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central value, which represents our best estimate of the parameter, and lower and higher <br /> values to represent the distribution around the best estimate. <br /> Source Geometry <br /> In the probabilistic analyses, it is assumed that earthquakes of a certain magnitude may occur <br /> randomly along the length of a given fault or segment. The distance from an earthquake to <br /> the site is dependent on the source geometry, the size and shape of the rupture on the fault <br /> plane, and the likelihood of the earthquake occurring at different points along the fault <br /> length. The distance to the fault is defined to be consistent with the specific attenuation <br /> relationship used to calculate the ground motions. The distance, therefore, is dependent on <br /> both the dip and depth of the fault plane, and a separate distance function is calculated for <br /> each geometry and each attenuation relationship. The size and shape of the rupture on the <br /> fault plane are dependent on the magnitude of the earthquake, with larger events rupturing <br /> longer and wider portions of the fault plane. We modeled the rupture dimensions following <br /> the magnitude-rupture area and magnitude-rupture length relationships of Wells and <br /> Coppersmith (1994). <br /> Recurrence <br /> The recurrence relationships for the faults are modeled using both the exponentially truncated <br /> Gutenberg-Richter relationship and the characteristic earthquake recurrence model. For the <br /> source zones, only an exponential recurrence relationship is assumed appropriate. We have <br /> used the general approach of Molnar (1979) and Anderson (1979) to arrive at the recurrence <br /> for the exponentially truncated model. A 70% probability has been included in the model <br /> that the faults rupture with a "characteristic" magnitude on specific segments. This model <br /> is described by Aki (1983) and Schwartz and Coppersmith (1984). We have used the <br /> numerical model of Youngs and Coppersmith (1985) for the characteristic model. These <br /> exponential and characteristic models are weighted to represent our judgment on their <br /> applicability to the sources (Figure 1). The number of events exceeding a given magnitude, <br /> N(m), for the truncated exponential relationship is <br /> H:\CONfRACr\TENMII.E\5 5 M0412951500 <br />