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<br />7. PROBABILISTIC SEISMIC HAZARD ANALYSIS <br />Seismic hazard analyses aze typically conducted using one of two <br />methods: (1) deterministic analysis or (2) probabilistic analysis. in <br />some cases, the methods may be combined to provide a check on <br />assumptions inherent in both methods. In general, deterministic and <br />probabilistic results agree only in highly active seismogenic areas, for <br />example southern California. In areas of low to moderate seismicity <br />like Colorado, deterministic methods tend to seriously overestimate <br />seismic risk. In our opinion, probabilistic analysis is the curtent state- <br />of-the-art in the assessment of seismic hazards for engineering projects <br />in low to moderate seismicity areas and is replacing traditional deter- <br />ministic methods. Both methods have advantages and disadvantages <br />as discussed below. <br />Deterministic methods use discrete models to arrive at scenazio-like <br />earthquake hazard assessments (Reiter, 1990). The analysis takes the <br />form of a specified ground motion at a given site resulting from an <br />earthquake (usually the Maximum Credible Earthquake or MCE} at a <br />given hypocentral/epicentral distance. The elements required in a <br />deterministic analysis are: (1) the site location; (2) the earthquake <br />source; (3) the associated earthquake magnitude; (4) the distance <br />between the source and the site; and (5) an appropriate attenuation <br />relation. The product of the analysis is a ground motion parameter, <br />usually the peak horizontal ground acceleration. No consideration is <br />given to the probability that the maximum earthquake (MCE) will <br />occur during the life of the project. Likewise, uncertainty in geologic <br />and seismologic interpretations is not accounted for. Because the <br />deterministic method is perceived to define the "worst case" scenario, <br />it has found acceptance among designers and is easily understood by <br />the public. Drawbacks are [hat the deterministic method may produce <br />unrealistically conservative hazazds assessments and is unresponsive to <br />uncertainty and rapidly evolving understanding of earthquake hazards. <br />The method also produces unreasonable results in areas of low to <br />moderate seismicity where seismicity cannot be associated with <br />specific geologic structures and hypocentraUepicentral distances are <br />highly uncertain. The typical response to these problems is to place <br />the maximum earthquake directly beneath the site which results in <br />unrealistically high ground motions without respect to the probability <br />of occurrence. <br />24 <br />