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North Fork VaileV Seismic Network - Third Quarter 2009 <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 unit based on comparisons with magnitudes of <br />larger events as estimated by the USGS National Earthquake Information Center in Golden, <br />Colorado (D. Wolny, personal communication, 2007). Mc are also automatically calculated by <br />Earthworm, based on a USGS northern California formula. Mc values are approximately <br />equivalent to ML <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated (ML >_ 1.7) in the third quarter. A total of only 22 <br />events were located in this quarter, the lowest count since the second quarter of 2007. The date <br />and origin time (hours, minute, and second) of the event, location (latitude, longitude, and <br />general area), magnitude, and largest PGA values (in g's; 1 g = 980 cm/sec 2) are listed. <br />Figure 2 shows the resulting locations for the mining; related seismic events of approximate ML <br />1.7 and larger. Note event locations are better constrained when they occur within the perimeter <br />of the seismic network (greater azimuthal coverage). The events are located in a rather diffuse <br />pattern in the mining areas with the majority of events concentrated along the southern boundary <br />of the Elk Creek Mine. The largest events to occur during the reporting period were two Mc 2.6 <br />events on 28 July and 19 September (Figure 2). The event on 19 September occurred at 13:47 <br />GMT within the Bowie Mine permit area. The second event on 28 July at 1:48 GMT is a <br />tectonic earthquake and occurred north of the West Elk Mine. This event had a indeterminate <br />depth and exhibited all compressional first motions ("up") at some of the NFVSN stations. Such <br />earthquakes are typical of seismicity in western Colorado. Only 4 events occurred within the <br />West Elk Permit Area (Figure 2). The largest event to occur in the West Elk Mine was a ML 2.3 <br />event on 25 September 2009. <br />Seismic Event Magnitudes <br />Of the 21 mining-related events, all were assigned magnitudes (Table 3). ML values were <br />assigned to 12 events by Dave Wolny (written communications, 2008) as determined from the <br />MSSN. Coda duration magnitudes (Mc) values assigned by Earthworm are also listed in <br />Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values are generally recorded for all triggered events on the three-component <br />accelerometers. We have included the largest PGA recorded for each relocated seismic event in <br />Table 3 when available, along with the station that recorded the ground motion and the event-to- <br />UM WAX_WUS\PROJECTS\WEST ELK-MTN COAL\2009 Q3\Q3_2009_MCC.D0C\13-NOV-09\\OAK 4