Laserfiche WebLink
Results-General <br />January was the most seismically active month of the quarter. Magnitudes were assigned to 81 <br />events during the quarter. Ground motions were tabulated for these 81 events and an additional <br />46 events derived from consideration of large arrivals on TCR, the station closest to the Terror <br />Creek Reservoir, or MCD, the station closest to the Minnesota Creek Dam. <br />Results-Magnitudes <br />Twenty-four events with sufficient arrivals were located during the first quarter. Magnitude <br />assignments from the MSS network were available for only a portion of the events at the date of <br />this report. No events in this general area were located by the National Earthquake Information <br />Center during this quarter. Preliminary analysis of the ground motion recordings do not indicate <br />that any events reached a magnitude threshold of 2.5 during the quarter. Yeh (2006) indicated <br />that "a minimum earthquake magnitude of 4.0 (ML) was required to cause a landslide." <br />Results-Accelerations <br />Events with intensities from 50 to 150,000 digital counts have been recorded. For the events <br />located, the maximum horizontal component of ground motion recorded was about 0.0065 g. <br />These results are based on the preliminary analysis of the data. <br />Yeh (2006) used a horizontal ground acceleration of 0.0158 for modeling the effect of a seismic <br />event on the Bruce Park landslide. Ground motions (including time histories) with average <br />maximum accelerations of over 0.16 g (GEI Consultants, 2002) were used to evaluate the Bruce <br />Park Dam. The dam was judged to be safe under that input of seismic loading. <br />The maximum horizontal component recorded at TCR, the nearest station to the Bruce Park Dam, <br />was 0.000258. <br />Results-Locations <br />For a selected set of the larger events preliminary locations were calculated. These locations are <br />shown on Figure 2. Though the locations are preliminary, two areas of activity are present, one <br />SW of station FGH and one north of W FW close to the mining activity by MCC during the quarter. <br />Four events east of FGH may or may not be due to Oxbow Mining activity. <br />CONCLUSIONS <br />The seismic network is operational. The seismic activity in the North Fork Valley is being recorded <br />and the largest events are being processed. Station delays to account for station elevation <br />differences will be necessary to precisely correlate the events with the mining activity. Measured <br />ground accelerations are lower than those used in the stability analysis for the Bruce Park dam <br />and landslide. <br />U <br />