Abstract

Seismic potential investigation of Tehran as the capital of Iran is an essential issue because their accumulation around a fault may indicate its seismic potential. Stress trajectories for this estimate are useful. In this research, fault slip data is used for paleo stress analysis. Base on that, the study area divided into 6 stable stress regions and the mean stress tensor related to each region determined. Then the mean stress tensor rotated based on Anderson’s theory representing a compressional tectonic regime. The Stress trajectory map drew based on rotated mean stress tensor acting on the regions during geological time. The resulted map showed the arrangement of sigma1 trajectories in the area obeyed the overall tectonic regime in Iran and limited converge through the junction ignoring addition in stress magnitude and seismic hazard in the junction of major faults.

Given the importance of Tehran as the political-economic capital of the country, and its location in Alborz Basin with high faults density. and due to the seismic background of the area, the necessity of seismic risk assessment in this area becomes more evident. In this research, we have attempted to produce and present a map of faults in the Tehran wide area, focusing on faults in the eastern part of Tehran, Mamlouk, Ghasre Firozeh and the margins, with accurate structural elements and drawing of the stress trajectories, convergence of the trajectories, and stress accumulation at convergence sites, assess seismic hazard at this location based on longitudinal stress data (Katsushi Sato, 2011; Yamada and Yamaji, 2002; Yamaji, 2000; Sippel et al., 2009).

Based on field observations and data collected, scratch faults were selected for collecting and analysis of longitudinal paleo stresses as they record all deformation stages. After collecting the fault data, we stabilized them using the Multiple Inverse Method (MIM) and zone boundaries, and by drawing a Mohr's circle (without scale) for each range, seismic potential analysis was performed (Katsushi Sato, 2011; Yamada and Yamaji, 2002; Yamaji, 2000; Sippel et al., 2009).

To separate the stress phases, obtain the reduced stress tensor, obtain different stress and stress parameters, and plot the stress trajectories, the study area had to be divided into smaller ranges. It is not possible to determine the size of the stress components and the principal stresses by longitudinal stress methods and it is not possible to draw a scaled circle. Therefore, it is possible to draw a circle without scales for fault data only. This circle enables the overall analysis of the field shape, the arrangement of the data in the graph, and the comparison of the relative components of the fault data stress. By the Mohr's circle (without scale) method, the principal minimum stress and the maximum stress difference (s₁ - s₃) are considered as base (0) and unit (1), respectively, and assume the same size with respect to the relation (F = (s₂ - s₃) / (s₁ - s₃)) between the calculation of the middle stress field shape and the field shape factor. Studies show that tensile tectonic structures are not dominant structures in the region. For the kinetic analysis of fault data, precise rock mechanics such as the internal friction angle and the Amonton-Columbus criterion cannot be used precisely. But given the arrangement of the fault data, a large degree of comparison can be made between the kinetic features and especially the fault dynamics of each range. Therefore, the main maximum stress must be horizontal. Assuming that all the faults are coherent and based on Anderson's theory of faulting that the main minimum stress is vertical in the compressive stress regime, the position of the principal stress axes of each range is returned to the conditions of the fault formation (vertical minimum stress). In all ranges, the principal minimum stress is near vertical. After rotation of the data and the vertical axis of the minimum stress was set, the trajectory maps were drawn for horizontal stresses (main and maximum stresses).

A study based on longitudinal stress studies and Andersen's theory introduces the main maximum stress trend N017E, which is in good agreement with the general crustal shortening trend of the Central Alborz (Vernant et al., 2004). Therefore, the major faults of the region do not have a significant impact on the disturbance of the stress field within the region and, in fact, the convergence of these faults does not lead to the convergence of stress trajectories. The positioning of the poles of the fault plates on the main stress plates indicates that along with the crustal deformation in this part of Alborz, the regional structures have been rotated and decomposed. In fact, the reason for the polarization of fault plates on the main stress sheets with zero shear stress is that the rotation and positioning of faults coincide with the rotation and deformation of other geological structures and phenomena such as folds and joints. The arrangement of the poles of the fault plates in the Mohr's circle indicates that the faults in zone 3 have less dynamic potential than elsewhere.

Keywords: Stress Trajectory, Multiple Inverse Method, Convergent Faults, Seismic Hazard, Mamlouk, Ghasre Firouzeh.