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North Fork Valley Seismic Network — Fourth Quarter 2010 <br />DATA PROCESSING <br />Identification of Events <br />Events are regarded as seismic in origin and logged by the Earthworm triggering system if they <br />appear on five or more stations with significant amplitude, impulsive onset and appropriate time <br />differences between stations. Triggering occurs within the two sub -nets (six stations in each) so <br />that important events are not missed. Earthworm also provides automatic locations for events <br />that have sufficient arrival time picks. During the fourth quarter, Earthworm detected and <br />located 3,129 events (not all of which were seismic events). This is larger than the number of <br />triggers from the previous quarter (1,249). The number of triggers steadily increased from <br />October (667) to December (1,366) Many more triggered waveform files were recorded of <br />which many have insufficient station data for locating the events. <br />We reviewed the daily summary or heliocorder records (these are continuous recordings of 12- <br />hour periods conveniently plotted on one page) for filtered traces at three stations (WTM, LLM <br />and WFW; Figure 1). We also reviewed the list of events automatically detected and located by <br />Earthworm. Events were chosen based on amplitudes on the heliocorder records at the three <br />stations. We measured the amplitudes on these heliocorder records for events known to be M <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 M 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 in [7938 ft]) and using a <br />weighted average velocity over the layers between the datum and the station elevation. <br />um W:\X_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q4 \Q4_2010_ MCC.DOC \18- MAR -1 1\ \OAK 3 <br />