Laserfiche WebLink
May 15, 1998 • 6 • 983-2348.135 <br />of 10 percent probability of exceedance in 50 years. This is the risk level and implied <br />design life that is expressed in the 1994 UBC Eor non-critical structures. <br />To assess the seismic risk at the site and to determine various levels of PGA versus return <br />period, a probabilistic seismic hazazd assessment (PSHA) was performed using the basic <br />approach outlined by Cornell (1968), and the seismogenic source (fault) activity data <br />presented in Table 1. In addition, the attenuation relationships developed by Sadigh et <br />al (1993) for rock or stiff sites were used to model the dependency of the PGA on the <br />earthquake magnitude and distance. The Sadigh et al (1993) attenuation relationships <br />were selected for use in the analysis because they aze current and incorporate recent and <br />more comprehensive strong motion data than previous relationships (including neaz- <br />field data), they address the source type (e.g., they distinguish between dip-slip and <br />strike-slip faults), and they address site foundation conditions (i.e., they distinguish <br />between rock and soil sites). <br />The activity of potential seismic sources was modeled as a truncated form of the <br />Gutenberg-Richter (1954) magnitude-frequency relation given by: <br />logN=a-bM <br />where N is the cumulative number of earthquakes with magnitude greater or equal to <br />M. The relation is truncated at the maximum credible earthquake (MCE) magnitude. <br />The a-and b-values are coefficients that aze dependent on the fault activity (Table 1) and <br />regional seismidty data, respectively. <br />The probability of exceedance, Pe (Z), at a given level of ground motion (i.e., PGA) at the <br />site, Z, within a specified time frame, t, is given as: <br />Pe (Z) = 1- exp(-V(Z)t ) <br />where V(Z) is the mean annual rate of exceedance of ground motion level Z. Different <br />probabilities of exceedance maybe calculated, depending on the required level of <br />structural performance. <br />To evaluate the seismic exposure at the site, the PSHA was implemented. This resulted <br />in the curve of PGA versus return period that is plotted on Figure 2. For comparison <br />purposes, Figure 2 also shows the plots of PGA versus return period for the results of <br />PSHA mapping in the site area taken from the U.S. Geological Survey (1996) and <br />Algermissen et al (1990). <br />Figure 2 shows that Eor a return period of 475 yeazs, which is equivalent to the <br />risk/design level of 10 percent probability of exceedance in a 50-yeaz period from the <br />1994 UBC, the PGA at the site is 0.08 g. Although this PGA is generated from the <br />influence of all the sources within 100 km (62 mi) of the site, it is dominated by the <br />contribution of the Oil Creek and Fourmile Creek faults that are 8 km (5 mi) and 9 km <br />(5.5 mi) from the site, respectively. The Oil Creek fault appears to contribute almost two- <br />thirds of the 0.08 g PGA, while the Fourmile Creek fault contributes almost one-third. <br />Golder Auoclcies <br />