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ax.:.`-?'..1?:r_..^.f rSXGSm.,._...^.?.__?...:-..-r:'.^:•sY!f1tl?•^.'.*fx?'r+.::-:'ssA..??:1?1.^.?:'FJ:.:r.-.`r.:.-,'-^.CJI::_rtla-.:^?•?: 'r___-x-:_'_: +'^__-_..--'.:'l _ _ n .J ?_? A'C}s.. _. __ .. rr:...._rs-.? '? ? rf- f.?-..._... n..Y.?^............s.. _.s ^._?......._.. _. I........,..._._ ...._ .rJit: <.s ?r? - _ ? .A :c ...,!'... X ,, ., _...f__f_.._, ,..,,.,- ?_.:- .,..,,...,,,.-.._. :-r.. :-_•.•... .:a,? :........... ... .... ....... _...r_. _. ?'__...... ....-'-------.._::..: ..: ?•:?1:?.-. -'-r._s4?s;:c 5=_....... .?:=d d'r:'3rF e,?o ...:. ...-; ....?._r.?.::.. ..,.._ ...e- North Fork Valley Seismic Network - Third Quarter 2007 stations. We measured the amplitudes on these heliocorder records for events known to be ML 1.5 or greater in size at the different mining locations and used that as a guide for selecting other events of approximate ML 1.5 and larger. Location Methodology We used the utility SUDSPICK in the program PC-SUDS (Banfill, 1996) to pick P- and S-wave arrival times within the DMX files (triggered waveform data files) for our selected list of events. Another utility within PC-SUDS, XTYHY71, was used to extract the arrival time picks from the waveform data files and convert them to the correct format to be used in the location program HYP071 (USGS, 1975). Only events with at least 8 arrival time picks were located. The location accuracy usually improves as the number of stations increases. A DOS version of the location program HYP071 was used to locate the earthquakes. HYP071 requires a plane-layered velocity model, indicating depths to the top of each constant velocity layer, as well as station locations and arrival times, in order to relocate the earthquakes. The velocity model (Table 2) was determined from a detailed seismic study at the West Elk Mine (Swanson and Koontz, 2006). In that study, the recording stations were directly above the mining panel and so all the seismic rays were considered to be direct rays. Station elevation corrections (Table 1) were incorporated into the location program based on the elevation differences between the station and the velocity model datum (6,500 ft [1.98 km/sec]) and the velocity of the uppermost layer (7470 ft/sec [2.21 km/sec]). Table 2. Velocity Model Used in Locating Earthquakes Top of layer (km) [mi] Velocity (km/sec) [ft/sec] 0.00 [0.00] 2.21 [7,470] 0.07 [0.04] 2.73 [9,227] 0.27 [0.17] 3.01 [10,174] 0.37 [0.23] 3.18 [10,748] 1.50 [0.94] 3.96 [13,385] 2.50 [1.56] 4.40 [14,872] 6.00 [335] 6.00 [20,280] Magnitude Estimation Magnitudes in terms of Mi. were estimated for all mining-related events of ML 1.5 and greater based on peak amplitudes as recorded on the Mesa State Seismic Network (MSSN) operated by David Wolny. The standard Richter magnitude formula was used to calculate the Mi, values. Magnitude uncertainties are typically 0.1 to 0.2 magnitude unit based on comparisons with magnitudes of larger events as estimated by the USGS National Earthquake Information Center in Golden, Colorado (D. Wolny; personal communication, 2007). Coda duration magnitudes are also automatically calculated by Earthworm. MR WAX WCFSNPROJECTSIWEST ELK-MTN COAL\2007 03\03_2007_MCC_FINAL DOCl29-JA"M%OAK 3