SUMMARY
Lithospheric deformation throughout Anatolia, a part of the Alpine–Himalayan orogenic belt, is controlled mainly by collision‐related tectonic escape of the Anatolian Plate and subduction ...roll‐back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle structure of Anatolia using teleseismic, finite‐frequency, P‐wave traveltime tomography. We use data from several temporary and permanent seismic networks deployed in the region. Approximately 34 000 P‐wave relative traveltime residuals, measured in multiple frequency bands, are inverted using approximate finite‐frequency sensitivity kernels. Our tomograms reveal segmented fast seismic anomalies beneath Anatolia that corresponds to the subducted portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify these anomalies as the subducted Aegean and the Cyprus slabs that are separated from each other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere. The eastern termination of the subducting African lithosphere is located near the transition from central Anatolia to the Eastern Anatolian Plateau or Arabian–Eurasian collision front that is underlain by large volumes of hot, underplating asthenosphere marked by slow velocity perturbations. Our tomograms also show fast velocity perturbations at shallow depths beneath northwestern Anatolia that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The associated velocity contrast across the NAFZ persists down to a depth of 100–150 km. Hence, our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary.
Overall, the resolved upper‐mantle structure of Anatolia is directly related with the geology and tectonic features observed at the surface of the Anatolian Plate and suggest that the segmented nature of the subducted African lithosphere plays an important role in the evolution of Anatolia and distribution of its tectonic provinces.
The 30 October 2020 Samos earthquake (Mw 7.0) ruptured an east–west striking, north dipping normal fault located offshore the northern coast of Samos Island, previously inferred from the bathymetry ...and regional tectonics. This fault, reported in the fault-databases as the
North Samos
and/or
Kaystrios Fault
, ruptured with almost pure dip-slip motion, in a region where both active extension and strike-slip deformation coexist. Historical information for the area confirms that similar ~ Mw7 events had also occurred in the broader Samos area, though none of the recent (last ~ 300 years) mainshocks appears to have ruptured the same fault. The spatial and temporal distribution of relocated aftershocks indicates triggering of nearby strike-slip and normal fault segments, situated in the areas where static stress has increased due to the mainshock generation. The relocated aftershocks and the slip model indicate that the sequence ruptured the upper crust (mainly the depth range 3–15 km). The top of the rupture plane nearly reached the sea bottom, located at a depth of < 1 km. Slip is confined in mainly two asperities, both located up-dip from the hypocenter and at shallow depths. The average displacement is ~ 1 m and the peak slip is ~ 3.5 m for a shear modulus of 3.2e10 N/m
2
. While it is difficult to constrain the rupture velocity in the inversions, the model suggests a slow rupture speed of the order of 2.2 km/s. The resolved source duration is ~ 16 s, compatible with the ~ 32 km length of the fault that ruptured.
Due to the unique soil and morphological conditions prevailing in Izmir Bay basin, structural damage has been governed by site effects. Consistently, during October 30, 2020 M7.0 Samos Earthquake, ...which took place offshore of Samos Island, structural damage and life losses were observed to be concentrated in Bayrakli region of Izmir Bay, despite the fact that the fault rupture was at a distance of 65–75 km from the city of Izmir. Additionally, strong ground motions recorded in Izmir Bay showed unique site amplifications that were observed surprisingly at both rock and soil sites. Soil amplifications and duration elongations were mostly due to site effects governed by the response of very deep alluvial deposits of low plasticity. Similarly, due to very extensive faulting-induced fracturing and unusually stratified nature of rock sub-layers, unexpected long period amplifications were also observed at rock sites. These earthquake and site resonance effects were more pronounced in the period range of 0.5–1.5 s. When they were superposed with relatively coinciding natural period of 7–9 story residential buildings of Izmir City, it was concluded that the triple resonance effects among incoming rock ground motions, soil deposits, and the damaged buildings, amplified and prolonged the overall system response. Within the confines of this manuscript, the governing role of site effects leading to increased seismic demand was assessed, through a series of 1D equivalent linear, total stress-based site response assessments, the results of which clearly highlighted the variation of seismic demand in Izmir Bay.
•Samos Island earthquake produced rich long period rock spectral accelerations.•Deep soil sites in Izmir Bay, amplified these long period rich rock motions.•Due to resonance effects, 7-9 story buildings were subjected to larger shakings.•Site effects increased seismic demand and prolonged shaking duration.•These, along with poor design-construction practices caused structural damage.
SUMMARY
3‐D P‐wave velocity structure and Vp/Vs variations in the crust along the North Anatolian Fault Zone (NAFZ) in north‐central Anatolia were investigated by the inversion of local P‐ and S‐wave ...traveltimes, to gain a better understanding of the seismological characteristics of the region. The 3‐D local earthquake tomography inversions included 5444 P‐ and 3200 S‐wave readings obtained from 168 well‐located earthquakes between 2006 January and 2008 May. Dense ray coverage yields good resolution, particularly in the central part of the study area. The 3‐D Vp and Vp/Vs tomographic images reveal clear correlations with both the surface geology and significant tectonic units in the region. We observed the lower limit of the seismogenic zone for north‐central Anatolia at 15 km depth. Final earthquake locations display a distributed pattern throughout the study area, with most of the earthquakes occurring on the major splays of the NAFZ, rather than its master strand. We identify three major high‐velocity blocks in the mid‐crust separated by the İzmir‐Ankara‐Erzincan Suture and interpret these blocks to be continental basement fragments that were accreted onto the margin following the closure of Neo‐Tethyan Ocean. These basement blocks may have in part influenced the rupture propagations of the historical 1939, 1942 and 1943 earthquakes. In addition, large variations in the Vp/Vs ratio in the mid‐crust were observed and have been correlated with the varying fluid contents of the existing lithologies and related tectonic structures.
The 2020 M7.0 Samos earthquake had occurred on the north of Samos Island; however, structural damage was observed in İzmir-Bayraklı, which is located approximately 65 km away from the epicenter. ...Strong ground motions recorded in İzmir Bay showed unique site amplifications, mostly due to the interaction between the basin and deep alluvial deposit response. The objective of this study is to evaluate the predictive performance of current ground motion models (GMMs) for estimating the recorded strong motions, especially the recordings over or near the Bayraklı-Bornova basin. 66 strong motion stations from Turkey with rupture distance (RRUP) < 200 km are used in the residual analysis, considering the ambiguities in the magnitude and depth to the top of the rupture estimations. Event terms of the earthquake for tested GMMs are found to be small and lie within the expected scatter, except for T = 0.5–1.5 s spectral accelerations. Event-specific distance attenuation for RRUP<100 km is consistent with the median predictions of current GMMs; however, the distance scaling for 100 km<RRUP<200 km are significantly different at high frequencies, indicating faster attenuation for Southwestern Anatolia. Relatively long period (0.5–1.5 s) spectral energy is present in both soft sites on the Bayraklı-Bornova basin and rock/stiff-soil sites on the basin edge and these stations contribute significantly to the positive event terms at T = 0.5–1.5 s. For sites within the basin or close to the basin edge, factors such as the direction of the rupture front, basin width and depth, and the seismic structure of the basin contribute to the ground motion variability.
•The structural damage from 2020 Samos Eq. was observed in İzmir-Bayraklı, which is located 65 km away from the epicentre.•The event terms of the earthquake are found to be small, except for T=1 sec, based on the analysis of 66 stations from AFAD.•Distance scaling at high frequencies indicates faster attenuation when compared to current GMMs for large distances.•Mid-period (0.5-1.5 sec) spectral energy is present in soft sites over the İzmir basin and rock sites on the basin edge.•For sites within the basin or close to the basin edge, the variability in recorded ground motions is significant.
The Burdur Basin is a late Miocene to Pliocene fluvio-lacustrine basin in SW Anatolia. It is developed within the postulated Fethiye-Burdur Fault Zone, which was argued to be a sinistral strike-slip ...fault zone developed in response to propagation of the Pliny-Strabo STEP fault into SW Anatolia (Turkey). In order to assess the presence and tectonic characteristics of the fault zone, we conducted a paleomagnetic study in the Burdur basin that involved rock magnetic experiments, Anisotropy of Magnetic Susceptibility (AMS) measurements and developing a magnetostratigraphy for dating purposes. The obtained age model constrains most part of the tectonic evolution of the basin. The well exposed (~270 m thick) Burdur section revealed 3 normal and 2 reverse polarity magnetozones. We propose that the Burdur Formation spans most of the Gauss Chron (~3.4–2.5 Ma) which implies a sedimentation rate of >18 cm/kyr. The AMS results in the section indicate NW-SE directed extension.
In addition, we have also conducted kinematic analyses from 1790 fault slip data collected at 44 sites distributed within the supposed Fethiye Burdur Fault Zone in the region. The results indicate that the region has been developed under a NW-SE directed extensional deformation regime and was dominated by NE-SW striking normal faults from late Miocene to recent. Few NW-SE striking normal faults with strike-slip components are categorized as transfer faults, which accommodated differential stretching between the Burdur and Çameli basins. Stretching amounts increase southwards demonstrating a dextral transtensional character of the transfer faults.
We have not observed any significant strike-slip motion along the NE-SW striking faults, which challenges the presence and sinistral transcurrent nature of the supposed Fethiye Burdur Fault Zone.
•Pliocene lacustrine succession of the Burdur Fm. is dated magnetostratigraphically.•Kinematic data indicate extensional deformation prevailed in the region since the Pliocene.•No supporting evidence found for the existence of a Fethiye-Burdur Fault Zone
Convergence between the Eurasian and the African plates in the West Anatolian‐Aegean region results in a trench retreat due to slab roll‐back and tearing of the subducted African lithosphere. The ...upper plate response of this process gave way to back‐arc extension in the region. We have conducted a very detailed anisotropy of magnetic susceptibility (AMS) study on the Neogene rocks in SW Anatolia to unravel the style and magnitudes of deformation. For this purpose, from 83 sites in 11 structurally homogeneous domains, 1,680 paleomagnetic samples were analyzed. The results show that AMS fabrics are related to the tectonic deformation and that the magnetic lineation (maximum susceptibility axis, k1) is parallel to inferred maximum extension, while minimum susceptibility (k3) is typically normal to the bedding plane, corresponding to a preserved compaction associated with deposition fabric. The intermediate axis (k2) is parallel to a second extension direction and indicates that the region has been under the control of multi‐directional extension during the Neogene. Two main magnetic lineation directions are identified and represent Oligocene to middle Miocene E‐W, and late Miocene to Pliocene NW‐SE oriented extension. The magnetic lineation directions are dominantly parallel or perpendicular to the general strikes of the normal faults. The results show that the deformation in the region resembles two differentially stretched rubber sheets under the influence of SW oriented extension, exerted by the southward retreating Eastern Mediterranean subduction system.
Plain Language Summary
The tectonic style and amount of crustal deformation in SW‐Anatolia are revealed by sets of anisotropy of magnetic susceptibility data obtained from SW Anatolia. The orientation of principal strain axes changes gradually although the shape of the strain ellipsoid among all the rocks in late Miocene to Pliocene domains remains the same. Based on these results and published information, we conclude that the SW Anatolia is under the control of multi‐directional extension associated with counterclockwise rotation exerted by the southward retreat of the Eastern Mediterranean subduction system (Hellenic‐Pliny‐Strabo and Cyprian Trenches). The retreat resulted in stretching of SW Anatolia, the over‐riding plate, to accommodate the retreat of the trench as a non‐rigid, stretched rubber‐sheet like deformation style, which seems to be pulled from a single point toward the SW. The Büyük Menderes‐Denizli‐Baklan grabens and Dinar‐Aksu faults mark the northern boundary of this peculiar and active deformation zone.
Key Points
Anisotropy of magnetic susceptibility data from Neogene rocks in SW Anatolia yield orientations of principal tectonic strain
SW Anatolia underwent E‐W, and then NW‐SE oriented extension in the Oligocene to middle Miocene and late Miocene to Pliocene, respectively
Deformation is the result of SW directed stretching of the over‐riding lithosphere above the southward retreating subducted African oceanic slab
The subducting African Plate in the easternmost Mediterranean is actively tearing and deforming beneath the Anatolian Plate as the margin transitions from long‐lived subduction to collision. In ...central Anatolia, the subducting slab is characterized by both lateral and vertical slab tears. We investigate patterns of mantle flow around the edges of a contorting and fragmenting African slab segment, called the Cyprean slab, using measurements of shear wave splitting. We observe three distinct regions of coherent shear wave splitting that correlate with the segmentation boundaries of the Cyprean slab. Regionally coherent mantle flow occurs near both the eastern and western the edges of the slab. These regions of coherent splitting are separated by an area of null splitting that encompasses the Central Anatolian Volcanic Province near the easternmost edge of the slab. The null measurements likely result from mantle upwelling due to the displacement of asthenosphere from the vertical Cyprean slab.
Plain Language Summary
The subducting African Plate in the easternmost Mediterranean has been tearing beneath central Turkey as the region switches from subduction to collision. This area provides one of the best chances to study how tearing of a subducting plate impacts patterns of mantle flow during the last stages of subduction. We study mantle flow around the edges of the fragmented Cyprean slab using measurements of shear wave splitting, which provide information on the direction of mantle flow via the fast splitting direction and delay time. We see three distinct regions of mantle dynamics that relate to the boundaries of the Cyprean slab. We observe coherent flow near both the eastern and western edges of the slab that align with background regional dynamics. The two regions with coherent flow are separated by an area of anomalously absent splitting. This likely indicates that mantle is flowing upward next to the vertical Cyprean slab segment in central Anatolia, including just below the Central Anatolian Volcanic Province.
Key Points
Shear wave splitting in central Anatolia shows evidence of mantle upwelling around the Cyprean slab
Weak toroidal return flow exists along the western edge of the Cyprean slab and not along the eastern boundary
Background mantle flow driving the Anatolian Plate is seen at many Continental Dynamics: Central Anatolian Tectonics stations
We present a dataset of 77 strong ground motion records within 200 km epicentral distance from the 30 October 2020, M7.0 Samos Island (Aegean Sea) earthquake, which affected Greece and Turkey. ...Accelerograms from National Networks of both countries have been merged into a single dataset, including metadata that have been uniformly derived using a common preliminary source model. Initial findings from the analysis and comparative examination of acceleration time histories, Fourier amplitude spectra and 5%-damped response spectra are discussed along with significant source, propagation path and site effects. The long-period amplifications observed in most records in Izmir bay triggered failures and severe damages in weak structures. Yet, the spectral accelerations are observed to lie below the current and previous design spectra corresponding to the damaged regions. Peak ground motions are used to construct a purely instrumental-based macroseismic intensity map, which is capable of reflecting the actual earthquake damage caused by this considerably large event. Finally, peak ground motions are compared to various ground motion models (GMMs) and deviations are highlighted. Our overall preliminary analysis reveals a strong energy signature of the Samos earthquake in the period range 0.5–1.5 s at many sites, both on rock and soil, whereas records in the heavily hit Izmir city, at an epicentral distance circa 70 km, provide strong indication for additional amplification due to basin effects. At relatively large distance from the earthquake source (> 120 km), several recorded amplitudes are significantly lower than those predicted by many GMMs, implying that further studies are necessary toward the improvement of regional attenuation models.
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