2009-05-01_REPORT - C1980007 (4)

Home Browse Search

Results-General January was the most seismically active month of the quarter. Magnitudes were assigned to 81 events during the quarter. Ground motions were tabulated for these 81 events and an additional 46 events derived from consideration of large arrivals on TCR, the station closest to the Terror Creek Reservoir, or MCD, the station closest to the Minnesota Creek Dam. Results-Magnitudes Twenty-two events with sufficient arrivals were located during the fourth quarter. Magnitude assignments from the MSS network were available for only a portion of the events at the date of this report. No events in this general area were located by the National Earthquake Information Center during this quarter. Preliminary analysis of the ground motion recordings do not indicate that any events reached a magnitude threshold of 2.5 during the quarter. Results-Accelerations Events with intensities from 50 to 150,000 digital counts have been recorded. For the events located, the maximum horizontal component of ground motion recorded was about 0.0064g(less than 3/4 of 1 percent). These results are based on the preliminary analysis of a sample of the data. Ground motions (including time histories) with average maximum accelerations of over 0.16 g (GEI Consultants, 2005) were used to evaluate the Minnesota Creek Dam. The dam was judged to be safe under that input of seismic loading. The maximum horizontal component recorded at MCD, the closest station to Minnesota Creek Dam, was 0.00053 g. Results-Locations For a selected set of the larger events preliminary locations were calculated. These locations are shown on Figure 2. Though the locations are preliminary, two areas of activity are present, one SW of station FGH and one north of WFW close to the mining activity by Mountain Coal Company during the quarter. Four events east of FGH may or may not be due to Oxbow Mining activity. The cluster of events located north of the first-quarter mining activity by MCC are located based on the preliminary velocity model, without station delays, and are outside the Mountain Coal subarray. Systematic offsets of calculated event locations from their actual positions are often caused by 1) Differences between the layered velocity model and the actual geologic conditions in the ground or 2) Station timing differences caused by elevation differences and other station-specific time delays or advances 3) the particular array geometry use to record the events. Such errors are amplified when the events originate outside of the areal bounds of the seismic stations and are reduced when the stations surround the source of seismic activity (Johnson and Butler, 1975). Changes in array parameters (such as station delays and the velocity model) should be based on good coverage of the area of interest with events and on a large number of events. As the mining by MCC progresses to the south, these criteria will be more closely satisfied and adjustments can be made.