عنوان مقاله [English]
نویسندگان [English]چکیده [English]
In this study, a modified “probabilistic seismic hazard assessment” (PSHA) method is used to estimate the level of the potential seismic ground motion in Firoozkouh. A problem that may be encountered in probabilistic studies of seismic hazard for a specific site, for engineering purposes, is the selection of design earthquakes corresponding to a given hazard value. In order to derive a seismic scenario consistent with the results of PSHA for a site and determine the relative contribution of events to the overall seismic hazard, the concept of disaggregation was introduced. The disaggregation of seismic hazard is an effective way to identify the scenario events that contribute to a selected seismic-hazard level. In other words, the disaggregation process separates the contributions to the mean annual rate of exceedance (MRE) of a specific ground-motion value at a site due to scenarios of given magnitude M, distance R, and the ground motion error term, ε. Disaggregation results could change with the spectral ordinate and return period, thus more than one single event may dominate the hazard especially if multiple sources affect the hazard at the site. These results can provide useful information for better defining the design scenario and selecting corresponding time histories for seismic design. In most cases, as the probability decreases, the hazard sources closer to the site dominate. Larger, more distant earthquakes contribute more significantly to hazard for longer periods than shorter periods. In this study, the seismic hazard disaggregation process is performed to identify dominant scenarios in “peak ground acceleration” (PGA) and 5% damped 0.2 and 2.0 s spectral accelerations corresponding to mean return periods (MRPs) of 50 yr, and 475 yr (hazard levels of 63% and 10% probability of exceedance in 50 yr, respectively) in Firoozkouh city. In this regard, potential seismic sources and their seismicity parameters have been estimated based on the concept of spatial distribution function in 34º–37ºN latitudes and 52º–55º E longitudes in grid intervals of 0.1º. For each point using proper attenuation relationships, PGA, 0.2 and 2.0 s spectral acceleration values with 63% and 10% probabilities of exceedance in 50 yr have been calculated using the EZ-FRISK (version 7.43) code. The hazard can be simultaneously disaggregated in different types of bin. The result of seismic hazard disaggregation are presented in terms of 1-D M, R and ε bins and 2-D M-R bins. Bins of width 0.4 in magnitude, 10 km in distance, and 0.2 in ε are selected. The disaggregation results in terms of probability density function (PDF) are reported, which is obtained by dividing the probability mass function (PMF) contribution of each bin by the bin’s size, thus the PDF representation is independent of the bin’s amplitude. The results identify the distribution of the earthquake scenarios that contribute to exceedance of PGA and 5% damped 0.2 and 2.0 s spectral accelerations for 50 yr and 475 yr MRPs, in terms of magnitude and distance (M-R). Dominant scenarios are identified for interest hazard levels in Firoozkouh city.