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• The velocity model given in Table 3 was generated for use in a very detailed seismic study in the <br />North Fork Valley in 2005 (Swanson and Koontz, 2006). Note that for that study the stations were <br />directly above a mining panel and all rays were assumed to be direct. At larger distances <br />refraction of rays from deeper layers will occur and the deeper layers will be more important. As <br />additional work is completed the velocity model will be refined. Additionally, the elevation <br />differences between stations (Table 1) are accounted for by station time-delay corrections <br />(Table 1). Delay times are calculated using 6500 feet as a datum elevation and a velocity of 7500 <br />feet/second. After calculation, empirical adjustments were made to select stations to reduce the <br />residuals for events in the BRL mining area. <br />Peak Ground Acceleration (Intensity) <br />Intensity is the actual earthquake effect at a spot removed from the epicentral point, regardless of <br />the magnitude of the earthquake. The sensors used in this array are three-component <br />accelerometers. The sensors report the acceleration of the ground at their installation point. The <br />complete characterization of the ground motion (what the ground was doing at every point in time) <br />is called a time history of the event at that station. Time histories are often used by structural <br />engineers to predict the performance of a structure if the ground at or under the structure should <br />experience that time history (intensity). <br />To calculate the largest amplitude of ground acceleration from a time history one would take the <br />square root of the sum of the squares of the three orthogonal accelerations for each time point <br />(resolving the amplitude of the vector of the ground motion) and pick out the largest value as a <br />function of time. That value, usually reported as a fraction of the vertical acceleration due to <br />gravity, is the amplitude of the largest ground acceleration due to that event. <br />In order to summarize a time history, a single measurement of ground motion is often used. While <br />many measures (summaries) of a time history are possible (peak particle velocity, average <br />maximum acceleration, duration of motion, etc.) an often used 'summary number' is peak ground <br />acceleration. The term "peak ground acceleration" is reserved in the literature (Wald, et. al., 1999) <br />for the largest horizontal amplitude of ground motion. The peak ground acceleration correlates <br />closely with the Modified Mercalli Intensity and is used to interpolate intensity maps for public <br />distribution. Another summary number, called the largest calculated accelereation, simply <br />resolves the largest value on each component. The largest calculated acceleration (LCA) <br />overestimates the actual ground acceleration because it is unlikely that the largest values on each <br />component occur simultaneously. <br />Magnitudes <br />Magnitude is a measure of the size or energy of the event released at the hypocentral point and <br />should not be confused with intensity (see above). The two quantities required to calculate <br />magnitude are the specific ground motions (intensities) as recorded at a point and the distance <br />from the point 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 events have <br />been assigned magnitudes by MSSN (Dave Wolny, personal communication). Discrepancies of <br />