عنوان مقاله [English]
The study area is a part of the Alpine-Himalayan orogen. It is formed by the Greater Caucasus Mts., the Lesser Caucasus Mts., the Talesh Mts., the Kura Basin, and the South Caspian Basin. Present-day structures of the study area are controlled by the ongoing collision of the Arabian and African plates with Eurasia. The study area extends over 1342 km2 located in Northwest of Iran, which can be regarded generally as the continuation of structural grain of Lesser Caucasus. However, the existence of rigid block of South Caspian Basin and the different lithology ofAzerbaijan including Quaternary volcanic masses such as Sahand and Sabalan has resulted in a complex distribution of deformation in the studied area.Northwest Iran is a region of vigorous inflection, deformation and seismicity situated between two thrustbelts namely Lesser Caucasus to the north and the Zagros thrust belt to the south.What is known about Seismotectonics of the area located in north of Tabriz fault is limited to the recent works done by Masson et al. (2006). Using dense GPS, they determined that the deformation in NW Iran is characterized by ~ 8 mm/yr of right-lateral movement on the North Tabriz fault, and ~ 8 mm/yr of extension within Talesh Mnt. The duplicate Ahar-Varzeghan earthquake focal mechanisms contradicted the GPS results. Although both main shocks have probable fault planes that strike roughly east–west, it is likely that the mapped surface faulting should be associated with the first main shock because field observations record nearly pure strike-slip motion that would be inconsistent with the transpressional mechanism of the second main shock (Ghods et al., 2015). The fault study of Coopley et al. (2013)introduced three Segments with a length of 400 m. till 8 km. In 2012 August 11 (12:23 UTC), a moderate earthquake with MW=6.4 (USGS) occurred between Ahar and Varzeghan towns in Azerbaijan Province at northwest of Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. In order to solve this problem, Ghods et al. (2015) have introduced a model to resolve this earthquake focal mechanisms and many active fault earthquake rupture in the range of Ahar -Varzeghan.A combined study of active tectonic parameters such as geomorphic indices and stress tensor measurement, allowed to recognize anew fault in the northern part of North Tabriz Fault. In addition to the seismicity of North Tabriz fault, seismicity map of Azerbaijan shows the same distribution ratio in the NW part of the North Tabriz fault. In the absence of active faults, the relation of this seismicity is not known. This study was an attempt to introduce one of the active faults at the North of the North Tabriz fault in order to improve the understanding of seismotectonic characteristic of this area.In order to inquire the relevance between rock rigidity and SL index based on a simplified geological map of the area and field observation, rocks were categorized by their resistance as below: Very low (young alluvial deposits), Low(old alluvial deposits, poorly consolidated conglomerates, marl), moderate(gypsum, gypseous marl), High(limestone, sandstone, dolomite, shale, conglomerate, tuff, schist, flysch sediments), Very high (andesites, trakiandesite, gabbro,dacite) (El Hamdouniet al., 2007). Based on the mentioned category, the distribution map of lithological resistance was obtained usingGIS. To assess tectonic activities in the area, geomorphic indices such as: the stream length-gradient index (SL) and hypsometric integral (Hi) have been used to reveal vertical active movements along a particular fault, namely Nahand fault. This fault lying in north of Tabriz fault strikes parallel to it with a length of 168 Km. The combination of morphometric studies and kinematic study show clear vertical movement along Nahand fault. Determination of stress tensor and geomorphic indices has shownthat the Nahand fault is a right-lateral strike-slip with a minor vertical component. Field observations and the inversions of stress tensor (geologic and seismologic fault kinematics) revealed that the Nahand fault is an active fault, acting in a transpressional tectonic regime, with the Sh-max oriented NW-SE.