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North Fork Valley Seismic Network - First 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 [24-20m]) 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). Mc are also <br />automatically calculated by Earthworm based on a USGS northern California formula. Mc <br />values are approximately the equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />• <br />Table 3 lists the events recorded and relocated in the first quarter (ML >_ 1.7). A total of only 57 <br />events were located in this quarter, the lowest count since 1 July 2007. The date and origin time <br />(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. There were network <br />problems in the first half of March (1 to 12) where data files were not telemetered correctly. <br />Pete Swanson will be downloading backup files from each station for six events that occurred <br />during this period. The locations for these events will be added in a revised edition of this report. <br />Figure 2 shows the resulting locations for the 57 seismic events of approximate ML 1.7 and <br />larger. The majority of mining-related seismic events occurred in two areas: one cluster <br />probably associated with the active longwall operations at a southern corner of the Elk Creek <br />Mine. The largest event to occur during the reporting period, a ML 2.9 event, also occurred in <br />this cluster on 22 March. A second smaller diffuse group located just within the Bowie Permit <br />area probably associated with the Bowie longwall operations. There were 11 events located <br />within or the Bowie Mine Permit area representing 19% of the total activity in this quarter <br />(Figure 2). The largest event to be recorded in the Bowie Mine was a ML 1.9 on 12 February <br />2009. No mining-related seismic events were located in the vicinity of Bruce Park Dam and the <br />adjacent slump area (Figure 2). <br />During the first quarter, one small probable tectonic earthquake, ML 1.8, was recorded on 1 <br />February 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 />0 <br />UM WAx_wcfs\PROJECTS\North Fork-BoWe\Quartedy reports\2009 Q1\Q1_2009_BRL.doc\0AK 4