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North Fork Valley Seismic Network — Fourth Quarter 2010 <br />Table 2. Velocity Model Used in Locating Earthquakes <br />Depth to Top of layer (km) [mil <br />Velocity (km /sec) ift /sect <br />0.00 [0.00] <br />2.21 [7,251] <br />0.07 [0.04] <br />2.73 [8,957] <br />0.27 [0.17] <br />3.01 [9,876] <br />0.37 [0.23] <br />3.18 [10,434] <br />1.00 [0.62] <br />3.9 [ 12,796] <br />1.50 [0.93] <br />4.60 [15,093] <br />2.50 [1.55] <br />5.20 [17,061] <br />3.50 [2.17] <br />6.00 [19,686] <br />6.00 [3.73] <br />6.5 [21,327] <br />Magnitude Estimation <br />ML magnitudes are generally provided for the larger events recorded (ML >_ 3.0) and located by <br />the National Earthquake Information Center (NEIC). Mc values are automatically calculated by <br />Earthworm, based on a USGS northern California formula. M values are approximately <br />equivalent to ML. <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the 75 events recorded and relocated (Mc >_ 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 2 ) are listed. <br />Figure 2 shows the resulting locations for the mining- related seismic events of approximate Mc <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 and Oxbow <br />Mine area with seven events in the West Elk Mine Permit Area. The largest event to occur <br />during the reporting period was a M 3.0 event on 11 December (Figure 2). The event occurred <br />at 11:55 GMT within the Bowie Mine Area. The largest event within the West Elk Permit Area <br />was a M 2.6 on 20 October at 18:56 GMT. This event was the second largest event this quarter. <br />Four questionable tectonic events occurred within the Oxbow Mine on 2 and 4 October and 13 <br />and 17 December with magnitudes of M 2.4, 2.0, 1.8, and 1.8, respectively (Figure 2). <br />However, all of the events occurred at depths shallower than 1.3 km suggesting a mining- <br />induced origin (Table 3). The events exhibited some compressional first motions ( "up ") at most <br />of the NFVSN stations. The mining- related seismic events that have been observed to date by <br />the NFVSN generally exhibit all dilatational ( "down ") first motions, typical of coal mine <br />seismicity in Utah (e.g., Wong and McGarr, 1990). <br />um W:\X_WCFS \PROJECTS \WEST ELK -MTN COAL \2010 Q4 \Q4_2010_ MCC.DOC \18- MAR -1 1\ \OAK 4 <br />