Deep Lithospheric Controls on Surface Deformation and Seismicity around the East Anatolian Fault Zone and A3 Triple Junction

The structural relationship between the East Anatolian and Dead Sea fault zones in southeastern Turkey

The East Anatolian Fault Zone (EAFZ) is a major tectonic feature in Anatolia, moving westward relative to the Eurasian plate, primarily due to the compressional behavior of the African, Sinai, and Arabian plates. The EAFZ has been historically significant as the epicenter of several major seismic events, including the recent devastating earthquakes in Kahramanmaraş on February 06, 2023. In this study, we provide a comprehensive tectonic analysis of the EAFZ, focusing specifically on the Hatay Triple Junction (HTJ), where the EAFZ meets with the continuation of the Dead Sea Fault Zone and the Cyprus Arc. Through the synthesis of pre-seismic evaluations and co-seismic data derived from our comprehensive geodetic network, we aim to extend the understanding of the seismic cycle and fault behavior in this tectonically complex region. In the pre-seismic phase, we employed a dense GNSS network, including the Turkish National Fundamental GPS Network (TUTGA) and various campaign sites, to analyze the strain accumulation and fault kinematics at the HTJ. Analyses revealed that the EAFZ and Karataş-Osmaniye fault exhibit complete locking at depths of 15 km and 7 km, respectively, while the Karasu Fault (KF) demonstrates locking up to a depth of 7 km. Our kinematic models indicated significant slip rates, suggesting a high potential for large earthquakes. Remarkably, during the revision of our manuscript in “Tectonophysics”, two major earthquakes (Mw 7.7 and Mw 7.6) occurred near the KF and EAFZ in Pazarcık and Elbistan, Kahramanmaraş, respectively. These earthquakes, happening in close succession, dramatically corroborated our predictions for seismic activity near the HTJ.  In the co-seismic phase, following the Kahramanmaraş earthquakes, we utilized a high-resolution GNSS network with 73 permanent and 40 campaign stations to assess surface displacements and fault slip distributions. Significant displacements were recorded, with the largest being 466 cm, and substantial fault ruptures were observed along the EAFZ and Çardak fault segment, including the left-lateral slips of 494 cm and 391 cm in the first and second earthquakes, respectively.   Our ongoing projects will integrate pre-earthquakes geodetic data with new GNSS measurements to analyze the postseismic deformation following the Kahramanmaraş earthquakes, including afterslip and its relation with aftershocks, as well as stress perturbations on the neighboring faults. This integrated approach is expected to further refine our ability to understand the behavior of the fault in the post-seismic phase and delineate the complete seismic cycle along the EAFZ.    

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

Lateral movement of lithospheric fragments along strike-slip faults in response to collision (escape tectonics) has characterized convergent settings since the onset of plate tectonics and is a mechanism for the formation of new plates. The Anatolian plate was created by the sequential connection of strike-slip faults following ≥10 m.y. of distributed deformation that ultimately localized into plate-bounding faults. Thermochronology data and seismic images of lithosphere structure near the East Anatolian fault zone (EAFZ) provide insights into the development of the new plate and escape system. Low-temperature thermochronology ages of rocks in and near the EAFZ are significantly younger than in other fault zones in the region, e.g., apatite (U-Th)/He: 11–1 Ma versus 27–13 Ma. Young apatite (U-Th)/He ages and thermal history modeling record thermal resetting along the EAFZ over the past ~5 m.y. and are interpreted to indicate thermal activity triggered by strike-slip faulting in the EAFZ as it formed as a through-going, lithosphere-scale structure. The mechanism for EAFZ formation may be discerned from S-wave velocity images from the Continental Dynamics–Central Anatolian Tectonics (CD-CAT) seismic experiment. These images indicate that thin but strong Arabian lithospheric mantle extends ~50–150 km north beneath Anatolian crust and would have been located near the present surficial location of the Bitlis-Zagros suture zone (co-located with the EAFZ in our study area) at ca. 5 Ma. Underthrusting of strong Arabian lithosphere facilitated localization of the EAFZ and thus was a fundamental control on the formation of the Anatolian plate and escape system.

The North and East Anatolian Fault Zones represent plate-bounding transform faults that enable the westward tectonic escape of the Anatolian Plate away from the Arabian-Eurasian collisional zone. These fault zones are both capable of hosting large (Mw > 7) seismic events, as most recently demonstrated by the extremely damaging February 2023 Kahramanmaraş earthquake sequence. This earthquake sequence highlighted that plate boundary forces in this area are distributed over a very broad region, however what controls the location, distribution, and character of this plate-bounding strike-slip system remains enigmatic. To better understand potential contributions to deformation, we compare seismic images of the lithosphere (e.g., crustal and lithospheric mantle thickness and velocity) to deformational features and seismicity near the EAFZ, as well as further west where it joins with the Anatolia-Arabia-Africa (A3) triple junction along the southeastern margin of the Anatolian escape system. We interpret that although controls on surface deformation are commonly linked to stress in the brittle upper crust, the complex deformation and seismicity patterns in this region are likely related to variations in the location and extent of the strong lithospheric mantle of the Arabian plate, which currently underthrusts Anatolia as far north as the Sürgü-Çardak fault zone (~50 km). In addition, the Arabian lithospheric mantle extends at least as far west as at least the central Adana Basin, coincident with a zone of relatively deep (>30 km) strike-slip seismogenesis that has produced Mw > 6 earthquakes. By investigating the relationship between recent geological deformation since the inception of the East Anatolian Fault (ca. 5 Ma) and the modern record of seismic structure and seismicity, we infer that the Sürgü-Çardak fault zone and its associated near-orthogonal bend reaching into the Adana Basin will be the future southeastern boundary of the Anatolian Plate escape tectonic system.

An earthquake of MW = 6.1 occurred in the Elazig region of eastern Turkey on 8 March 2010 at 02:32:34 UTC. The United States Geological Survey (USGS) reported the epicenter of the earthquake as 38.873°N-39.981°E with a focal depth of 12 km. Forty-two people lost their lives and 137 were injured during the event. The earthquake was reported to be on the left-lateral strike-slip east Anatolian fault (EAF), which is one of the two major active fault systems in Turkey. Teams from the Earthquake Engineering Research Center of the Middle East Technical University (EERC-METU) visited the earthquake area in the aftermath of the mainshock. Their reconnaissance observations were combined with interpretations of recorded ground motions for completeness. This article summarizes observations on building and ground damage in the area and provides a discussion of the recorded motions. No significant observations in terms of geotechnical engineering were made. The major tectonic structure in Turkey is the north Anatolian fault zone (NAFZ), with right-lateral faulting extending from Istanbul in the west to Karliova in the east. During the twentieth century this fault zone has produced several large earthquakes ( MS > 7) with surface rupturing with a westward migrating sequence as demonstrated in Figure 1 (Barka 1996; Utkucu et al. 2003). Around the Karliova region, NAFZ joins the southwest-trending east Anatolian fault zone (EAFZ). The EAFZ is predominantly left-lateral strike-slip in nature, but its faulting is less continuous and less localized than that of the NAFZ (Ambraseys 2009). The EAFZ has nucleated relatively small magnitude earthquakes in the twentieth century (Figure 1). Recent GPS data indicates that the slip rate in the EAFZ has an upper bound of 8±1 mm/year (Ambraseys 2009). The epicenter (by USGS) of the 8 March 2010 Elazig-Kovancilar earthquake is in the segmented fault region …

The southwestern continuation of the East Anatolian Fault Zone (EAFZ), specifically its relationship with Iskenderun Basin, the Dead Sea Fault Zone (DSFZ) and Cyprus Slap is still enigmatic. In 2023, nearby splays of EAFZ in the southwest are ruptured by two large earthquakes that are nine hours apart. At first, Pazarcık earthquake (M7.8) initiated at a secondary fault, later jump to the main strand of EAFZ and propagated bilaterally producing a surface rupture exceeding 315 km in length. The Ekinözü earthquake (M7.7) triggered nine hours later also displayed bilateral rupture propagation and produced a 140 km long surface rupture. Surface deformations associated to both events that ruptured multiple fault segments with left-lateral strike-slip mechanism, are mapped in detail using satellite images and field observations. Surface offsets of both events are highly variable, reaches up to 8 m, and controlled mainly by subsurface slip. The accuracy of mapped active faults prior to the doublet, reduce significantly along plains where distributed deformations are common and occasionally surface rupture follows mapped inactive faults suggesting reactivation of old faults or unrecognized active faulting in the area.Large 19th century earthquakes previously associated to the faults ruptured during this doublet, are likely mislocated and these segments were accumulating stress at least for 500 years. Earthquake mechanisms recorded before and after the doublet revealed strike-slip regime corresponding well with EAFZ but towards south, extensional events become abundant. Based on the computed stress field, east-west striking Çardak fault ruptured during the second event, is not optimally oriented for left-lateral failure but suffered from noticeable static stress increase and rate-and-state friction based simulations including both static and dynamic effects suggested that it was at the end of its seismic cycle. Static stress changes resulted from the doublet also indicate pronounced increases, especially along Malatya, Savrun, Türkoğlu and Antakya fault segments which are remained as seismic gaps.GPS based slip models along multiple profiles constrained left-lateral slip rates of ruptured faults and suggested an increase in slip rate from south to north across EAFZ. During Pazarcık earthquake, rupture made a sharp bend towards south rather than following parallel fault segments towards Adana which are previously proposed as the western continuation of EAFZ. Our field observations indicated a fault traversing the Amanos mountains parallel to EAFZ along which fault kinematics and compiled GPS data together suggest left-lateral motions. Based on these findings, alternative regional kinematic models assuming Iskenderun and Maras blocks as independent or intact are established and later utilized for probabilistic seismic hazard analysis throughout the Adana basin by considering variable site conditions and basin effect in long spectral periods.

On 6 February 2023, the MW 7.8 Pazarcik and the MW 7.5 Elbistan earthquakes occurred in southeastern Turkey, close to the Syrian border, causing many deaths and a great deal of property destruction. The Pazarcik earthquake mainly damaged the East Anatolian Fault Zone (EAFZ). The Elbistan earthquake mainly damaged the Cardak fault (CF) and the Doğanşehir fault (DF). In this study, Sentinel-1A ascending (ASC) and descending (DES) orbit image data and pixel offset tracking (POT) were used to derive surface deformation fields in the range and azimuth directions induced by the Pazarcik and Elbistan earthquakes (hereinafter referred to as the Turkey double earthquakes). Utilizing GPS coordinate sequence data, we computed the three-dimensional surface deformation resulting from the Turkey double earthquakes. The surface deformation InSAR and GPS results were combined to invert the coseismic slip distribution of the EAFZ, CF, and DF using a layered earth model. The results show that the coseismic ruptures of the Turkey double earthquakes were dominated by left-lateral strike-slips. The maximum slip was 7.76 m on the EAFZ and about 8.2 m on the CF. Both the earthquakes ruptured the surface. The Coulomb failure stress (CFS) was computed based on the fault slip distribution and the geometric parameters of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake’s epicenter. The CFS change resulting from the Pazarcik earthquake suggests that the subsequent Elbistan earthquake was triggered by the Pazarcik earthquake. The Antakya fault experienced an increase in CFS of 8.4 bars during this double-earthquake event. Therefore, the MW 6.3 Uzunbağ earthquake on 20 February 2023 was jointly influenced by the Turkey double earthquakes. Through stress analysis of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake’s epicenter, the Ecemis segment, Camliyayla fault, Aadag fault, Ayvali fault, and Pula segment were all found to be under stress loading. Particularly, the Ayvali fault and Pula segment exhibited conspicuous stress loading, signaling a higher risk of future seismic activity.

The powerful earthquake that struck eastern Türkiye on February 6th 2023 is the most devastating earthquake of the past century in the region. Here we present our first-hand field measurements of the ground offsets and the high resolution (centimeter level) drone-mapped surface ruptures 10 days after the first shock. It is clear that the initial rupture was on the Dead Sea fault zone (DSFZ), yet maximum displacements and energy release (Mw 7.8) occurred 24 sec later when rupture transferred to the East Anatolian fault zone (EAFZ). Seven hours later, a Mw 4.5 aftershock at the junction of the EAFZ with the east-west striking Çardak-Sürgü fault (Ç-SF) triggered the second large (Mw 7.5) earthquake, causing another round of the damage in the region. The maximum ground offsets are around 47.5 kilometers away from the epicenter in this event on the EAFZ. The surface ruptures directly cut young basins and mountains, as well as activating some pre-existing surfaces. Our observation provides important data on surface deformation during large continental strike-slip earthquakes, rupture propagation mechanisms, and how slip may be transferred between complex fault systems. We also provide insight into how slip along linked fault systems accommodates global plate motions.

On 6 February 2023, an earthquake with magnitude M w c . 7.0 on the Narlı Fault, a fault sub-parallel to the East Anatolian Fault Zone (EAFZ), initiated a chain of large earthquakes on the EAFZ. The earthquakes occurred in a seismic gap with low geodetic strain rates and low background seismicity, where deformation is distributed across a wide fault zone and a long recurrence time of historical earthquakes. The c. 50 km long rupture of the Narlı Fault towards Pazarcık led to an M w 7.8 left-lateral strike-slip earthquake breaking a c. 300 km section of the c. 600 km long EAFZ bilaterally with a total duration of more than 80 s. Toward the SW, the rupture propagated on the c. 100 km long Amanos segment with a peak surface offset of 5 m, before diminishing toward the Hatay graben. In the NE direction, the rupture reached a peak surface offset of 7 m before sharply declining at the termination of the 2020 M w 6.8 Sivrice earthquake rupture. A second large earthquake with M w 7.6 occurred 9 h later on the Çardak Fault, located at the western margin of (and sub-parallel to) the EAFZ and breaking the surface with almost 9 m of left-lateral slip (average of c. 4 m). Following these large earthquakes, the increase in the regional stress led to a rapid seismic activation in a broad region from central to eastern Anatolia, loading the faults at various scales and increasing seismic hazard. Two weeks after the initiation of the seismic crisis, a third earthquake with M w 6.4 occurred at the southern boundary of the Hatay graben, near the southwestern termination of the Amanos rupture. The earthquakes caused significant loss of human life, devastating 12 cities. We evaluate the observations prior to the ruptures, and present preliminary seismological results with surface displacements from sub-pixel correlation of optical satellite images and the stress perturbations computed on the nearby faults based on preliminary slip models. The re-evaluation of the seismic potential in light of the recent and historical earthquakes provides some new insight on seismic hazard assessment. The recent series of events on the EAFZ is an important reminder that large faults can generate very large earthquakes on multiple segments. The seismic potential of large earthquakes on these fault zones can be estimated only by considering multiple seismic cycles and moment deficits from very large earthquakes. Supplementary material : Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6567094

Paleoseismology of the Palu–Lake Hazar segment of the East Anatolian Fault Zone, Turkey

Evolution of the Gölbaşı basin and its implications for the long-term offset on the East Anatolian Fault Zone, Turkey

Failures of masonry dwelling triggered by East Anatolian Fault earthquakes in Turkey

An evaluation of recent seismicity behaviors in the East Anatolian Fault Zone, eastern part of Turkey, is made by using Gutenberg-Richter b-value and fractal dimension Dc-value. Reasenberg’s algorithm is used in order to separate the dependent events from independent ones. The completeness magnitude in study region is calculated as 2.8. b-value is calculated as 1.02 /-0.01 by maximum likelihood estimation and matches the Gutenberg-Richter law with a b-value typically close to 1. Dc-value is found as 2.34 /-0.04 with 95% confidence limit by linear regression and this large value suggest that seismicity is more clustered at larger scales in the East Anatolian Fault Zone. The smallest b-values and the highest Dc-values are found in the same areas covering the East Anatolian Fault Zone, Askale Fault, Sancak-Uzunpinar Fault Zone, Pulumur Fault, Goynuk Fault Zone and Genc Fault, in and around Karatas-Osmaniye Fault Zone and in the southeast part of the study region. Thus, low b-value in these regions may be an indication of low degree of heterogeneity, high-strain due to the active tectonics and stress to build up over time and to be released by events. Thus, special attention should be given to these regions where low b-value and large Dc-value are observed.

One of Turkey's most important neotectonic structures East Anatolian Fault Zone (EAFZ), has occurred many earthquakes. One of these earthquakes, the 6.8 Mw Sivrice-Elazig earthquake dated January 24, 2020, was felt in various provinces, especially in Elazig and Malatya, and caused the death of 44 people. It is critical to investigate this earthquake, which caused significant economic damage, and to identify possible hazards on the EAFZ. One of the remote sensing methods DInSAR was used in this study. By choosing two Sentinel 1A (Single Look Complex) descending datasets, dated 16/01/2020/01 and 28/01/2020/01 respectively (pre and post earthquake), the surface deformation and time series were determined. In addition, using the data obtained from the DInSAR results, Elastic Dislocation Modelling has been performed by applying linear and nonlinear inverse solutions to determine the slip amount of the fault structure, the fault surface slip distribution, and determine the strain area. According to the DInSAR results, while there is an offset of approximately 26 cm (away from the satellite direction) on the left block of the EAF, 19 cm offset (towards the satellite direction) are observed in the right block, respectively. Elastic Dislocation Modelling shows that the observed deformation pattern can be explained by the slip on a single plane fault of the Elazig earthquake. This fault plane was identified as a southwest strike-slip fault segment, which lies within the upper crustal region and extends to a depth of approximately 10 km. According to the results obtained by elastic modelling; slip amount (slip) was calculated as 1.95 m, Mw 6.75, rupture length 34.78 km, focal depth 10 km, width 7.4 km, strike 240.27°, slope 69.19°, rake 0.19°. Overall, the study reveals the strike-slip of the Sivrice-Elazığ earthquake, shows the deformation after the earthquake, and the elastic half-space fault model.