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work is completed the velocity model will be refined. Additionally, the elevation differences <br />between stations (Table 1) will be accounted for by station time-delay 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 the ground was doing at every point in time) is called a time history of the event at <br />that station. Time histories are often used by structural engineers to predict the performance of a <br />structure if the ground at or under the structure should experience that time history (intensity). <br />To calculate the largest amplitude of ground acceleration from a time history one would form 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. That value, <br />usually reported in percent of the vertical acceleration due to gravity, is the amplitude of the <br />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 />more closely with the Modified Mercalli Intensity and is used to interpolate intensity maps for public <br />distribution. <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 quantities 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 events have <br />been assigned magnitudes by MSSN (Dave Wolny, personal communication). Discrepancies of <br />+/- 0.3 are typical for local micro magnitude scales (Thatcher, 1973), and are due not only to <br />instrument variation but also to uncertainties in the basic definitions of magnitude scales. <br />Magnitude values for detected and located events should be analyzed with consideration of these <br />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.