discontinuities with 1 km or greater lengths and bends with 5° or greater angles are chosen. Based on these limits, seven segments and associated earthquakes (with magnitudes equal or greater than 5.0) are recognized. From this data set it appears that fault geometry, such as bends and stepovers along the main fault trace, plays an important role in controlling the location of moderate to large earthquakes along the fault zones. Certain geometrinc patterns, such as restraining stepovers, are particularly common and can be viewed as responsible for strain accumulation along portions of fault zone. Moderate to large earthquake epicenters often occur near restraining stepovers.

Iran is a wide compressional deformation and seismic activity zone along the Alpine-Himalayan orogenic belt resulting from the conversion motion between the stable Arabian and Eurasian plates. Northwestern Iran is part of a complex tectonic system within the Arabia-Eurasia collision zone. The main active fault of northwestern Iran is the Gailatu–Tabriz strike-slip fault system (GTFS) that extends ~400 km in length from north of Mianeh (a town in the East-Azarbaijan province of Iran) to the southwest and south of Kaghsman (in Turkey) to the northwest. It has a conspicuous history of seismicity and a controlling role in the geodynamics of the region. In this study, we utilize satellite images, DEM images, field evidence, earthquake information, and GPS data, to investigate the active tectonic characteristics of the GTFS. From the southeast to the northwest, GTFS consists of three main fault zones, named: North Tabriz Fault, Mishu-Tasuj Fault, and Gailatu-Siah Cheshmeh-Khoy Fault. Near Kaghsman, the northwestern end of the GTFS forms a horsetail splay structure, with many faults having normal components, and to the east, GTFS merges with the Bozghush fault zone. GTFS shows a variety of transtension and transpression tectonic structures (stepovers, bendings, pull-apart basins, and splay structures) formed in the dextral shear zone. North Tabriz fault zone is characterized by three main NW striking right-stepping en echelon segments (Bostan Abad, Shebli, and Tabriz fault segments) and is known as the causative fault of three destructive historical earthquakes on 1042/11/04 (Mw 7/6), 1721/04/26 (Mw 7/7), and 1780/01/08 (Mw 7/7). To the east, it joins the Bozghush thrust fault zone that caused the 2019/11/07 (Mw 6/0) earthquake on its Shalgun-Yelimsi left-lateral strike-slip fault segment. In the central part of the GTFS, the Mishu-Tasuj fault zone is formed as a transpressional bend. The macroseismic epicenter of the 1786/10 (Mw 6/2) earthquake is located near this fault zone. Thrust faults in the southern part of the Mishu-Tasuj fault zone are parallel with close distances, and have uplifted the land masses; probably representing the migration of thrust faulting into the southern plains; similar to the Esfarayen and Sabzevar thrust faults in northeastern Iran. Four pull-apart basins have been created due to the movement of fault segments along the Gailatu-Siah Cheshmeh-Khoy fault. The current kinematics of the GTFS plays a key role in the tectonic of northwestern Iran and accommodates part of the convergence movement between the Eurasian and Arabian plates. Earthquake history and geometry of different segments of the GTFS imply seismic gap, especially on the North Tabriz fault, and faults interaction (e.g., between Shalgun-Yelimsi left-lateral strike-slip fault and south Bozghush thrust fault, and Gailatu-Siah Cheshmeh-Khoy right-lateral strike-slip fault and Tasuj thrust fault) which are important issues in seismic hazard in northwestern, especially for Tabriz City with a population of about 1.5 million.

An array of en-echelon strike-slip faults in eastern Iran results in the formation of releasing and restraining bends or stepovers, within which some faults are hidden in the extensional and contractional parts of the structures. This is investigated in the East Neh-Esmaeilabad left- stepping restraining stepover. Transpressional deformation in the transverse structure associated with the stepover is expressed as folding and uplifting in the Late Neogene, Quaternary and recent deposits, revealing the Shusf fault as a hidden and blind thrust. Processing of magnetic anomaly maps shows the existence of the Shusf magnetic lineament, which consistently well correlates with the Shusf fault and its hidden segments. Also the hidden part of the fault was investigated by the differential GPS profiles constructed perpendicular to the trend of the Shusf fault scarp. In this study, we used surface morphology surveys, fan median method and satellite images to calculate the cumulative horizontal and vertical offsets related to the Late Quaternary active tectonics along the Shusf fault, measuring mean values of 92 and 4.25 m, respectively. Analysis of the velocity vector recorded in the Nehbandan geodynamic station across the Shusf fault indicates the present-day evolution of the stepover expressed as uplift and left-lateral displacement.