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Abstract:In this paper, the state of stress in the northern Tabas block in east-central Iran is analyzed based on the systematic inversion of aftershock focal mechanisms from the 1978.09.16 Tabas earthquake, to characterise the stress regime that controls most earthquakes in this area. Here, stress inversions of double-couple focal mechanisms of earthquakes recorded during the 30 days following the main shock have been carried out. The calculated average stress regime indicates dominant major 226 • to 237 • trending compression for the Tabas region. The dominating regime in east-central Iran is thrusting with a minimum stress axis, σ 3 , close to vertical. The reconstruction of the main seismotectonic stress in east-central Iran with a NE-SW compression is consistent with independent information of the active plate convergence related to Arabia-Eurasia convergence. Most earthquakes in the mentioned area occur near or around concealed Quaternary thrust faults with their activity being controlled by the NE-SW compression. Where Φ, the ratio of principal stress differences, is 0.5, a small difference between σ 2 σ 3 and σ 1 and small amounts of deviatoric stress is indicated. Therefore, for small deviatoric horizontal σ 1 it is not possible to increase and reactivate small sections of basement thrust faults and create secondary basement aftershocks. Reconstructed stress regimes in this study for sedimentary cover (237) and basement (226) of Tabas are similar. Therefore, it seems that the basement and cover were coupled together, possibly along the 2-4 km of upper Precambrian low-grade metamorphic rocks. Then these segments of the fold-and-thrust belt were involved in similar seismic activity under a similar stress regime. IntroductionIran is located within the convergence domain between the Arabian and Eurasian plates. The central parts of * E-mail: b.zamani@tabrizu.ac.ir, Tel: +98 411 3392700, fax: +98 411 4448914Iran move northwards with respect to western Afghanistan at˜15 mm yr −1 [1]. This northward motion introduces 15 mm yr −1 of north-south right-lateral shear across eastern Iran which is accommodated on right-lateral strike-slip fault systems on both the western (GowakNayband fault system) and eastern (Neh and Zendan fault systems) margins of the aseismic Dashte-e Lut desert [2,3]. The study area is situated in the eastern part of central Iran, and northern part of the Tabas block ( Fig. 1). Tabas is a town with approximately 30,000 inhabitants, 950 kilometers southeast of Tehran. The city was hit by a major destructive earthquake on September 16 th 1978, during which about 22,000 lost their lives. The recorded earthquake magnitude was between 7.5 and 7.9 on the Richter scale. Despite the major tectonic hazard in the Tabas region, relatively few studies have attempted to analyze the local-scale seismotectonic stress regime. Consequently, this research aims to characterise the stress regime which controls most of the earthquakes occurring in this area through an inversion of the earthquake focal mechanisms...

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