2011-06-01_REPORT - C1980007 (2)

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North Fork Valley Seismic Network — Third Quarter 2010 1,249 events (not all of which were seismic events). This number of events is larger than the number of triggers from the previous quarter (934). Many more triggered waveform files were recorded of which many have insufficient station data for locating the events. We reviewed the daily summary or heliocorder records (these are continuous recordings of 12- hour periods conveniently plotted on one page) for filtered traces at three stations (WTM, LLM and WFW; Figure 1). We also reviewed the list of events automatically detected and located by Earthworm. Events were chosen based on amplitudes on the heliocorder records at the three stations. We measured the amplitudes on these heliocorder records for events known to be M 1.7 or greater in size at the different mining locations and used that as a guide for selecting other events of approximate M 1.7 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 FIYPO71 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 (2420 m [7938 ft]) and using a weighted average velocity over the layers between the datum and the station elevation. Table 2. Velocity Model Used in Locating Earthquakes Depth to Top of layer (km) [mi] Velocity (km /sec) [ft /sec] 0.00 [0.00] 2.21 [7,251] 0.07 [0.04] 2.73 [8,957] 0.27 [0.17] 3.01 [9,876] 037 [0.23] 3.18 [10,434] 1.00 [0.62] 3.9 [12,796] 1.50 [0.93] 4.60 [15,093] 2.50 [1.55] 5.20 [17,061] 3.50 [2.17] 6.00 [19,686] 6.00 [3.73] 6.5 [21,327] UM WAX_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q3 \Q3_2010_MCC.DOC\11- JAN -11\ \OAK 3