Quaternary Journal of Iran

Studying the Site effect on the earthquake acceleration distribution in Karaj city

Document Type : Original Article

Authors

Mehrdad Mohammadi 1
Ali Beitolahi 1
Bahram Akasheh 1
Arezo Dorostian 1
Khalil Rezaei 2

1 Department of Geophysics, North Tehran Branch, Islamic Azad University, Tehran, Iran

2 Department of Geology, Kharazmi University, Tehran, Iran

10.22034/irqua.2025.2043978.1031
Abstract
Introduction
As one of the most seismic regions in the world, Iran has two main tectonic belts (Berberian and King, 1981). The Zagros belt, which starts from the eastern end of the Makran subduction zone in the Oman Sea. The trend of this belt is southeast-northwest and its other side is limited to the eastern corner of the North Anatolian fault (Nissen et al., 2019). The Alborz tectonic belt starts from this point with an east-west trend and extends to the Kepe Dagh fault in the northeast corner of Iran (Alavi, 1996). The central part of the Alborz belt is surrounded by active faults that have caused the destruction of cities in the region in the past centuries (Zanchi et al., 2007). Investigating the activity of these faults and how they function during a major earthquake event is very important (Berberian and Yeats, 2016). On the other hand, the effect of the construction in the dispersion of the waves of such earthquakes can significantly affect the destruction of the structures (Habibi et al., 2023). On average, from 1900 to 2014, 3 people died every hour in Iran due to earthquakes (Berberian, 2014). Such a shocking statistic makes it even clearer that it is necessary to pay more attention to the conditions of the building.

Materials and Methods
Systematic records of earthquakes that occurred in Karaj and its surrounding areas have been prepared from the catalog of various international sources and published studies. Sources of earthquake records include the International Seismological Center (ISC) comprehensive catalog from 1908 to 2023, the Global Earthquake Model (GEM) catalog from 1903 to 2019, and historical earthquake records from published studies (Ambraseys and Melville, 1982; Berberian, 1995; Berberian et al. ., 1983, 1985) was collected for an area with a radius of 150 km from Karaj. It was tried to add the relevant earthquakes to the catalog in cases where the length of active faults in the region is outside this radius. Seismic records are collected, reformatted, and stored chronologically to produce a uniform composite seismic catalog.
The catalog of clustered earthquakes for the period 1762 to 2023 is not complete. Seismic rates predicted using the earthquake events in this list may underestimate the occurrence of future earthquakes in these regions. A reliable mean seismic rate can be predicted by identifying the period over which the catalog is complete for a given magnitude range. Therefore, completeness periods for different magnitude ranges are determined using the Step method (Stepp, 1972).
Results and Discussion
In addition, during the return periods of 475 and 975 years, the values of earthquake acceleration from the PGA period have an increasing trend until 0.2 seconds and then decrease. But in the return period of 2475 years, the upward trend is observed even up to a period of 0.3 seconds.
In order to control and check this point, it is necessary to use earthquake risk separation charts. The relative contribution of seismic sources (isolation model) in seismic hazard (PGA) and period of 0.3 s for three return periods of 475,974,2475 years for the central point of Karaj city are shown in Figures 6 and 7. Also, the numerical values obtained from these graphs are presented in Table 4. The percentage values of the contribution of each of the seismic sources in the total risk are displayed as a pie chart. In all the analyzes of Table 4, the share of faults in the total risk is North of Karaj, North of Tehran, Mosha, Taleghan, Peshva, and Firouzkuh, respectively. But clearly, in the return period of 2475 years in a period of 0.3 seconds, the contribution of seismicity of North Tehran fault increases compared to other faults. These diagrams show the role of distant springs in the changes of spectral acceleration values. In this way, the effect of the faults that are far away can be clearly seen in increasing the acceleration of the earthquake for long return periods and long periods.

Conclusions
The most challenging part of seismic hazard analysis is identifying seismic sources. The most important limitation for the description of seismic sources is the uncertainty in determining the location, geometry, size of the earthquake and the open distance of the occurrence of each source. Large earthquakes (Mw > 7) occur at intervals of hundreds to thousands of years. But the historical records of earthquakes in these areas go back to 1830 AD Masha fault. The proper identification of the epicenter of the earthquake was not possible until the beginning of the 20th century, when the recording of earthquakes began. Therefore, the foci of historical earthquakes were identified based on the severity of damage to the structures. The rupture of smaller and deeper earthquakes does not appear on the surface of the earth. Large, shallow crustal earthquakes generally cause surface ruptures, but due to the lack of understanding of the earthquake mechanism, the ruptures of those earthquakes are not well documented. Fault slip in prehistoric and historic earthquakes can be estimated through paleoseismological studies. Therefore, in this study, while examining recent paleoseismological studies, the geomorphic evidence of past earthquakes was determined. Also, the statistics of historical and prehistoric earthquakes in all the faults of the region were accurately identified. Then, the Probabilistic Seismic Hazard Analysis (PSHA) has been performed using recently published data and information. A shallow crustal fault model has been used to estimate seismic hazard using the recently published Ground Motion Prediction Equations (GMPEs). Epistemic uncertainties have been calculated using the logical tree approach. The value of spectral acceleration was estimated for the return period of 475, 975, 2475 years in PGA periods of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75 and 1.0 seconds. In these calculations, the shear wave velocity map of the region has been used for the first time.

Keywords

Maximum ground acceleration (PGA)
spectral acceleration (SA)
far field source
probabilistic seismic hazard analysis (PSHA)
Karaj

Subjects

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Quaternary Journal of Iran
Volume 10, 1 , 2
September 2025
Pages 62-87

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