Paleoseismological and morphological evidence of slip rate variations along the North Tabriz fault (NW Iran)

International audienceNorthwest Iran is characterized by a high level of historical and instrumental seismicity related to the ongoing convergence between the Arabian and Eurasian plates. In this region, the main right-lateral strike-slip fault known as the North Tabriz fault (NTF) forms the central portion of a large crustal fault system called the Tabriz fault system (TFS). The NTF is a major seismic source along which at least three strong and destructive earthquakes have occurred since 858 AD. The two most recent destructive seismic events occurred in 1721 AD and 1780 AD, rupturing the SE and NW fault segments, respectively. This paper reports paleoseismological and quantitative geomorphologic investigations on the SE segment of the NTF, between the cities of Bostanabad and Tabriz. These observations help to improve our understanding of the seismic hazard for Tabriz city and its surrounding areas. Our field investigations revealed evidence of successive faulting events since the Late Quaternary. Paleoseismic investigations indicate that since 33.5 kyr, the SE segment of the NTF has experienced at least three major (M>7.5) seismic events, including the 1721 AD earthquake (M=7.6–7.7). Along the NW segment of the fault, however, our results suggest that the amount of strong (M~7.5) seismic events during the same period is significantly greater than along the SE segment. One possible explanation of such a difference in seismic activity is that the Late Quaternary-Holocene coseismic slip rate is decreasing along the NTF from the northwest to the southeast. This explanation contradicts the former hypothesis of a constant slip rate along the whole length of the NTF. In addition, more distributed deformation along several parallel fault branches, in a wider fault zone of the SE segment of the NTF may be considered as additional evidence for the estimation of lower rate of deformation along the fault segment. Such a slip distribution pattern can explain the existence of smaller (~300 m) Pliocene-Quaternary cumulative dextral offsets along the SE fault segment than the measured cumulative offsets along the NW segment (~800 m) of the NTF

Seismic hazard assessement for Tehran, Tabriz and Zandjan cities (NW Iran) based on morphotectonical and paleoseismological approaches

La faille de Mosha est une des grandes failles actives dans le sud de l'Alborz Central. Cette faille de 190 kilomètres de long et orientée N110 E est décrite comme étant la source de plusieurs tremblements de terre historiques et se caractérise par une signature morphologiques remarquable. Située à seulement 12 km au nord-est de la capitale Téhéran (15 millions d'habitants), la faille de Mosha représente une source potentielle de violent tremblement de terre. Afin de mieux évaluer l'aléa sismique associé à la faille de Mosha, nous avons procédé à une étude morphotectonique et paléoseismologique, en nous concentrant sur l'est et la partie centrale de la faille (entre la vallée de Tar à l'est et la région de Lavasanat à l'ouest), où sont observées des preuves claires d'une activité récente. Le nord-ouest de l'Iran se caractérise par un fort niveau de sismicité historique et une densité de population élevée. Afin de mieux évaluer l'aléa sismique dans le nord-ouest Iran, nous avons concentré nos études sur deux régions (Tabriz et Zandjan). Dans la région de Tabriz, douze tremblements de terre historiques destructeurs ont été enregistrés depuis 858 AD. La plupart d'entre eux ont été associés à la faille Nord Tabriz. Le dernier événement a eu lieu en 1721 AD sur le segment SE de la faille (à l'ouest de Bostanabad) et a affecté fortement la ville de Tabriz (population actuelle de plus de 1.200000). Tout au long de ce segment, nous avons observé dans la morphologie de nombreux témoignages de glissement latéral dextre. Nous avons effectué et exploité quatre tranchées. Les premiers résultats montrent que, depuis 33,5 K années, ce segment de faille a connu au moins quatre grands (M = 7.2-7.4) événements sismiques, y compris le dernier en 1721. Selon les mesure GPS des mouvements interseismiques (Masson et al., 2006; Vernant et al., 2006), long-terme de taux de glissement de la faille nord-Tabriz pourrait atteindre 5-6 mm / an. D'autre part, des études paleoseismologiques (Hessami et al., 2003 et la présente étude) montrent une différence importante concernant le comportement sismique de la faille entre les segments NW et SE . Sur la base des résultats des études mentionnées, le temps de récurrence des grands événements sismiques est plus court sur le segment NW de la faille que sur le segment SE. Cette observation pourrait indiquer que la faille a un taux de glissement qui diminue entre le secteur NW et SE.Selon les catalogues de sismicité historique, la région de Zandjan peut être considérée comme une lacune sismique, même si des magnitudes modérées (M <5.5) ont été enregistrées au cours du siècle dernier. Sur la base de modèles numériques de terrain (MNT), d'images satellites et de photos aériennes ainsi que les observations de terrain, nous avons mis en évidence un important réseau de failles actives. Il est principalement constitué par plusieurs failles décrochantes et inverses. Cette zone de failles peut être subdivisée en trois parties affectant Zandjan (510000 habitants). Basée sur les caractéristiques géométriques (longueur,pendage, segmentation...) et la cinématique, la faille de Zandjan peut produire des séismes modérés à forts et engendrer de graves dommages dans les zones avoisinantes. L'étude de ces failles est donc d'un intérêt majeur pour l'évaluation des risques sismiques dans la province de Zandjan dont la population totale dépasse 1,5 millions d'habitants, mais aussi pour mieux comprendre la géodynamique actuelle du NW de l'IranThe Mosha fault is one of the major active faults in the southern Central Alborz between Longitudes E 50 45' and E 52 30'. This 190 kilometres long, N110E trending fault is described as the source of several strong historical earthquakes, and is characterized by an outstanding morphological signature. Situated for its closest part, just 12 km at the north-eastern vicinity of the Tehran capital (15 million peoples with suburbs areas), the Mosha fault represents a potential source for strong earthquake that would certainly damage the Iranian metropolis. In order to better assess the seismic hazard associated to the Mosha fault, we carried out a morphotectonics and paleoseismological study, that we focused on the eastern and central parts of the fault (between Tar valley in the east and Lavasanat region in the west), where are observed clear evidences of a recent activity. The northwest of Iran is characterized by high level of historical seismicity and population density. To better evaluate the seismic hazard in NW Iran, we focused our studies on two regions (Tabriz and Zandjan). In the Tabriz region, twelve destructive historical earthquakes have been recorded since 858 AD., most of them were associated to the North Tabriz strike slip Fault. On the SE segment of the fault (west of Bostanabad) the last macroseismic event occurred in 1721 AD and hit strongly the Tabriz city (present population more than 1200000). Along this segment, we observed in the morphology many evidences of nearly pure right lateral strike slip movements and we dug four trenches. The first results show that since 33.5 K years this place experienced at least four major (M=7.2-7.4) seismic events including the last one in 1721. According to interseismic measurements (Masson et al., 2006; Vernant et al., 2006), long-term slip rate of the North Tabriz Fault could reach 5-6 mm/y. On the other hand, paleoseismological studies (Hessami et al., 2003 and the present study) show an important difference concerning fault seismic behaviour between NW and SE segments. Based on the results of mentioned studies; the amount of large seismic events along NW segment of the fault is significantly greater than along the SE segment. This observation could indicate that fault slip rate along the Tabriz fault decreases from NW to SE. According to historical seismicity catalogues, the Zandjan region can be considered as a seismic gap even if a few low (M<5.5) magnitude earthquakes have been recorded in the last century. Based on Digital Elevation Models (DEMs), satellite images and aerial photo interpretations together with detailed field investigations, we have discovered a major active fault network. It is mainly constituted by several thrust faults with right lateral slip components trending NW-SE to N-S and it can be divided into three major fault zones affecting Zandjan city (present day population over 510000) and part of the Zandjan-Mianeh basin more to the west. Based on fault geometrical characteristics (fault length,...) and kinematics, Zandjan fault system can produce moderate to large earthquakes and serious damages in surrounding areas. The study of these faults is, then, of major interest for seismic hazard assessment in Zandjan province where total population exceed 1.5 million but also to better understand present day geodynamic of NW IranMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Active-couple indentation in geodynamics of NNW Iran: Evidence from synchronous left- and right-lateral co-linear seismogenic faults in western Alborz and Iranian Azerbaijan domains

International audienceSituated within the central portion of the Arabia-Eurasia collision zone, NNW Iran exhibits an interesting active tectonic context characterized by synchronous left- and right-lateral co-linear seismogenic faults along the adjacent WNW-striking West-Central Alborz (e.g. the Rudbar earthquake fault) and Iranian Azerbaijan (e.g. the North Tabriz seismogenic fault). These structural domains are deforming as a single deformable geomechanical territory between the nearly rigid Central Iran and South Caspian domains to the SSW and NNE, respectively. In this paper, we analyze tectonic interactions of these active structural domains and their influence on the geodynamics of NNW Iran based on morphotectonic and seismological investigations. Indentation tectonics is suggested to play an important role in the geodynamics of this territory. At a plate tectonic scale, the rigid Arabian plate acts as the main indenter which bulldozes the less rigid crustal domains ahead into folded belts (within Zagros and Caucasus to the north) and pushes other blocks aside. In this deformation system, the South Caspian domain acts as a backstop against southern and western deformation zones. The structural domains of Alborz (to the east) and Iranian Azerbaijan Caucasus (to the west) are separated by the NNW-striking Astara-Talesh dextral transpressional zone. Analysis of morphotectonic features and focal mechanisms conducted in this central portion of NNW Iran confirms dextral faulting localized along the NNW-striking deformation zone, which is in agreement with the observed reverse earthquake faulting on west-dipping planes. We also discuss the contraction trajectories derived from kinematics and geometry of active folding and faulting features observed within NNW Iran. Our study highlights two prominent sets of fan-shaped trajectories dominating in the west and east sides of the N–S zone in the Astara-Zanjan direction. We propose a couple-indentation geodynamic model to explain the fan-shaped pattern of these two laterally convergent sets of trajectories within the Talesh-Azerbaijan and Alborz domains