Laserfiche WebLink
North Fork Valley Seismic Network - First Quarter 2008 <br />magnitudes of larger events as estimated by the USGS National Earthquake Information Center <br />in Golden, Colorado (D. Wolny, personal communication, 2007). Coda duration magnitudes <br />(Mc) are also automatically calculated by Earthworm, based on a USGS northern California <br />formula. MC values are approximately the same as ML <br />RESULTS <br />Seismic Event Locations <br />Table 3 lists the events recorded and relocated in the first quarter (ML >_ 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 = 980cm/sec2 ) are listed. <br />Figure 2 shows the resulting locations for the 143 mining-related seismic events of approximate <br />ML 1.7 and larger. These events cluster in four areas, one at Bowie No. 2 Mine, two at Elk Creek <br />mine, and one at West Elk Mine, indicating where the mining activity occurred during this <br />quarter. The largest events to occur during the reporting period were four ML 2.3 events, two of <br />which occurred at the Bowie No. 2 Mine and two at the Elk Creek Mine. Approximately 31 <br />events can be associated with the West Elk mining operations. The activity at West Elk Mine <br />corresponds occurs in the vicinity of mining activity during the quarter (Figure 2). The largest <br />events, two ML 2.2 events occurred on 26 January and 23 February 2008. Events are located <br />reasonably close to the active mining despite the fact that they are occurring outside of the <br />NFVSN station configuration. Event locations are better constrained when they occur within the <br />perimeter of the seismic network (greater azimuthal coverage). <br />During the first quarter, five small tectonic earthquakes were located south of the NFVSN, four <br />of which occurred near Mt. Lamborn and are shown in Figure 2, and the one near Black Canyon <br />of the Gunnison. These events occurred at greater depths (around 5 km at Mt. Lamborn and 7 km <br />at Black Canyon of the Gunnison) than the mining-related seismic events and their magnitudes <br />were between ML 1.9 and 2.4. These are naturally occurring events, events similar to which have <br />been observed before and are typical of the pattern of seismicity in western Colorado. <br />Seismic Event Magnitudes <br />Of the 143 mining-related events, all were assigned magnitudes (Table 3). ML values were <br />assigned to 112 events by Dave Wolny (written communications, 2008) as determined from the <br />MSSN. Coda duration magnitudes (Mc) assigned by Earthworm are also listed in Table 3. <br />Recorded Peak Ground Accelerations <br />PGA values (in g's; 1 g = 980cm/sec2) were also recorded for all triggered events on the three- <br />component accelerometers. We have included the largest PGA recorded for each relocated <br />seismic event in Table 3 when available, along with the station that recorded the ground motion <br />and the event-to-station distance. The largest PGA recorded during the quarterly period was <br />0.01g at station MFB for a ML 1.9 event at a distance of approximately 0.9 km [0.6 mil. This <br />event occurred on 9 January 2008 at the Bowie No. 2 Mine. The largest PGA recorded at the <br />tw WAX_WCFS\PROJECTS\WEST ELK-MTN COAL\2008 Q1\Q1_2008_MCC_FINAL.DOC\16-MAY-08\\OAK 4