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North Fork Valley Seismic Network - Second Quarter 2009 <br />model will be refined as further information is made available, particularly for recorded events <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 [2,120m]) 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 />• <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated in the second quarter (ML >_ 1.7). A total of only <br />45 events were located in this quarter, the lowest count since 1 July 2007. The date and origin <br />time (hours., minute, and second) of the event, location (latitude, longitude, and general area), <br />magnitudes, and largest PGA values (in g's; 1 g = 980 cm/sec2) are listed. <br />Figure 2 shows the resulting locations for the 45 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. The largest event to occur during the reporting period, a MC 2.3 event on 7 <br />June, also occurred in this cluster. There were only 3 events located within the Bowie Mine <br />Permit area representing 7% of the total activity in this quarter (Figure 2). The largest event to <br />be recorded in the Bowie Mine was a ML 2.1 on 28 May. No mining-related seismic events were <br />located in the vicinity of Bruce Park Dam and the adjacent slump area (Figure 2). <br />During the second quarter, one small probable tectonic earthquake, MC 2.0, was recorded on 5 <br />May by NFVSN and located south of Bowie Mine (Figure 2 and bolded in Table 3). This event <br />exhibited compressional first motions ("up") at most of the NFVSN stations indicating a double- <br />couple mechanism. Such earthquakes are typical of the pattern of seismicity in western <br />Colorado. <br />0 <br />M WAx_wcfs\PRQJECTS\North Fork-Bowie\Quarterly reports\2009 Q2\Q2_2009_BRL.d0C\0AK 4