ANNALS OF GEOPHYSICS, 56, 5, 2013, S0561; doi:10.4401/ag-6343 S0561 Disaggregation of probabilistic ground motions in two cities of Western Iran, Kermanshah and Sanandaj Elham Shabani * , Noorbakhsh Mirzaei, Mehrdad Pakzad Institute of Geophysics, University of Tehran, Tehran, Iran ABSTRACT This article presents the results of disaggregation of ground motion hazard obtained for two of Iran’s urban centers, Kermanshah and Sanandaj. Disaggregation of peak ground acceleration (PGA) and spectral acceleration hazard corresponding to mean return periods of 475 and 50 years is performed. 12 area seismic sources in the study re- gion as well as 15 area seismic sources in a 150-km distance from the region are delineated. The scenario earthquakes are characterized by bins of magnitude, M, source-to-site distance, R, and number of stan- dard deviations, ε, that the ground-motion parameter is away from its median value for that M-R pair as estimated by a prediction equation. In most cases, the sources closer to the site dominate. Larger, more dis- tant earthquakes contribute more significantly to hazard for longer pe- riods than for shorter periods. Disaggregation plots can provide useful information on the distance and magnitude of predominant sources, which can be used to generate scenario earthquakes and select corre- sponding time histories for seismic design. 1. Introduction For many earthquake and/or geotechnical engi- neering analyses, time histories compatible with local seismic activity are desired. It is desirable the selection of these time histories to be based on a probabilistic seismic hazard assessment (PSHA) consistent with the seismicity of the region. PSHA aggregates ground mo- tion contributions from all earthquakes of all possible magnitudes, as probability at all significant distances from the site of engineering interest. Therefore, the PSHA results are not representative of a single-design earthquake. For a given site of engineering interest, the disaggregation of the PSHA results provides a tool to understand the relative contribution of each source to the overall seismic hazard. Precisely, the disaggregation process extracts the combinations of magnitude (M), source-to-site distance (R) and the number of standard deviations (ε) by which a ground motion parameter dif- fers from the median value predicted by a ground mo- tion prediction equation (GMPE) given magnitude, M, and distance, R. The hazard disaggregation portrays the design or controlling earthquake from either the mean or modal values of M and R only, or ε as well. The probabilistic disaggregation methodology is expli- cated in great detail on the studies of Chapman [1995], McGuire [1995], Cramer and Petersen [1996], Frankel et al. [1996], Bazzurro and Cornell [1999], Harmsen et al. [1999], Frankel et al. [2000], Harmsen and Frankel [2001], Campos Costa et al. [2002], Montilla et al. [2002], Harmsen et al. [2003], Barani et al. [2009], Sousa and Costa [2009], Chioccarelli and Iervolino [2013] and Sa- betta [2013]. Based on the seismic hazard study conducted by Shabani and Mirzaei [2007], the hazard assessment re- sults have been disaggregated in terms of magnitude, distance and epsilon, to investigate the scenario earth- quake contributing the most to the ground motion haz- ard. The disaggregation was undertaken in accordance with the procedure recommended by McGuire [1995]. Shabani and Mirzaei [2007], assuming that earth- quakes follow a Poisson model, used a probabilistic procedure to calculate seismic hazard for different re- turn periods in the Kermanshah-Sanandaj region, an area encompassed by 46-48°E longitudes and 34-36°N latitudes. 12 seismic sources are delineated in the Ker- manshah-Sanandaj region as well as 15 other seismic sources in a 150-km distance from the region. The major aim of this study is the disaggregation of seismic hazard for the two cities; Kermanshah and Sanandaj in the west of Iran. In this regard, the quan- Article history Received April 23, 2013; accepted October 30, 2013. Subject classification: PSHA, Scenario earthquake, Disaggregation, Main Recent Fault, Zagros, Iran.