Search

SUMMARY The East Anatolian Fault (EAF) is a 700-km-long left-lateral transform fault located between the Anatolian and Arabian plates. The proximity of the Euler Pole to the Arabia–Anatolia Plate boundary leads to rapid changes in plate velocity along the boundary, which is manifested by the decreasing slip rates from east (10 mm yr–1) to west (∼1–4 mm yr–1). The EAF displays heterogeneous seismicity patterns with seismic gaps, localized clusters and broad diffuse zones. In this study, in order to understand the origin of these complexities and quantify the seismic hazard along the EAF, we present an improved seismicity catalogue with more than 26 000 earthquakes and 160 focal mechanisms from regional moment tensor inversion between 2007 and 2020. The focal mechanisms and seismicity show that the EAF dips towards north and forms a well-defined plate boundary in the east between Palu and Çelikhan with almost pure left-lateral motion. Further west, the boundary becomes broader with activity along subparallel faults. Focal mechanisms show heterogeneous stress orientations in consistence with geodetically determined strain rate field. The stress orientations show a transition from strike-slip to extension towards the west of Çelikhan. Amongst all segments of EAF, the Pütürge segment, which holds the near-repeating earthquakes in the vicinity of the nucleation of the 2020 Mw 6.8 earthquake, is distinguished with its steady and high rate of seismicity. Further east, the neighbouring Palu segment is characterized by several distinct moderate earthquakes. We do not observe any change in the seismicity rate on these segments of the EAF following large earthquakes. In order to quantify the seismic hazard along the EAF, we calculate the recurrence time and maximum magnitude for each segment by using an extended seismicity catalogue of 150 yr including the large historical earthquakes and the geodetic strain rate. The results show ∼150 yr recurrence time with Mmax∼6.7–7.0 along the seismically active Palu and Pütürge segments on the east, while relatively silent western segments yield longer recurrence times; 237–772 for Pazarcık and 414–917 for Amanos segments with slightly larger magnitudes (Mmax ∼7–7.4). We infer that the seismicity patterns and strain-rate field along the EAF are shaped by several factors such as strong geometrical irregularities, heterogeneous coupling and complex plate motion leading to rapid change of fault slip rate.

SUMMARYIn spite of growing evidence that many earthquakes are preceded by increased seismic activity, the nature of this activity is still poorly understood. Is it the result of a mostly random process related to the natural tendency of seismic events to cluster in time and space, in which case there is little hope to ever predict earthquakes? Or is it the sign that a physical process that will lead to the impending rupture has begun, in which case we should attempt to identify this process. With this aim we take a further look at the nucleation of two of the best recorded and documented strike-slip earthquakes to date, the 1999 Izmit and Düzce earthquakes which ruptured the North Anatolian Fault over ∼200 km. We show the existence of a remarkable mechanical logic linking together nucleation characteristics, stress loading, fault geometry and rupture speed. In both earthquakes the observations point to slow aseismic slip occurring near the ductile-to-brittle transition zone as the motor of their nucleation.

SUMMARYThe 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 ≥ Mw 3.5 in a week. The Mw 5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 Mw 6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an Mw 4.7 foreshock within ∼2 d. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the Mw 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 Mw 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.

Author(s): Konca, A Ozgun; Guvercin, Sezim Ezgi; Ozarpaci, Seda; Ozdemir, Alpay; Funning, Gareth J; Dogan, Ugur; Ergintav, Semih; Floyd, Michael; Karabulut, Hayrullah; Reilinger, Robert | Abstract: SUMMARY The 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gokova 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 Gokova. 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.

The seismicity along the Main Marmara Fault (MMF) below the Marmara Sea is analyzed during the 2007–2012 period to provide insights on the recent evolution of this important regional seismic gap. High precision locations show that seismicity is strongly varying along strike and depth providing fine details of the fault behavior that are inaccessible from geodetic observations. The activity strongly clusters at the regions of transition between basins. The Central basin shows significant seismicity located below the shallow locking depth inferred from GPS measurements. Its b-value is low and the average seismic slip is high. All observations are consistent with a deep creep of this segment. On the contrary, the Kumburgaz basin at the center of the fault shows sparse seismicity with the hallmarks of a locked segment. In the eastern Marmara Sea, the seismicity distribution along the Princes Island segment in the Cinarcik basin, is consistent with the geodetic locking depth of 10 km and a low contribution to the regional seismic energy release. The assessment of the locked segment areas provide an estimate of the magnitude of the main forthcoming event to be about 7.3 assuming that the rupture will not enter significantly within creeping domains.

International audience; The engines of surface deformation in the Anatolia-Aegean region are a matter of debate, including the origin of the high elevations of the Anatolian plateau. Recent publications based on geological and thermomechanical modelling emphasize the role of dynamic topography in the plateau uplift. However, quantitative estimates of the contribution of dynamic topography are affected by large uncertainties due to insufficient knowledge of the crustal structure, in particular crustal thickness and density. To reduce these uncertainties, we provide a new accurate crustal thickness map of the Anatolia-Aegean domain computed from a large volume of broadband seismic data. In addition, we display high-resolution seismic sections of the internal structure of the crust in Western and Central Anatolia. Density contrasts are derived from the same seismic data set and Bouguer gravity anomaly computed from the EGM2008 model. Our crustal thickness model is highly correlated with the topography suggesting that the Anatolian plateau is close to isostatic equilibrium. The average density difference between crust and upper mantle computed from our crustal model and Bouguer gravity anomaly is low compared to the global average, ∼0.315 ×10 3 kg m −3. The ratio of surface elevation to crustal thickness is lower than average, 1/9.4, which also indicates a low-density crust. Differences between isostatic topography and observed topography are overall small (

International audience; A better understanding of what drives surface motion in the rapidly deforming Aegean–Anatolia region requires the comparison of mantle circulation models with reliable and densely spaced seismic anisotropy data. We present a new set of 4279 high-quality splitting data of core-refracted shear waves measured at 216 permanent and temporary broadband seismic stations in Turkey and Greece, and their neighboring countries. When combined with previously published observations, our dataset provides unprecedented dense spatial coverage of the area. The delay time between the fast and slow shear waves is highest in the northern Aegean Sea and northwestern Anatolia (average, 1.5 ± 0.4 s) and lowest in the southern Aegean Sea (average 0.6 ± 0.4 s) The fast-wave polarization axes are oriented NE–SW over most of Anatolia and the northern Aegean Sea. These show steady counterclockwise rotation of 1° per degree of longitude from eastern Anatolia to the northern Aegean. The only exceptions to this uniform pattern are NNW–SSE to NW–SE orientations in mainland Greece, and NW–SE orientations in the southwestern corner of Anatolia. The overall anisotropy pattern can be explained by instantaneous density–driven mantle flow with additional local effects, such as slab rollback in the Aegean Sea and a slab window beneath southwestern Anatolia.

International audience; Western Turkey provides spectacular examples of the two end-member models of deformation of the continental lithosphere, with strain localization along the North Anatolian fault and distributed north–south extension along the Aegean coast. To provide constraints on the mechanisms of continental deformation, we present a new high-resolution image of lithospheric structure along a ∼650 km transect crossing Western Anatolia at 28°E longitude from the Black Sea to the Mediterranean. More than 2600 receiver functions are computed from records of teleseismic earthquakes at 40 broadband seismic stations with an average spacing of 15 km. Lateral variations of crustal thickness and Vp/Vs ratio are inferred from both H-k and common conversion point stacks. We observe long-wavelength variations of Moho depth from ∼31 km in the Thrace basin to ∼25 km beneath the Marmara Sea, ∼32 km beneath the Izmir–Ankara suture, ∼25 km beneath the Menderes Massif and ∼20 km on the coast of the Mediterranean. No mid- or lower-crustal interface is visible in the migrated depth section and seismic discontinuities are confined to the shallow crust. The small-amplitude and long-wavelength lateral variations of the Moho topography suggest that viscous flow in a hot lower crust has smoothed out the lateral variations of crustal thickness induced by Cenozoic continent–continent collision. The crust–mantle boundary is flat beneath the central and southern Menderes Massif. The rougher Moho topography and more heterogeneous crust of the Marmara region likely result from transtension in the North Anatolian Fault Zone superimposed onto Aegean extension. The Moho of the subducted African lithosphere is observed dipping northward between ∼40 and ∼60 km depth at the southern end of the profile. The abrupt termination of the subducted slab only 50 km to the north of the Mediterranean coast confirms the slab tear inferred from previous tomographic studies.

[1] We investigate the effect of dynamic and static stress changes produced by the 1999 Izmit earthquake, on four preexisting seismic clusters located in the eastern Marmara sea, beyond the western termination of the earthquake rupture. These four clusters show long-lasting modifications in their seismicity rate. We observe that these seismic activity variations are related to stress changes. Dynamic stress pulses activate strike-slip faulting instantaneously, but in the absence of a concomitant static Coulomb stress increase, this activation is short lived. Indeed, a large dynamic stress combined with a negative static Coulomb stress may result in an immediate activation followed by the occurrence of a seismicity shadow. In contrast, the activation of extensional clusters begins slowly and takes a few days to fully develop. It is also remarkably long lasting and does not follow a classical Omori decay. More than 10 years after the earthquake, the extensional clusters located near the termination of the rupture, where static stress and pressure changes were high, are still activated.

SUMMARY This study provides new constraints on the upper-mantle structure from western Greece to central Anatolia using seismic data of permanent broad-band networks recently installed in Greece and Turkey and from a two-year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad-band stations with a typical interstation distance of 60–100 km across the study area. The high-resolution 3-D shear wave velocity model of the mantle is obtained by inversion of fundamental-mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damping. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3-D model (0.02–0.1 km s−1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper-mantle structure never reached at regional scale in the area. The overall structure is characterized by a low-velocity zone (80–200 km depth) reflecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low-velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high-velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high-velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low-velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia.

SUMMARY This study presents an analysis of Pn traveltimes to determine lateral variations of velocity, anisotropy of uppermost mantle and crustal thickness beneath Turkey and adjacent regions. From 1999 to 2010, more than 50 000 Pn arrivals are compiled from 700 regional earthquakes and 832 stations of permanent and temporary networks operated in the study area. We used a regularized least-squares inversion to estimate the uppermost mantle parameters. The results reveal features that correlate well with the geology and the active tectonics of the region. Overall,thePnvelocitiesshowveryfast(>8.4kms –1 )andveryslow( 8.3 km s –1 ), western Black Sea Basin (>8.3 km s –1 ), Adriatic Sea (>8.3 km s –1 ) and Zagros Suture (>8.3 km s –1 ). Pn anisotropy has maximum amplitude of ±0.6 km s –1 in the study area. The largest and coherent anisotropic anomalies are observed in the western Anatolia, Aegean Sea and Cyprian Arc. Pn anisotropy in western Anatolia, Aegean Sea and Greece correlate well with the present deformation of N–S regional extension. No clear correlation between anisotropy and the shear deformation along the North Anatolian Fault was observed. The majority of the stations in central Anatolia show small station residuals indicating the average crustal thickness of 35 ± 2 km. Western Anatolia and the Aegean Sea have crustal thicknesses between 28 ± 2 and 33 ± 2 km. In Greece, the crustal thicknesses are increasing from 33 ± 3 km from the western coast to a maximum of 48 ± 3k m beneath Dinarides-Hellenides. The large crustal thicknesses are also observed along southern coast of Anatolia (40–48 km).

SUMMARY We present the analysis of a Love-wave dispersion study performed across Turkey and the surroundings. Group velocity dispersion curves were obtained from the local and regional earthquakes recorded at both permanent and temporary stations operated in the region. Lovewave group velocity maps in the period range of 10–50 s were computed using a tomographic inversion method. The study shows the existence of significantly different crustal types in the area. The group velocity anomalies are associated with the major geological structures in the region. Low group velocities at shorter periods (10–20 s) are observed in the local sedimentary basins, the eastern Mediterranean and the Black Sea. The eastern Anatolia region is also characterized by low group velocities while Pontides and Bitlis–P¨ ot¨ urge massif display higher group velocities. The central Anatolia exhibits uniform velocity distribution indicating more homogenous crust. The Isparta angle is marked by a wedge-shaped low group velocity anomaly. The low velocities observed in the Isparta angle are related to crustal thickening and subduction-related complex. High velocities observed on the maps are associated with metamorphics, magmatic arcs along the orogenic belts of Pontides, P¨

SUMMARY Three main shocks M-1, M-2 and M-3 (17 October 2005 at 05:45 UTC, M w 5.4; 17 October at 09:46 UTC, M w 5.8 and 20 October at 21:40 UTC, M w 5.9) and their associated after- shocks within the Gulf of Si˘ gacik, 50 km southwest of Izmir, Turkey were studied in detail. A temporary seismic network deployed during the activity allowed the hypocentre of M-3 and subsequent aftershocks to be determined with high accuracy. A relative relocation technique was used to improve the epicentres of M-1 and M-2. All three main shocks have strike-slip mechanisms which agree with the linear trends of the aftershock locations. Two distinct zones were illuminated by the aftershock locations. The zones contain clear echelon patterns with slightly different orientations from the trend of the aftershock distribution. M-2 and M-3 rup- tured along of the eastern rupture zone which aligns N45 ◦ E. However the strike direction of M-1 is not clearly identified. The alignment of the two rupture zones intersect at their southern terminus at an angle of 90 ◦ . The fault zones form conjugate pair system and static triggering is considered as a probable mechanism for the sequential west to east occurrence of M-1, M-2 and M-3. This earthquake sequence provides seismological evidence for conjugate strike-slip faulting co-existing within a region dominated by north-south extension and well-developed east-west trending normal faults.

International audience; We present observations of ground accelerations recorded at a small array close to the fault during the Düzce earthquake and its early aftershocks. The records show the strong spatial variability of ground acceleration over distances of only a few hundred metres. During the main shock, the peak horizontal acceleration values ranged from 0.3 to about 1.0 g at stations distant of 1.5 km only. We attribute this spatial variability to a fault zone site effect as peak ground acceleration steadily increases as the distance to the fault trace decreases. The spectral ratio between the ground motion recorded near the fault and the one outside the fault zone shows a shift of the spectral peak to lower frequencies with increasing peak accelerations. Such an observation suggests a non-linear behaviour of the fault zone due to the strong ground shaking. As much as a 45 per cent reduction in the shear wave velocity is necessary for the observed shifts. The opening of pre-existing cracks throughout the fault zone is the proposed mechanism to account for the observed shear wave reductions. The observation that elastic fault zone properties are soon recovered following episodes of large strains shows that cracks and fissures close rapidly after the strong shaking is over.

International audience; We use the strong motion data recorded at near-fault accelerometers combined with Global Positioning System (GPS) measurements to infer the space and time evolution of the M w = 7.2 1999 November 12 Düzce (Turkey) earthquake. We confirm that the rupture is subshear towards the west whereas it overpassed the shear speed towards the east. The mechanism of the earthquake is predominantly right lateral on a 65 • north-dipping fault. The strike-slip we infer represents at least 75% of the total seismic moment. We stress that the use of strong motion data allows us to constrain a strong spatial variation of the slip direction during faulting: to the west of the hypocentral region, the slip is oblique accompanied by a significant normal component. In the central and eastern part of the activated fault the slip is almost pure right lateral. This result is consistent both with the surface offset observations and with the long-term morphology of the fault. Neither strong motion data nor GPS data can resolve the complexity of the eastern termination of the Düzce rupture.

We use continuous recordings at four stations installed near the eastern termination of the Izmit rupture to analyze the seismic activity there during the 5 hr leading to the Duzce earthquake. The records clearly show the presence of three groups of events. One of the groups originates from the middle of the Izmit rupture and consists of aftershocks of the M 5.7 Spanca Lake earthquake that had occurred a day earlier and was itself, to date, the second largest aftershock in the Izmit sequence. A second group of events is spread over the 30-km-long eastern (Karadere) segment of the Izmit rupture, and a comparison with the number of aftershocks expected, based on decay laws for similar sequences in other regions, suggests that seismic activity was high on the eastern segment in the hours leading to the Duzce earthquake. The third group of events occurs beyond the termination of the Izmit rupture and clusters around the hypocenter of the Duzce earthquake. It shows that, in the little over 5 hr leading to the Duzce earthquake, this destructive event was preceded by six precursory shocks ranging in magnitude from 0.9 to 2.6. Manuscript received 30 August 2000.

We used seismic stations that we deployed after the Izmit earthquake along the shores and islands of the Izmit Bay-Cinarcik basin to study the seismic activity that took place after the earthquake in the eastern Marmara Sea. The aftershock distribution indicates the existence of three clusters of activity there in the days that followed the earthquake. One of the clusters shows the extension of the E-W-trending Izmit rupture into the eastern Marmara Sea. The seismic activity there clearly outlines the fault plane of the main rupture and shows its termination 35 km beyond the Hersek peninsula. Two other clusters of activity are present in the region. One is in the Armutlu peninsula and the other one is located a few kilometers southwest of the Tuzla peninsula beneath the northern slope of the Cinarcik basin. The focal mechanism solutions indicate strike-slip faulting along the main branch of the Izmit rupture and normal faulting within the two swarms. The presence of the different mechanisms suggests the existence of slip partitioning in the region. A remarkable feature of the aftershock data is the absence of seismicity above 4 km in the Marmara Sea. Manuscript received 30 August 2000.

SUMMARY An indirect boundary element method has been used to compute seismic wavefields in layered media. The advantages of this technique over numerical grid methods are that it satisfies radiation conditions, incorporates free surface effects, solves viscoelastic problems and can handle arbitrary source types. The technique is more appropriate for models with high aspect ratios and a limited number of layers. The computational cost increases with the number of layers and frequency content. Examples are presented in this study to illustrate some of the features of the technique. Seismograms are computed for SH and P‐SV line sources in 2-D earth models. Finite-difference and the Cagniard‐ de Hoop methods are used to test the accuracy of the technique. Even though the computation time for the method is several times greater than finite differences, the technique becomes cost effective as the number of computed shot-gathers increases. The technique also allows computation of constant offset gathers at a small additional cost which is very expensive to do by using finite differences. Numerical grid methods (e.g. finite differences, finite elements and pseudospectral methods) have become the standard tool for seismic modelling. These techniques are extremely versatile in terms of model complexity that can be treated, although grid generation may still be problematic. These methods are also very demanding of computation time. They involve discretization of the entire model volume, at a grid spacing which is generally much shorter than the smallest wavelength present in the solution. The finite aperture of the grid also imposes difficulties in the approximation of radiation conditions on the solutions and thus the grid may need to be made much larger than the modelled region. A third set of difficulties is associated with the approximation of the solution in the vicinity of boundaries (both internal and free surface). All of the grid methods involve some smoothing of the solution that may be objectionable for some modelling purposes, but may in fact be an advantage in imaging or migration. Boundary methods, boundary integrals (BIM) and boundary elements (BEM) have emerged as alternatives to the grid methods in cases where better accuracy is required near boundaries and sources or where the domain is infinite in extent (Cruse 1980). Boundary methods discretize the model only on boundary surfaces, rather than throughout the model volume, so that advantages may be obtained for models with large aspect ratio

An extension of the boundary integral method for SH waves is given for transversely isotropic media. The accuracy of the method is demonstrated for a simple flat interface problem by comparison to the Cagniard-de Hoop solution. The method is further demonstrated for a case with interface topography for both surface and vertical seismic profiles. The new method is found to be both accurate and effective.

We use the records of the ground motion obtained at near-fault accelerometers to study the space and time evolution of rupture and faulting during the Izmit earthquake. We find that the rupture propagated at the sub-Rayleigh speed of about 3 km/sec on the western and eastern segments of the fault, but that the central segment (Izmit-Sapanca Lake-Sakarya), nearly 50 km long, broke at the supershear speed of about 4.8 km/sec. This value, within the range of uncertainties, is the one theoretically predicted ( \(\sqrt{2}V_{\mathrm{S}}\) ) in fracture dynamics for stable shear crack growth at intersonic speed. We infer an average fault slip of about 2.9 m over a total rupture length of about 150 km, with the largest values (of up to 6 m) occurring in the Golcuk area to the west and in the Sakarya region to the east. The strong-motion data also indicate that the slip diminished gradually to the west beyond the Hersek peninsula over about 30 km, whereas it stopped abruptly at depth at the termination of the eastern (Karadere) segment. The slip duration is between 2 and 4 sec, except in the hypocentral area, which slipped in about 1 sec. The seismic moment inferred is about 2.5 × 10 20 N m.