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North Fork Valley Seismic Network — Fourth Quarter 2009 0 <br />A DOS version of the location program HYP071 was used to locate the seismic events. <br />HYP071 requires a plane - layered velocity model, indicating depths to the top of each constant <br />velocity layer, as well as station locations and arrival times, in order to relocate the seismic <br />events. The velocity model (Table 2) was determined from a detailed seismic study at the West <br />Elk Mine (Swanson and Koontz, 2006). In that study, the recording stations were directly above <br />the mining panel and so all the seismic rays were considered to be direct rays. This velocity <br />model will be refined as further information is made available, particularly for recorded events <br />with known locations. Station elevation corrections (Table 1) were incorporated into the location <br />program based on the elevation differences between the station and the velocity model datum <br />(7938 ft [2420m]) and using a weighted average velocity over the layers between the datum and <br />the station elevation. <br />Magnitude Estimation <br />Magnitudes in terms of ML were generally estimated for all mining - related events of M 1.7 and <br />greater based on peak amplitudes as recorded on the Mesa State Seismic Network (MSSN) <br />operated by David Wolny. For some events, their recordings were too noisy to calculate a ML. <br />The standard Richter magnitude formula was used to calculate the ML values. Magnitude <br />uncertainties are typically 0.1 to 0.2 magnitude units based on comparisons with magnitudes of <br />larger events as estimated by the USGS National Earthquake Information Center (NEIC) in <br />Golden, Colorado (D. Wolny, personal communication, 2007). Mc values are also automatically <br />calculated by Earthworm based on a USGS northern California formula. Mc values are • <br />approximately the equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the 47 events recorded and relocated in the fourth quarter (M ? 1.7). 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 (in g's; 1 g = 980 cm/sec 2 ) are listed. <br />Figure 2 shows the resulting locations for the 46 mining - related seismic events of approximate <br />M 1.7 and larger. Note event locations are better constrained when they occur within the <br />perimeter of the seismic network (greater azimuthal coverage). The events are located in a rather <br />diffuse pattern in the mining areas with the majority of events concentrated in the Elk Creek <br />Mine and the Bowie Mine Permit Areas (Figure 2). The largest induced event to occur during <br />the reporting period, a M 2.1 event on 29 December, occurred at the Bowie Mine. There were <br />21 events located near the Bowie Mine Permit area representing 46% of the total activity in this <br />quarter (Figure 2). No mining - related seismic events were located in the vicinity of Bruce Park <br />Dam and the adjacent slump area (Figure 2). <br />During the fourth quarter, one possible tectonic earthquake, ML 1.8, was recorded on 14 October <br />by NFVSN at the very western edge of the Bowie Mine at a focal depth of 1.9 km (Figure 2 and <br />bolded in Table 3). This event exhibited compressional first motions ( "up ") at all of the NFVSN • <br />stations. The mining - related seismic events that have been observed to date by the NFVSN <br />UM WAx_wds \PROJECTS \North Fork - Bowie \Quarterly reports\2009 Q4 \Q4_2009_BRL.d0C \0AK 4 <br />