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• <br />North Fork Vaney seismic Network - Second 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 />(NEIL) 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 second 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 /> <br />Figure 2 shows the resulting locations for the 81 seismic events of approximate ML 1.7 and <br />larger. The majority of mining-related seismic events cluster in two areas: one cluster <br />associated with the active longwall operations at West Elk Mine and a second cluster located just <br />east of the Bowie Permit area within the Elk Creek Mine area also probably associated with the <br />Oxbow longwall operations. Fourteen events are located within or very near the Bowie Mine <br />Permit area (Figure 2). Most of the events do not locate near the active longwall but they are still <br />probably associated with the continuing redistribution of stress in the mine area. No mining- <br />related seismic events were located in the vicinity of Bruce Park Dam and the adjacent slump <br />area (Figure 2). <br />The largest event to occur during the reporting period was a ML 2.7 event at the West Elk Mine <br />area on 9 May. This was followed by a ML 2.5 aftershock on the same day in nearly the same <br />location. <br />During the second quarter, two small probable tectonic earthquakes, ML 1.9 and 2.0, were <br />recorded by NFVSN and located (Figure 2 and Table 3). These events occurred at greater depths <br />(> 5 km) than the mining-related seismic events and they also exhibited compressional first <br />motions ("up") at some of the NFVSN stations indicating a double-couple mechanism. Such <br />earthquakes are typical of the pattern of seismicity in western Colorado. <br />U WAx_wcfs\PROJECTSWorth Fork-BoWe\Quarterly reports\2008 Q2\Q2_2008_BRL.d0C\0AK 4