Seismic hazard zoning in Iran and estimating peak ground acceleration in provincial capitals



  Growing environmental and social concerns, both on the part of decision makers and public opinion, have brought a new perspective to the perception of hazard assessments a valid alternative in the long-term, and an effective complement in short and medium terms, to traditional design procedure for a resistant and safe environment. Results of the gradual development of research on the probabilistic seismic hazard assessment (PSHA) in the past 40 years make a framework that could be used for estimation of probability of
occurrences of earthquakes, at certain return periods on each site. The primary advantage of the PSHA over alternative representations of the earthquake threat is that PSHA integrates over all possible earthquake occurrences and ground motions to calculate a combined probability of exceedance that incorporates the relative frequencies of occurrence of different earthquakes and ground-motion characteristics. Features of the PSHA allow the ground-motion hazard to be expressed at multiple sites consistently in terms of the earthquake sizes, frequencies of occurrence, attenuation, and associated ground motion. Potential seismic sources, seismicity models, ground motion prediction equations (GMPE) and site effects, are the most important factors in seismic hazard studies. In this research, a modified probabilistic seismic hazard assessment, developed by Chinese researchers, is used to estimate the level of the potential seismic ground motion in Iran. A unified catalog of de-clustered earthquakes containing both historical and recent seismicity until late 2012 in the area encompassed by 22-42ºN and 42-66ºE is used. An area source model which contains 238 potential seismic sources within 5 major seismotectonic provinces in the study region has been delineated. Considering magnitude uncertainty and incompleteness of the earthquake data, the seismicity parameters of the seismotectonic provinces are determined. Spatial distribution function is used to determine occurrence rates of potential seismic sources for different magnitude intervals. Also, the background seismicity has been determined for each province. Seismic hazard assessment of Iran for a grid of over 40,000 points with 10 km interval is carried out using OpenQuake software by three different GMPEs and two models of seismicity for potential seismic sources in a logic tree.  The peak ground horizontal acceleration (PGA) and spectral accelerations (SA) for 5% damping ratio at 0.2 and 2 seconds corresponding to 10% and 63% probability of exceedances within 50 years (475- and 50-years mean return periods, respectively) are calculated. The resultant seismic hazard maps display a probabilistic estimate of PGA and 0.2 and 2 sec SA for different mean return periods of 50 and 475 years. Resultant peak ground horizontal acceleration for 475-years return period varies from 0.63g in North-East of Lorestan to 0.1g in central Iran. The resultant PGAs for the 475-year return period in provincial capitals indicate the maximum value (0.35g) in Bandar Abbas and Tabriz, and the minimum one (0.11g) in Esfahan and Yazd. Comparison of the results of this study with the last map of seismic hazard in the Iranian code of practice for seismic resistance design of buildings, seismic macrozonation hazard map of Iran, Standard 2800, shows significant differences. Seismic hazard levels estimated in this study in southern Iran, Sistan-Baluchestan, Hormozgan and Fars provinces, show significantly higher values.