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North Fork ValleV Seismic Network - Fourth Quarter 2009 <br />Depth to Top of layer (km) <br />[mi] Velocity (km/sec) [ft/sec] <br />2.50 [1.55] 5.20 [17,061] <br />3.50 [2.17] 6.00 [19,686] <br />6.00 [3.73] 6.5 [21,327] <br />Magnitude Estimation <br />Magnitudes in terms of ML were generally estimated for all mining-related events of ML 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 unit based on comparisons with magnitudes of <br />larger events as estimated by the USGS National Earthquake Information Center in Golden, <br />Colorado (D. Wolny, personal communication, 2007). Mc are also automatically calculated by <br />Earthworm, based on a USGS northern California formula. MC values are approximately <br />equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the 17 events recorded and relocated (ML >_ 1.7) in the fourth quarter. The date and <br />origin time (hours, minute, and second) of the event, location (latitude, longitude, and general <br />area), magnitude, and largest PGA values (in g's; 1 g = 980 cm/seC ) are listed. <br />Figure 2 shows the resulting locations for the mining-related seismic events of approximate ML <br />1.7 and larger. Note event locations are better constrained when they occur within the perimeter <br />of the seismic network (greater azimuthal coverage). The events are located in a rather diffuse <br />pattern in the mining areas with the majority of events concentrated in the Bowie Mine area and <br />the Elk Creek Mine Permit Areas. The largest event to occur during the reporting period was a <br />ML 2.1 event on 29 December (Figure 2). The event occurred at 06:38 GMT within the Bowie <br />Mine Permit Area. <br />One possible tectonic event occurred beneath Bowie Mine on 14 October. This Mc 1.8 event <br />had a focal depth of 1.9 km but exhibited all compressional first motions ("up") at the NFVSN <br />stations. The mining-related seismic events that have been observed to date by the NFVSN <br />generally exhibit all dilatational ("down") first motions, typical of coal mine seismicity in Utah <br />(e.g., Wong and McGarr, 1990). Only 7 events occurred within the West Elk Permit Area <br />(Figure 2). The largest event to occur in the West Elk Mine was a ML 2.0 event on 15 November <br />2009. <br />UM WAX_WCFS\PROJECTS\WEST ELK-MTN COAL\2009 Q4\Q4_2009_MCC.DOC\2-MAR-10\\OAK 4