Abstract We use local and teleseismic earthquakes to analyze shear wave splitting within the Pamir‐Hindu Kush region, north of the western syntaxis of the India‐Asia collision zone. These two data sets allowed us to map the distribution of azimuthal anisotropy, to put constraints on the depth range where it is accumulated, and to deduce characteristics of ongoing deformation. From 1,073 SKS (core‐mantle refracted phases) measurements at 107 stations, we derived time delays of 0.7–2.25 s and dominantly ENE‐WSW oriented fast polarization directions. Fast polarization directions only deviate adjacent to the subducting slabs and major strike‐slip faults, aligning parallel to these structures. From 461 direct S measurements along a transect perpendicular to the Pamir seismic zone, we obtain fast directions parallel to those from SKS measurements but smaller delay times (average 0.4 s), which vary depending on depth. Time delays exhibit 0.1–0.3 s crustal contribution and increase to 0.8 s in a narrow domain coinciding with the inferred subcrustal contact of the two colliding plates. We find measurements from the same event‐station paths at different filter frequencies to be frequency‐independent, allowing a comparison with SKS results along the studied profile. The smaller average time delays of local events imply that the crust and uppermost mantle only make a minor contribution to the SKS splitting. Thus, the coherent fast direction pattern suggests a strain field dominated by the indentation of India and the escape of sublithospheric material north of the indenter. Crustal anisotropy is likely also controlled by this regional deformation pattern with locally highest strain rates closest to the continental subduction front.
Abstract Compressional and extensional tectonics following northward plate convergences since the Miocene have formed the major surface features in Turkey, such as faulting and orogeny. Despite increasing efforts in last few decades aiming to elucidate the current architecture of the crust and mantle beneath Turkey, several issues regarding the depth extent of the deformation zones, crust‐mantle interaction (e.g., coupling and decoupling) in relation to the deformation, and stress transmission in the lithosphere remain elusive. Here we present high‐resolution 3‐D P wave isotropic and azimuthal anisotropic velocity models of the crust and uppermost mantle beneath Turkey by inverting 204,531 P wave arrival times of 8,103 local crustal earthquakes. Our results reveal low‐velocity anomalies or velocity contrasts down to the uppermost mantle along the North and East Anatolian Fault Zones. The fast velocity directions (FVDs) of azimuthal anisotropy in the lower crust and uppermost mantle are parallel to the regional maximum extensional directions in western Turkey, and the FVDs in the crust and uppermost mantle are parallel to the surface structures in southeastern Turkey. These results indicate that vertically coherent deformation between the crust and uppermost mantle occurs in western and southeastern Turkey. However, in central northern Turkey, the FVDs in the uppermost mantle are oblique to both the FVDs in the lower crust and the maximum shear directions derived from GPS measurements, suggesting that the crust and lithospheric mantle are decoupled there.
Abstract The North Anatolian Fault Zone (NAFZ) is a prominent tectonic structure with a significant impact on the observed active deformation in Türkiye. Detailed knowledge of the seismic anisotropy in the crust and mantle along this nascent shear deformation zone provides insights into the kinematics associated with past and present tectonic events. We employed teleseismic earthquakes observed by the Dense Array North Anatolia seismic network to map 3‐ D variations in crustal and mantle anisotropy in/around the NW segment of the NAFZ. To achieve this, we first performed a harmonic decomposition analysis of P‐receiver functions. The results were then used as a priori information to conduct an anisotropic receiver function inversion with the Neighborhood Algorithm that enabled imaging of the actual orientation and geometry of anisotropic structures. SKS splitting measurements are further used to make a comparison between the anisotropic behavior of crustal and mantle structures. Crustal anisotropy parameters estimated in our analyses/models well identify the signature of deformation caused by accumulated strain in the earthquake cycle through the strike of shallow cutting faults in the brittle crust beneath the NAFZ. Diffuse intense anisotropic energy at lower crustal depths was attributed to lattice preferred orientation of crystals or partially molten lenses elongated along the shear direction. Strong harmonic energy variations beneath the northern part of the Istanbul Zone likely reflect imprints of LPO‐originated frozen fabric at shallow depths (0–20 km) associated with the palaeotectonic Odessa Shelf, Intra‐Pontide Suture Zones or remnants of the Tethys Ocean.
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.
The eastern Mediterranean which is one of the most tectonically active collisional regions where Eurasian, African and Arabian plates converge, provides an excellent opportunity to investigate the evolution of various scales of deformation throughout the Earth. In such a region with highly complex and active tectonic structures, a detailed study of geodynamic processes and related mantle kinematics is required to better understand the development of complex structures at the surface. For example, the region of study, the Anatolian plate and surroundings host several complicated deformation regimes with two large transform faults (North and East Anatolian Faults; NAF and EAF, respectively), regions of extensional and compressional tectonics in the west and east of Anatolia. Seismic anisotropy provides a robust link between seismic observations and geodynamic processes which play a key role for controlling the past and/or present deformations in the mantle lithosphere and asthenosphere. In this study, we perform shear wave splitting analyses on teleseismic core-refracted S-waves (e.g. SKS and SKKS phases) recorded by ~600 broad-band seismic stations located in the region. We estimate seismic anisotropy parameters (e.g., fast polarization direction; FPD and delay time; DT) beneath each seismic station by employing conventional shear wave splitting (e.g., transverse energy minimization and eigenvalue) and splitting intensity approaches. Exploiting a large earthquake dataset, spanning through 2000-2022 with Mw ≥ 5.5 events, that covers a wide range of back-azimuths enables the reliable estimates of complex anisotropic models, such as two-layer and dipping anisotropy models. Our preliminary results largely indicate the NE-SW directed FPDs throughout the study area, except for SW Turkey (NW-SE) and central parts of Anatolia (E-W) that can be mainly explained by the lattice-preferred orientation (LPO) of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. Findings from our two-layer grid search algorithm indicated strong evidences for two-layer anisotropy models beneath the seismic stations in eastern Aegean and western Anatolia, in particular close to the western branches of NAF in the Aegean.
<p>Seismic anisotropy studies can provide important constraints on geodynamic processes and deformation styles in the upper mantle of tectonically active regions. Seismic anisotropy parameters (e.g. delay time and fast polarization direction) can give hints at the past and recent deformations and can be most conventionally obtained through core-mantle refracted SKS phase splitting measurements. In order to explore the complexity of anisotropic structures in the upper mantle of a large part of the Aegean region, in this study, we estimate splitting parameters beneath 25 broad-band seismic stations located at NW Anatolia, North Aegean Sea and Greece mainland. To achieve this we employ both transverse energy minimization and eigenvalue methods. Waveform data of selected earthquakes (with M<sub>w</sub> &#8805; 5.5; 2008-2018 and with epicentral distances between 85&#176;&#8211;120&#176;) were retrieved from Earthquake Data Center System of Turkey (AFAD; http://tdvm.afad.gov.tr/) and European Integrated Data Archive (EIDA; http://orfeus-eu.org/webdc3/). A quite large data set, the majority of which have not been studied before, were evaluated in order to estimate reliable non-null and null results. In general, station-averaged splitting parameters mainly exhibit the NE-SW directed fast polarization directions throughout the study area. These directions can be explained by the lattice-preferred orientation of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. We further observe that station-averaged splitting time delays are prone to decrease from north to south of the Aegean region probably changing geometry of mantle wedge with a strong effect on&#160; the nature of mantle flow along this direction. The uniform distribution of splitting parameters as a function of back-azimuths of earthquakes refers to a single-layer horizontal anisotropy for the most part of the study area. However, back azimuthal variations of splitting parameters beneath most of northerly located seismic stations (e.g., GELI, SMTH etc.) imply the presence of a double-layer anisotropy. To evaluate this, we performed various synthetic tests especially beneath the northern part of study region. Yet, it still remains controversial issue due to the large azimuthal gap and thus requires further modelling which may involve the use of joint data sets.</p>
ÖzetBu çalışmada, telesismik (uzak) depremlere ait dalga formu kayıtları üzerinde gözlenen SKS fazı kullanılarak Marmara Bölgesi'nin altında kalan manto yapısının deformasyonu ile ilişkili anizotropik bulgular incelenmiştir.Bu amaçla kullanılan 34 adet istasyonda, tek-tabakalı anizotropi modeli varsayılarak hesaplanan 572 adet iyi kalitede SKS ayrımlaşması parametresi, hızlı ve yavaş S dalgaları arasındaki zaman gecikmelerinin 0.97 sn ile 2.17 sn aralığında değiştiğini göstermektedir.İstasyon ortalamaları alınarak hesaplanan hızlanma polarizasyonu yönleri K10°D ile K63°D arasında değişerek büyük çoğunlukla KD-GB yönlü bir dağılıma işaret etmektedir
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)