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SUMMARYThe 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gökova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gökova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.

We computed a 3-D shear wave velocity model of the Marmara Sea region from ambient noise tomography. The correlations of up to 8 yr of vertical-component seismic recordings from 80 broad-band stations provided Rayleigh wave group velocity measurements in the period band 6–21 s at more than 1400 selected virtual source–receiver pairs. Rayleigh wave group velocity maps were used to derive a shear wave velocity model through simulated annealing inversion. The resulting crustal model provides coverage of the Marmara Sea along with its surrounding regional tectonic features. This allows for an investigation of the spatial extents of the Marmara Sea on a scale larger than that of basins. The low-velocity structures of the Marmara Sea and the Thrace Basins are coeval to a depth of approximately 9 km. The crustal velocities beneath the Marmara Sea basins exhibit a low vertical gradient and smooth horizontal variations. The regional tectonic structures, such as Istranca Massif, Istanbul and Sakarya Zones, display sharp velocity contrasts with the lower velocity crust beneath the Marmara Sea. The observed low crustal velocities, along with depth variations of the velocity isosurfaces (i.e. 3.4 km s−1) indicate that the Marmara region is a structural depression much deeper and larger than the three basins of the North Marmara Trough. The North Anatolian Fault Zone is unlikely to be the primary factor contributing to the origin of this significant depression, as the basin's development appears to have occurred before the fault propagated into the region. [ABSTRACT FROM AUTHOR]

Abstract: We document the establishment of the Aegea-Anatolia/Eurasia plate boundary in Pliocene-Pleistocene time. Before 2 Ma,nolocalized plate boundary existed north of the Aegean portion of the Anatolia plate and the [...]

The North Anatolian Fault is a 1200 km long strike-slip fault system connecting the East Anatolian convergent area with the Hellenic subduction zone and, as such, represents an intracontinental transform fault. It began forming some 13-11 Ma ago within a keirogen, called the North Anatolian Shear Zone, which becomes wider from east to west. Its width is maximum at the latitude of the Sea of Marmara, where it is 100 km. The Marmara Basin is unique in containing part of an active strike-slip fault system in a submarine environment in which there has been active sedimentation in a Paratethyan context where stratigraphic resolution is higher than elsewhere in the Mediterranean. It is also surrounded by a long-civilised rim where historical records reach well into the second half of the first millennium BCE (before common era). In this study, we have used 210 multichannel seismic reflexion profiles, adding up to 6210 km profile length and high-resolution bathymetry and chirp profiles reported in the literature to map all the faults that are younger than the Oligocene. Within these faults, we have distinguished those that cut the surface and those that do not. Among the ones that do not cut the surface, we have further created a timetable of fault generation based on seismic sequence recognition. The results are surprising in that faults of all orientations contain subsets that are active and others that are inactive. This suggests that as the shear zone evolves, faults of all orientations become activated and deactivated in a manner that now seems almost haphazard, but a tendency is noticed to confine the overall movement to a zone that becomes narrower with time since the inception of the shear zone, i.e., the whole keirogen, at its full width. In basins, basin margins move outward with time, whereas highs maintain their faults free of sediment cover, making their dating difficult, but small perched basins on top of them in places make relative dating possible. In addition, these basins permit comparison of geological history of the highs with those of the neighbouring basins. The two westerly deeps within the Sea of Marmara seem inherited structures from the earlier Rhodope-Pontide fragment/Sakarya continent collision, but were much accentuated by the rise of the intervening highs during the shear evolution. When it is assumed that below 10 km depth the faults that now constitute the Marmara fault family might have widths approaching 4 km, the resulting picture resembles a large version of an amphibolite-grade shear zone fabric, an inference in agreement with the scale-independent structure of shear zones. We think that the North Anatolian Fault at depth has such a fabric not only on a meso, but also on a macro scale. Detection of such broad, vertical shear zones in Precambrian terrains may be one way to get a handle on relative plate motion directions during those remote times. La faille nord-anatolienne est constituee d'un systeme de failles decrochantes d'une longueur de 1200 km qui relie le secteur convergent anatolien a la zone de subduction hellenique elle represente ainsi, une faille transformante intracontinentale. Elle a commence a se former il y a environ 13 a 11 Ma dans une ceinture deformee dominee par des deplacements de decrochement (<< keirogene >>), nomme la zone de cisaillement nord-anatolienne; elle s'elargit d'est en ouest. Elle atteint sa largeur maximale de 100 km a la latitude de la mer de Marmara. Le bassin de Marmara est unique en ce qu'il contient une partie d'un systeme actif de failles de decrochement dans un environnement sous-marin ou ilya eu de la sedimentation active dans un contexte de mer Paratethys et ou la resolution stratigraphique est plus elevee qu'ailleurs en Mediterranee. La faille est aussi entouree par une bordure ou, grace aux civilisations de longue date, il existe des documents historiques datant de la deuxieme moitie du premier millenaire avant notre ere. Dans la presente etude, nous avons utilise les donnees obtenues par 210 profils de reflexion sismique multicanale, ce qui donne un total de 6210 km de profils; nous avons aussi utilise de la bathymetrie a haute resolution et des profils de fluctuation de longueur d'onde disponibles dans la litterature pour cartographier toutes les failles plus recentes que l'Oligocene. Pour ces failles, nous distinguons celles qui recoupent la surface et celles qui ne la recoupent pas. Pour ces dernieres, nous avons cree un tableau temporel de generation de failles basee sur la reconnaissance de la sequence sismique. Les resultats sont surprenants; en effet, peu importe l'orientation des failles, elles contiennent toutes des sousensembles actifs et des sous-ensembles non actifs. Cela suggere qu'a mesure de revolution de la zone de cisaillement, les failles de toutes directions sont activees et desactivees d'une maniere qui semble presque aleatoire. Cependant, nous notons une tendance a restreindre le mouvement general a une zone qui se retrecit avec le temps depuis de debut de la zone de cisaillement, c.-a-d. tout le<>, a sa pleine largeur. Dans les bassins, avec le temps, les bordures se deplacent vers l'exterieur alors que les zones surelevees gardent leurs failles libres de couverture sedimentaire, ce qui complique la datation. Toutefois, de petits bassins a leur sommet permettent d'obtenir une datation relative. De plus, ces bassins permettent de comparer l'histoire geologique des zones surelevees a celle des bassins avoisinants. Deux creux, situes a l'ouest dans la mer de Marmara, semblent etre des structures heritees de la collision entre le fragment Rhodope-Pontide et le continent Sakarya. Cependant, ils ont ete grandement accentues par le soulevement des zones surelevees durant Devolution du cisaillement. Lorsque nous partons de l'hypothese qu'a une profondeur superieure a 10 km, les failles qui forment actuellement la famille de failles de Marmara pourraient avoir des largeurs approchant les 4 km, l'image qui en ressort ressemble a une version agrandie d'une fabrique de zone de failles au facies des amphibolites. Cette inference concorde avec une structure de zones de cisaillement, independante de l'echelle. Nous croyons que la faille nord-anatolienne constitue un systeme qui a une telle fabrique en profondeur, non seulement a echelle moyenne mais aussi a grande echelle. La detection de telles larges zones verticales de cisaillement dans des terrains precambriens pourrait etre une maniere de comprendre les directions relatives des plaques a ces temps anciens. [Traduction par la Redaction], In pietate memoriam J. Tuzo Wilson Introduction In 1965, J. Tuzo Wilson defined a new kind of fault that could not be accounted for by Anderson's theory of faulting in [...]

The East Anatolian Fault (EAF) is a left‐lateral transform fault accommodating the relative motion between the Anatolian and Arabian plates. On January 24, 2020, Mw6.8 Sivrice (Elazığ) earthquake is the largest event that occurred along the EAF since the nineteenth century. The earthquake provides a unique opportunity to capture a critical stage of the seismic cycle from the interseismic deformation to co‐seismic rupture. In this study, we examine the relationship between the interseismic fault activity and co‐seismic behavior of the earthquake. A kinematic model of the earthquake obtained from strong‐motion, GNSS and teleseismic waveforms along with static displacements from GNSS and InSAR data shows that the mainshock ruptured only 45 km of the 95 km long Sivrice‐Pütürge segment. Rupture initiated adjacent to the interseismically weakly coupled northeastern section and propagated unilaterally toward southwest with a rupture velocity of ∼2.5 km/s, stopping ∼30 km before the southwestern segment boundary. The earthquake did not generate any surface offsets. We identified 4 long‐term near‐repeating earthquake clusters beneath the highest co‐seismic slip zone adjacent to the northeastern creeping section. The mainshock was dynamically triggered by a M ∼ 5.4 foreshock located within the zone of near‐repeating earthquakes. We suggest that creep along the weakly coupled section of the fault loaded the neighboring locked section leading to the repeating earthquakes below the locked zone. The earthquake partially ruptured a fault segment characterized by high rate of diffuse seismicity, structural complexities and heterogeneous coupling, leading to a source characterized by a complex source time function with relatively slow rupture velocity. Plain Language Summary: Understanding the magnitude, location and slip distribution of large earthquakes is a crucial problem in earthquake studies. In this study, we analyze the rupture process of the 2020 Mw6.8 Elazığ‐Sivrice earthquake that occurred along the East Anatolian fault, which is a strike‐slip fault in eastern Turkey. We then relate the earthquake to the fault behavior in the time period before the earthquake. Our results show that the earthquake ruptured only part of a fault segment, generating two patches of slip along a fault section with geometrical complexities. The earthquake did not generate any significant surface slip. The rupture pattern is consistent with previously identified heterogeneous coupling with patches of creeping, partially locked and locked sections. The seismicity of this segment is also different from the neighboring segments with more distributed and higher rate of earthquake occurrence. We also identified near‐repeating earthquakes along the transition from the creeping section and locked section right beneath the highest slip patch of the 2020 earthquake. The 2020 earthquake reflects this heterogeneity of the long term behavior, with relatively slow rupture velocity, patchy slip distribution along a geometrically complex fault segment. Key Points: 2020 earthquake partially ruptured a segment with heterogeneous coupling and high rate of seismicity without any surface slipThe asperities were loaded by interseismically creeping sections. Near‐repeater clusters are detected below the 2020 rupture zoneThe duration and complexity of the source time function reflects the complexity of the rupture process [ABSTRACT FROM AUTHOR]

The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behaviour along its 150 km length. In 2019 September, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes ≥  M w 3.5 in a week. The M w 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 M w 6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an M w 4.7 foreshock within ∼2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the M w 5.8 earthquake did not occur on the MMF, but it ruptured ∼60° north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the main shock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10–13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the M w 5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading. [ABSTRACT FROM AUTHOR]

The 2017 July 20, M w6.6 Bodrum–Kos earthquake occurred in the Gulf of Gökova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gökova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean. [ABSTRACT FROM AUTHOR]