SUMMARY The North Anatolian Fault (NAF) is a dextral strike‐slip fault which runs about 1400 km from east to west, from the Karliova triple junction to the Aegean domain. It was active during the Plio‐Quaternary as a consequence of the collision between the Eurasian and Arabian plates. The NAF intracontinental deformation zone contributes with the sinistral East Anatolian Fault to the westward extrusion of Anatolia as a consequence of the northward drift of Arabia. The Central NAF zone forms a northward convex bend where the eastern NAF N110°E‐trending segment extending from Karliova is connected to the western NAF N75°E‐trending segment running to Aegea. Analysing the slip occurring in major earthquakes around the NAF, we show that the present‐day stress pattern along the NAF agrees with stress magnitude and strike variations from east to west. The stress regime along the NAF is NNW‐trending σHmax (σ1) transpression in eastern NAF; this progressively changes westwards to NW‐trending σ1, transtension in central Anatolia. This lateral variation means a relative decrease of the σHmax magnitude, which evolves progressively to an extensional regime (σ1 vertical) around the westernmost NAF, characterized by a NNE‐trending σhmin (σ3). The drastic change in the tendency of the regional stress state in the North Anatolian block, from compressional to extensional, is effective within the central NAF bend. The stress regime determined by inversion of slip vectors measured on fault planes of various scales confirms the present‐day transtensional regime in the central part of the NAF bend. However, the fault kinematic analysis of Quaternary stress states within the Central NAF indicates that this stress state was not continuous throughout the Quaternary. Indeed, chronologies of fault slip vectors provide evidence for two distinct Quaternary regional strike‐slip stress states within this zone. Both states have consistent NE‐ and NW‐trending σ3 and σ1 axes, respectively, but have significantly different R values. The change in the strike‐slip stress regime probably occurred in the middle Pleistocene. The older mean stress state is characterized by a N142 ± 8°E‐trending σ1, a N52 ± 13°E‐trending σ3 and a mean arithmetic Rm value of 0.75, which indicates that the regional stress regime is transpressional. The younger strike‐slip regime is characterized by a N142°± 14°E σ1 axis, a N52°± 10°E σ3 axis and a mean Rm of 0.24, which indicates the transtensional character for this regime. The low R values of the stress deviators related to the recent stress state reflect normal‐component slips. These temporal and spatial stress changes along the NAF result from the coeval influence of forces due to the Aegean subduction in the west and to the northward drift of Arabia in the east. Over these boundary forces, shear is superimposed along the NAF, which accommodates the Anatolia extrusion. However, the timing of the temporal stress change permits the suggestion that the Quaternary stress regime variation in North Anatolia is mainly due to...

Summary In this study we determine the Plio‐Quaternary to present‐day stress regime acting in the Hatay region located at the northeastern corner of the East Mediterranean region. The modern state of stress is obtained from inversion of focal mechanism solutions of shallow earthquakes. This inversion identifies a dominantly extensional stress regime with a NE‐trending σHmin (σ3) axis at the present‐day. The stress regime determined from inversion of slip‐vectors measured on fault planes confirms that this regime is extensional in the studied area. Both the kinematics and chronologies of fault slip‐vectors show that the stress state changed from an earlier strike‐slip regime to a younger extensional stress regime with a consistent NE‐trending σHmin(σ3) axis. The change from strike‐slip to extensional stress regimes probably occurred during the Quaternary. Regionally, both stress regimes induce sinistral displacement on the East Anatolian Fault and Dead Sea Fault systems. The North Anatolian Fault (NAF) is a dextral strike‐slip fault which runs about 1400 km from east to west and has been active since collision between the Eurasian and Arabian plates. Together with the sinistral East Anatolian fault, the NAF intracontinental deformation zone contributes to the westward extrusion of Anatolia as a consequence of northward drift of Arabia. Consequently, the Late Cenozoic stress regimes acting in the Hatay region result from the coeval influence of forces due to: (1) the subduction processes in the west and southwest; (2) the continental collision in the east, and (3) the westward escape of the Anatolian Block. However, the timing of the temporal stress transition suggests that the Quaternary stress regime change resulted from subduction processes with the extensional stress regime in the Hatay region being mainly attributable to roll‐back of the Mediterranean subducted slab along the Cyprus Arc.

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In this study we determined the stress regime acting along the East Anatolian Fault Zone between Turkoglu (Kahramanmaras) and Celikhan (Adiyaman), from the Neocene to present-day, based on the inversion of striations measured on faults and on the focal mechanisms of earthquakes having magnitudes greater than 5.0. The inversions yield a strike-slip stress regime with a reverse component (i.e., transpression) operative in the Neocene to present with a consistent N-to NW-trending σ 1 axis 156 ± 11• and an E-to NE-trending σ 3 axis, 67 ± 9• σ 3 , producing left-lateral motion along the East Anatolian Fault Zone. The inversions of focal mechanisms yield a strike-slip stress deviator characterized by an approximately N-S (N1• W)-trending σ 1 and an approximately E-W (N89• E)-trending σ 3 axis. Both the kinematic analysis and structural observations indicate that the stress regime operating in the study area has had a transpressional character, giving rise to the Mio-Pliocene compressive structures (reverse faults, thrusts and folds) observed in the study area. Field observations allow estimation of a Pliocene age for the strike-slip East Anatolian Fault Zone.

In the eastern Mediterranean, plate motions occur between the Arabia/Anatolia, Africa/Arabia and Anatolia/Africa boundaries along the Amanos Fault, the Dead Sea Fault and the Cyprus Arc, and the extension of the latter on land. Detailed enhancement and classification procedures applied to SPOT XS imagery of the Hatay region, and centred on the Quaternary Amik Basin, have revealed a prominent NE-SW-trending tectonic lineament. Recent seismicity suggests that this NE-SW-trending lineament dies out in the Quaternary Amik Basin in the east and continues to the Cyprus Arc to the south-west. Structural lineament extensions derived from SPOT XS imagery using linear edge enhancement and unsupervised classification methods and the distribution of seismicity in the Hatay region show that this region has been affected by the East Anatolian Fault Zone, the Dead Sea Fault Zone and the Cyprus-Antakya Transform fault. These fault associations accommodate northerly movement of the African and Arabian plates toward the Eurasian plate. The Amik Basin appears to have been formed by interaction among the Amanos Fault interpreted here as a continuation of the East Anatolian Fault the left-lateral Dead Sea Fault, and the left-lateral Cyprus-Antakya transform fault. Active faults belonging to the aforementioned structural zones meet one another to form a triple junction at the Amik Basin near Antakya.

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Antakya city is at risk because of strong earthquakes occurring in the area, and different soil conditions that can produce variation of the ground motion amplification. Microzonation of cities provides a basis for site-specific hazard analysis in urban settlements. In particular, seismic microzonation can be provided by means of detailed seismic assessment of the area, including earthquake recordings and geological studies. In this paper, we propose a preliminary microzonation map for the city of Antakya, based on the variation of the dominant periods and shear velocities of the sediments covering the area. The periods are retrieved from microtremor measurements conducted at 69 sites, using the horizontal-to-vertical spectral ratio technique. The results of microtremor analysis were compared with data obtained from refraction microtremor (ReMi) measurements at four profiles crossing the studied area. According to the classification of dominant periods, Antakya city can be divided into five zones, probably prone to different levels of seismic hazard. The shorter natural periods are in inner Antakya and both the sides of Asi River (i.e., northern and southern parts). The eastern and western parts of Antakya have maximum dominant periods. The V s 30 values were calculated by using the ReMi method along the profiles. Antakya city has V s 30 values in the range of category C of the national earthquake hazard reduction programme site classification.

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