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use a layered-earth model as input to the location program. Using a layered-earth model, <br />velocities are constant over certain vertical intervals and change at discrete horizontal boundaries. <br />The travel path for this model is a series of line segments with abrupt emergence angle changes at <br />velocity boundaries. <br />Table 3 Preliminary Velocity Model <br />Layer Top Velocity in km/sec Velocity in ft/sec <br />0.00 2.21 7470 <br />0.07 2.73 9227 <br />0.27 3.01 10174 <br />0.37 3.18 10748 <br />1.50 3.96 13385 <br />2.50 4.40 14872 <br />6.00 6.00 20280 <br />The velocity model given in Table 3 was generated for use in a very detailed study at West Elk in <br />2005 (Swanson and Koontz, 2006). Note that for that study the stations were directly above a <br />mining panel and all rays were assumed to be direct. At larger distances refraction of rays from <br />deeper layers will occur and the deeper layers will be more important. As additional work is <br />completed the velocity model will be refined. Additionally, the large elevation differences between <br />stations (Table 1) will be accounted for by station corrections. <br />Peak Ground Acceleration <br />The sensors used in this array are three-component accelerometers. The sensors report the <br />acceleration of the ground at their installation point. The complete characterization of the ground <br />motion (what it was doing at every point in time) is called a time history of the event at that station. <br />Figure 2 is an example of a time history for a typical event recorded by this network. Time <br />histories are often used by structural engineers to quantify the performance of a structure if the <br />ground at or under the structure should experience that time history (intensity). <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, duration of <br />motion, etc.) an often used 'summary number' is peak ground acceleration. To calculate the peak <br />ground acceleration from Figure 2, one would form the square root of the sum of the squares of <br />the three orthogonal accelerations for each time point (resolving the amplitude of the vector of the <br />ground motion) and pick out the largest. That value, usually reported in percent of the vertical <br />acceleration due to gravity, is the peak ground acceleration. <br />None of the summary designations are completely satisfactory (for instance, the frequency of the <br />ground motions is important) but peak ground acceleration has the advantage that most persons <br />can relate to it-we all contend with 100% vertical acceleration at all times. Thus we can <br />personally appreciate that a 1 % peak ground acceleration is usually not perceptible by most <br />human beings.