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SEMUTWO Seismic Hazard Analysis MethN1olody <br /> p(Z>zlmi,rj) = probability that given an earthquake of magnitude mi at a distance of rj, the <br /> ground motion exceeds the specified level z. <br /> The calculations were made using the computer program HAZ38 developed by Norm <br /> Abrahamson. This program has been validated in the Pacific Earthquake Engineering Research <br /> (PEER) Center-sponsored "Validation of PSHA Computer Programs" Project (Thomas et al., <br /> 2010). <br /> 2.2 SEISMIC SOURCE CHARACTERIZATION <br /> Two types of earthquake sources are characterized in the seismic hazard analyses: (1) fault <br /> sources; and (2) background seismicity (Section 4.1). Fault sources are modeled as three- <br /> dimensional fault surfaces and details of their behavior are incorporated into the source <br /> characterization. Background seismicity can be treated as an areal source zone where <br /> earthquakes are assumed to occur randomly or the historical seismicity can be assumed to be <br /> stationary in space and hence smoothed using a Gaussian filter. Both approaches were used in <br /> this PSHA(Figure 4). <br /> The geometric source parameters for faults include fault location, segmentation model, dip, and <br /> thickness of the seismogenic zone. The recurrence parameters include recurrence model, <br /> recurrence rate (slip rate or average recurrence interval for the maximum event), slope of the <br /> recurrence curve (b-value), and maximum magnitude. Clearly, the geometry and recurrence are <br /> not totally independent. For example, if a fault is modeled with several small segments instead <br /> of large segments, the maximum magnitude is lower, and a given slip rate requires many more <br /> small earthquakes to accommodate a cumulative seismic moment. For areal source zones, <br /> earthquakes are modeled as point sources and only the areas, maximum magnitude, and <br /> recurrence parameters(based on the historical earthquake record)need to be defined. <br /> Uncertainties in the source parameters are included in the hazard model using logic trees <br /> (Figure 4). In the logic tree approach, discrete values of the source input parameters have been <br /> included along with our estimate of the likelihood that the discrete value represents the actual <br /> value. In this PSHA, generally all input parameters have been represented by three values; the <br /> values represent a distribution about the best estimate(Figure 4). <br /> 2.2.1 Source Geometry <br /> In the PSHA, it is assumed that earthquakes may occur randomly along the length of a given <br /> fault or segment. The distance from an earthquake to the site is dependent on the source <br /> geometry, the size and shape of the rupture on the fault plane, and the likelihood of the <br /> earthquake occurring at different points along the fault length. The distance to the fault is <br /> defined to be consistent with the specific ground motion prediction model used to calculate the <br /> ground motions. The distance, therefore, is dependent on both the dip and depth of the fault <br /> plane, and a separate distance function is calculated for each geometry and each ground motion <br /> prediction model. The size and shape of the rupture on the fault plane are dependent on the <br /> magnitude of the earthquake, with larger events rupturing longer and wider portions of the fault <br /> plane. For a given magnitude, the associated rupture surface is uniformly distributed along the <br /> fault length and width. Ruptures are constrained to occur entirely on the defined fault plane. We <br /> modeled the rupture dimensions following the magnitude-rupture area and magnitude-rupture <br /> width relationships of Wells and Coppersmith(1994). <br /> um 3 <br />