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;, .?. .ary-. .. _.<.r... «. F_.. t...-.._ E xrJ '^:?IE-:==?'-? r`Y.:=??'•i.{-.x>.:?.:.or-.:: =f<--_:??iii?-r.?.S.r"O?.'-i}7= r: r. '"°' 'fi'Y..-'r.:F!':i `s°. rr .__..n.: rs_. .. .v_.._ <br /> ? ,-. ...? <br />North Fork Vallev Seismic Network - Third Quarter 2007 <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated in the third quarter (ML >_ 1.5). The date and <br />origin time (hours, minute, and second) of the event, location (latitude, longitude, and general <br />area), magnitudes, and largest PGA values are listed. <br />Figure 2 shows the resulting locations for the 187 events of approximate ML 1.5 and larger. The <br />seismic events cluster in four distinct areas indicating where the mining activity occurred during <br />this quarter. The largest events to occur during the reporting period were two ML 2.6 events both <br />of which occurred at the West Elk Mine on 9 and 15 August (shown in red on Figure 2). <br />Approximately 61 events can be associated with the West Elk mining operations. The activity at <br />West Elk Mine corresponds reasonably well with the location of mining activity during the <br />quarter (Figure 2); however, there is more scatter in the locations than those located in the <br />westernmost cluster at the Bowie mine. This larger location uncertainty is related to the mining <br />activity being outside of the current NFVSN station configuration. Although event locations are <br />better constrained when they occur within the perimeter of the seismic network (greater <br />azimuthal coverage), the seismic activity in the West Elk area is still relatively well located <br />because it occurs close to the network perimeter. <br />Seismic Event Magnitudes <br />Of the 187 events relocated, 184 were assigned magnitudes (Table 3). ML values were assigned <br />to 117 events by Dave Wolny (written communications, 2007) as determined from the MSSN. <br />Coda duration magnitudes (Mc) assigned by Earthworm are also listed in Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values were also recorded for all triggered events on the three-component accelerometers. <br />We have included the largest PGA recorded for each relocated seismic event in Table 3 when <br />available, along with the station that recorded the ground motion and the event to station <br />distance. The largest PGA recorded during the quarterly period was 0.09 g at station FGH for a <br />ML 1.9 event at a distance of 0.7 km [0.4 mil. This event occurred on 15 September 2007 at the <br />Bowie No. 2 Mine. The largest PGA recorded at the West Elk Mine was 0.005 g at station <br />WFW resulting from a ML 2.6 seismic event that occurred on 9 August 2007 at a distance of 2.4 <br />km [1.5 mi]. <br />ACKNOWLEDGMENTS <br />The NFVSN data analysis and evaluation are jointly supported by MCC and BRL. Our thanks to <br />Wendell Koontz, MCC Project Manager, and Collin Stewart, BRL. Project Manager for their <br />support. Our appreciation to Pete Swanson (NIOSH/CDS) and Dave Wolny (Mesa State <br />College) for their assistance in the operation and maintenance of the network. Our thanks to <br />Melinda Lee for her assistance in the preparation of this report. the report was reviewed by Pete <br />Swanson and Collin Stewart. <br />Ulm WA _WCFSIPROJECTSiWEST ELK-MTN COAL@007 Q31Q3_2007_&ICC FINAL.DOC\20-JAN-M8 OAK 4