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personally appreciate that a 1% peak ground acceleration is usually not perceptible by most <br />human beings. <br />Magnitudes <br />Magnitude is a measure of the size or energy of the event released at the epicentral point. <br />Intensity is the actual earthquake effect at a spot removed from the epicentral point, regardless of <br />the magnitude of the earthquake. The two quanities required to calculate magnitude are the <br />specific ground motions (intensities) as recorded by seismic stations and the distance from the <br />station to the event. <br />A correlation of the magnitudes with values published by the USGS is used to normalize the local <br />magnitude scale. Events recorded by stations near Grand Junction (the Mesa State Seismic <br />Network{MSSN}) and by USGS regional stations are correlated to define a scale comparable to <br />published values. The current state of magnitude determination for this project is that 47 events in <br />August have been assigned magnitudes by MSSN (Dave Wolney, personal communication). <br />These events have preliminary magnitudes of 1.8 to 3.0 and the current values correlate within 0.1 <br />to 0.15 units with the six events that have been assigned magnitudes by NEIC (the National <br />Earthquake Information Center).._ Discrepancies of */- 0.3 are typical for local micro magnitude <br />scales (Thatcher, 1973), and are due not only to instrument variation but also to uncertainties in <br />the basic definitions of magnitude scales. Magnitude values for detected and located events <br />should be analyzed with consideration of these uncertainties. <br />This process is ongoing but the use of stable stations within MSSN will allow the correlations to be <br />continually refined. As recorded ground motions and station distances are applied to these <br />magnitude determinations, the magnitude scale can be extrapolated to the smaller, more <br />numerous events. <br />Results-General <br />August was the most seismically active month of the quarter. The area within the network were <br />unusually seismically active in August (Pete Swanson, personal communication). Collin Stewart <br />(personal communication) noted that the BRL longwall panel was undermining and caving a thick <br />sandstone channel during the month of August. Drill hole data indicated that this channel <br />sandstone was over 65 feet thick. Mining under this channel corresponded to increased floor <br />heaving in the longwall tailgate and more frequent bumping of the longwall coal face during <br />mining. <br />Results-Magnitudes <br />Twenty-two events of greater than magnitude 1.5 out of a total of 523 events recorded by the <br />triggered system occurred during the August period. <br />Results-Accelerations <br />Events from 50 to 100,000 digital counts have been recorded. Note that, as shown on Table 2, a <br />maximum ground motion of 100,000 counts is only about 0.003g.