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North Fork Valley Seismic Network - First Quarter 2008 <br />stations. We measured the amplitudes on these heliocorder records for events known to be ML <br />1.7 or greater in size at the different mining locations and used that as a guide for selecting other <br />events of approximate ML 1.7 and larger. <br />Location Methodology <br />We used the utility SUDSPICK in the program PC-SUDS (Banfill, 1996) to pick P- and S-wave <br />arrival times within the DMX files (triggered waveform data files) for our selected list of events. <br />Another utility within PC-SUDS, XTYHY71, was used to extract the arrival time picks from the <br />waveform data files and convert them to the correct format to be used in the location program <br />HYP071 (USGS, 1975). Only events with at least 8 arrival time picks were located. The <br />location accuracy usually improves as the number of stations increases. <br />A DOS version of the location program HYP071 was used to locate the earthquakes. HYP071 <br />requires a plane-layered velocity model, indicating depths to the top of each constant velocity <br />layer, as well as station locations and arrival times, in order to relocate the earthquakes. The <br />velocity model (Table 2) was determined from a detailed seismic study at the West Elk Mine <br />(Swanson and Koontz, 2006). In that study, the recording stations were directly above the <br />mining panel and so all the seismic rays were considered to be direct rays. Station elevation <br />corrections (Table 1) were incorporated into the location program based on the elevation <br />differences between the station and the velocity model datum (2420 m [7938 ft]) and using a <br />weighted average velocity over the layers between the datum and the station elevation. <br />Table 2. Velocity Model Used in Locating Earthquakes <br />Depth to Top of layer (km) <br />[mi] Velocity (km/sec) [ft/sec] <br />0.00 [0.00] 2.21 [7,251] <br />0.07 [0.04] 2.73 [8,957] <br />0.27 [0.17] 3.01 [9,876] <br />0.37 [0.23] 3.18 [10,434] <br />1.00 [0.62] 3.9 [12,796] <br />1.50 [0.93] 4.60 [15,093] <br />2.50 [1.55] 5.20 [17,061] <br />3.50 [2.17] 6.00 [19,686] <br />6.00 [3.73] 6.5 [21,327] <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 unit based on comparisons with <br />um WAX_WCFS\PROJECTS\WEST ELK-MTN COAL\2008 01\Q1_2008_MCC_FINAL.DOC\18-MAY-08\\OAK 3