فهرست مطالب سید حسن موسوی بفروئی

Most of the areas of the Iranian Plateau are high seismicity regions and there is a considerable difference between them. These differences in the eastern Iranian Plateau including the Lut block compared to the surrounding areas are so substantial that can be studied independently. A brief look at the seismic maps of this region shows that around the Lut block, the activity of strike-slip and reverse faults has caused many and devastating earthquakes but in the inner parts, it can be considered as a small rigid plate regarding the relative stability. In this research, using the most updated catalog as the main source, a catalog of 1760 historical and instrumental declustered earthquakes was provided for the area bounded in 56.5°-61°E and 28°-35.5°N in eastern Iran. The data was divided into a historical part and four instrumental parts of 1900 to 1963, 1964 to 1999, 2000 to 2005 and 2006 to 2022. Using the maximum curvature method for each of the four time intervals, the data completeness thresholds were determined to be 5.7, 4.7, 4 and 3.1, respectively. The seismicity parameters were estimated taking into account the uncertainty of the magnitudes. Substantially, the estimated b-value is equal to 1.06 and the mean annual occurrence rate for magnitude equal or greater than 3.1 is 58. The maximum magnitude of the earthquakes is estimated to be 7.9, which indicates the high seismicity of the region. The probability of annual exceedance for different magnitudes is estimated for the return periods of 50, 475, and 2475 years. Comparing the results of this study with the results of the other studies performed based on the same data and the same method for the Central-East Iran seismotectonic province confirms that the seismicity of the Lut block and its margin can be determined independently from the other parts of the Central-East Iran seismotectonic province.

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.

In this study, we used the recent comprehensive earthquake catalogue of Mousavi-Bafrouei and Babaie Mahani (2020), including historical and instrumental earthquakes until the end of 2018, to calculate Probabilistic Seismic Hazard Assessment (PSHA) at eight metropolitans with a population of more than 1 million. These metropolitans include Ahvaz, Isfahan, Karaj, Mashhad, Qom, Shiraz, Tabriz, and Tehran. Our approach was implemented using a MATLAB code that was compiled for the purpose of this study. The historical seismicity within a 300-km radius was considered around each city, and the seismicity parameters were obtained in each case. We provided the hazard curve and uniform hazard spectrum for peak ground acceleration (PGA) and pseudo-response spectral acceleration (PSA) at periods of 0.04, 0.1, 0.2, 0.3, 1.0, and 3.0 sec for the return periods of 50, 475, and 2475 years, respectively. For hazard calculations, we used four ground motion prediction equations with equal weights; Boor et al. (2014), Idriss (2014), Kale et al. (2015), and Farajpour et al. (2019). Our PSHA results show that the highest hazard occurs in the cities of Shiraz and Tabriz, whereas the lowest hazard level happens in the city of Isfahan. Specifically, the largest PGA values at the bedrock ( = 760 m/sec) condition and for the return periods of 50, 475, and 2475 years are 77 cm/sec2 (Shiraz and Tabriz), 203 cm/sec2 (Shiraz and Tabriz), and 535 cm/sec2 (Tabriz), respectively. On the other hand, the smallest PGA values for the same return periods occur for the city of Isfahan at 29 cm/sec2, 77 cm/sec2, and 125 cm/sec2. We also compared our results with other PSHA studies obtained by other researchers, including Mousavi Befrouei et al. (2014), Salahshour et al. (2018), and Shahbazi and Mansouri (2019). In general, we found that our results show lower values in terms of ground motion amplitudes. For example, Mousavi-Bafrouei et al. (1393) obtained higher values by up to ~30% than those obtained in this study. This difference is probably due to the inclusion of different datasets for source characterization and calculation of seismicity parameters. In the approach used in this study and the works of Salahshour et al. (2018) and Shahbazi and Mansouri (2019), historical seismicity is the only source of information for the determination of seismic sources and their parameters, which resulted in similar ground motion values. However, Mousavi Befroui et al. (2014) used geological, geophysical, and seismotectonic evidence along with historical seismicity for source characterization.

In this study, Seismic Hazard zoning was probed by studying seismogenic zones and earthquake data in a 150 km radius around the Fars province boundaries in the Zagros seismotectonic zone. Zagros is one of the most seismically active parts of Alpine-Himalayan seismic belt. Fars province located at latitude of 27.2° – 31.42° and longitude of 50.42° – 55.36°, respectively. In this study, seismotectonic zone was determined based on the location of interest, faults strikes, seismic records, and geological structure. In the first step of process, an earthquake catalogue of the instrumental and historical earthquakes was prepared by assimilating them to a uniform magnitude type (Mw) from 8th century until September 2016. Then, by using the Uhrhammer (1986) and Gardner and Knopoff (1974) methods in 1974, aftershocks and foreshocks were eliminated from the catalogue to have main earthquakes only. After the elimination of aftershocks and foreshocks, 2181 earthquakes remained as the main earthquakes in catalogue and their completeness magnitude was calculated by ZMap software. Afterwards, it was tried to recognize all the active faults around the sites; therefore, 23 area sources were introduced within the studied area from faults, historical and instrumental earthquakes. After introducing area sources, seismic parameters such as beta and lambda were calculated for each part using Kijko software. Similarly, space distribution function portion of each source was calculated. In addition, the results of six magnitude studies utilizing step method carried out by Mousvai et al. (2014) was used in this paper. In the deterministic approach, Pick Ground Acceleration (PGA) over bedrock was computed using two attenuation equations, Campbell and Bozorgnia (2008) and Boor and Atkinson (2008), using a MATLAB software for Fars province. Besides, in the probabilistic approach, PGA and pick ground velocity, PGV, were calculated using the same values in the attenuation equations that were used in the deterministic method using OpenQuake, one of the distinguished programs in seismic hazard analysis to determine the PGA on bedrock. The probabilistic approach was carried out on a grading area using a 10 km zonation map. PGA was calculated and covered 63% and 10% probability of exceedance in one life cycle of 50 and 475 years were presented, and hazard curves were calculated for 50 and 475 years using OpenQuake software. Probabilistic study resulted maximum PGA values in the periods of 50 and 475 years to be 0.23 g and 0.6 g respectively .These maximum PGA values centered on Kazeroon. After Kazeroon, Shiraz and Lamerd have the highest levels of hazard risk of cities in Fars province. Moreover, using the deterministic approach and the two attenuation equations, the obtained maximum PGA values fell between 0.68 g and 0.75 g. Finally, the results were compared with previous studies like Hamzeloo (2005), Ghafory-Ashtiany (1999) and Mousavi (2014).

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.

Based on the standards for design of structures, any structure should be designated for seismic loads and any combinations containing seismic loads. For spectral analysis of a structure, the site effect is taken into account by considering its effects on the design spectra. Because of different lateral and vertical stiffness of the soil layers underneath the structure, the design spectra are different from soil to soil. The stiffer the soil the higher the velocity results in. The shear and compressive wave velocities of the soil as continuum are criteria for categorizing the soil in stiffness point of view. Since, the structures are more affected by lateral forces than vertical ones, the shear wave velocity is more important than the compressive wave velocity. Moreover, the soil nearer to the structure affects the structure more than the soil far from it. Thus, in the standards for design of structures, the mean shear wave velocity of the upper 30 m of the soil layer is used for categorizing the soil underneath the structure in stiffness point of view. In the Iranian code of practice for seismic resistant design of buildings, the standard number is 2800, the sites have been categorized into four different types, which are rock (with the mean shear wave velocity of the upper 30 m denoted as larger than 750 m/sec), medium alluvium (where m/sec), soft alluvium (where m/sec), and very soft alluvium (with m/sec). To analyze the structures using design response spectra, specified horizontal and vertical spectra are needed for each category. Because of the increase in the number of strong motion accelerograms in recent years, in this research the horizontal and vertical design spectra for the first category (the rock site) are prepared based on Iranian data. To do so, all the existing horizontal and vertical acceleration time histories in different stations fixed on rock sites are gathered. The data are filtered and baseline corrected by Seismosignal software to remove the noise frequency components and to modify the magnitude of both the displacement and velocity. In addition, all the data are normalized for the Peak Ground Acceleration (PGA). According to the data, the quality of 60 vertical time histories and 71 horizontal time histories were acceptable. With this data, both the vertical and the horizontal response spectra are prepared for each time history and for four different damping ratios, which are %2, %5, %10 and %20. Averaging the response spectra, the unsmoothed design spectra are obtained. The smoothed design spectra are plotted in tripartite coordinate system and spectral acceleration-time system, as well. These procedures are done for the average plus one standard deviation of vertical and horizontal response spectra. Finally, the smoothed design spectra from the data of this research are compared with that of the Iranian code of 2800 regulation and also the Mohraz design spectra. It is shown that the results are in good agreement with the Mohraz design spectra except that in long periods, the spectral acceleration obtained in this study is smaller. Comparing the result of this research with that of 2800 regulation, it is seen that in short periods, the spectral acceleration in this study is higher than that in the 2800 regulation, while for long periods, the spectral accelerations in this study is much less than that given in 2800 regulation. It means that in the category of short period structures more strengthen structures may be needed.