitude, longitude, <br />and general area), magnitudes, and largest PGA values (in g's; 1 g = 980 cm/seC2) are listed. <br />Figure 2 shows the resulting locations for the 22 seismic events of approximate ML 1.7 and <br />larger. Note event locations are better constrained when they occur within the perimeter of the <br />seismic network (greater azimuthal coverage). The events are located in a rather diffuse pattern <br />in the mining areas with the majority of events concentrated along the southern boundary of the <br />Elk Creek Mine and within the Bowie Mine. The largest induced event to occur during the <br />reporting period, a ML 2.6 event on 19 September, occurred at the Bowie Mine. There were 8 <br />events located near the Bowie Mine Permit area representing 35% of the total activity in this <br />quarter (Figure 2). No mining-related seismic events were located in the vicinity of Bruce Park <br />Dam and the adjacent slump area (Figure 2). <br />During the third quarter, one small tectonic earthquake, also ML 2.6, was recorded on 28 July by <br />NFVSN and located north of West Elk (Figure 2 and bolded in Table 3). This event exhibited <br />compressional first motions ("up") at all of the NFVSN stations suggesting a double-couple <br />mechanism.. Such earthquakes are typical of the pattern of seismicity in western Colorado. <br />Seismic Event Magnitudes <br />All of the 21 mining-related events relocated were assigned magnitudes (Table 3). ML values <br />were assigned to 12 events by Dave Wolny (written communication, 2008) as determined from • <br />the MSSN. MC values assigned by Earthworm are also listed in Table 3. <br />UM WAx_wcfs\PROJECTS\North Fork-Bowie\Quartedy reports\2009 Q3\Q3_2009_BRL.doc\OAK 4