Probabilistic seismic hazard assessment and disaggregation of seismic hazard in golestan and adjacent areas

In the present study, probabilistic seismic hazard assessment (PSHA) and disaggregation of seismic hazard in Golestan and adjacent areas have been conducted. The study area includes Golestan province and surrounding areas to about 150 km away from the geographical borders of the province. For this purpose, at the first stage, geological and seismotectonic studies, including the tectonic features, faults dimensions, and faults mechanism in the study area have been studied. The seismicity parameters including b-value, annual mean occurrence rate (λ) and maximum possible magnitude (Mmax) of the study area are evaluated based on the uniform catalog containing historical and instrumental earthquakes from 400 BC to the end of 2021 AD with magnitudes ranging from 3.5 to 7.5 on the Mw scale. Based on the geological and seismological information of the study area, 21 areal seismic source zones have been identified. Using the spatial distribution function, the annual mean occurrence rate of earthquakes of each seismic source has been determined in different magnitude intervals. The latest three local ground motion prediction equation models of the Iranian plateau, have been employed to assess the peak ground acceleration (PGA) and spectral acceleration (SA) in periods of 0.2, 1, and 2 seconds for return periods of 50, 475 and 2475 years. A three branches logic tree with the same weight has been used to apply the GMPEs. The GMPEs are specific to Iran and used for the first time in the study area. The entire study area is divided into points that are 5 km apart. Subsequently, the PGA and SA have been calculated for each point with the OpenQuake software package. The results are presented in the form of seismic hazard zoning maps in Golestan province and spectral acceleration curves for Gorgan city. The calculations are on the bedrock site condition according to the Iranian code of practice for seismic resistant design on building (Standard No. 2800 4th edition). In order to determine the most effective seismic source in the PGA of the Gorgan, the disaggregation of PSHA has been conducted on the parameters of distance, magnitude, the number of standard deviations (ε), and latitude and longitude. The most probable scenario of maximum acceleration in Gorgan has been introduced. The results show that some areas of the Gorgan, Kord-Kouy, Agh-Ghala and Ali-Abad in southwest and Maraveh-Tapeh in northeast of the province can be affected by more ground acceleration than other areas so that the maximum ground acceleration in these areas for the return period of 475 years is 0.21g to 0.25g. The disaggregation of PSHA for Gorgan city shows that the Caspian and North Alborz faults have the most substantial contribution in the PGAs of Gorgan.