2011-06-06_REPORT - C1980007 - Laserfiche WebLink

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North Fork Valley Seismic Network — First Quarter 2011 Depth to Top of layer (km) [mi] Velocity (km /sec) [ft /sec] 1.00 [0.62] 3.9 [12,796] 1.50 [0.93] 4.60 [15,093] 2.50 [1.55] 5.20 [17,061] 3.50 [2.17] 6.00 [19,686] 6.00 [3.73] 6.5 [21,327] Magnitude Estimation M magnitudes are generally provided for the larger events recorded (M >_ 3.0) and located by the National Earthquake Information Center (NEIC). There were three events reported by the NEIC during the first quarter, two events on 6 January and one on 17 February. Mc values are automatically calculated by Earthworm, based on a USGS northern California formula. M values are approximately equivalent to M L . RESULTS Seismic Event Locations Table 3 lists the 86 events recorded and relocated (M >_ 1.7) in the first quarter. The date and origin time (hours, minute, and second) of the event, location (latitude, longitude, and general area), magnitude, and largest PGA values (in g's; I = 980 cm /sec are listed. Figure 2 shows the resulting locations for the mining - related seismic events of approximate M 1.7 and larger. Note event locations are better constrained when they occur within the perimeter of the seismic network (greater azimuthal coverage). The events are located throughout the study area but the vast majority are concentrated in the Oxbow Mine area, with only three events in the West Elk Mine Permit Area (Figure 2). The largest event to occur during the reporting period was a M 3.1 event on 17 February. The event occurred at 22:47 GMT within the Elk Creek Mine. The largest event within the West Elk Permit Area was a M 1.8 on 12 January at 02:43 GMT. Two possible tectonic events occurred within the Oxbow Mine on 25 January and 22 February with magnitudes of M 1.7 and 2.0, respectively (Figure 2 and Table 3). The events exhibited some compressional first motions ( "up ") at most of the NFVSN stations. The mining - related seismic events that have been observed to date by the NFVSN generally exhibit all dilatational ( "down ") first motions, typical of coal mine seismicity in Utah (e.g., Wong and McGarr, 1990). The focal depth for the event on 22 February was 5.4 kin consistent with a tectonic origin. The 25 January event remains questionable due to its shallow focal depth of about 1 kin, and its location within the area of active mining (Figure 2). Seismic Event Magnitudes Of the 84 mining - related events, all were assigned magnitudes (Table 3). M values assigned by Earthworm are also listed in Table 3. The events on 6 January and 17 February were reported during this quarter by NEIC as occurring within the bounds of Figure 2. Two NEIC recorded WAX_WCFS \PROJECTS \WEST ELK -MTN COAL \2011 Q1 \Q1_2011_MCC.DOC \27- MAY -11\ \OAK 4