Browsing by Author "Cinar, Hakan"

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Crustal S-Wave Structure Around the Lake Van Region (Eastern Turkey) From Interstation Rayleigh Wave Phase Velocity Analyses
(Tubitak Scientific & Technological Research Council Turkey, 2017) Cinar, Hakan; Alkan, Hamdi
This study focuses on the average crustal and the upper mantle structure throughout the Lake Van region of eastern Turkey. The study aimed to investigate the structure with the fundamental mode interstation Rayleigh wave phase velocities from the local and the regional earthquakes recorded by Kandilli Observatory and Earthquake Research Institute stations. Considering back azimuth differences of each source and station path, six different broadband station pairs and 27 earthquakes were selected to determine the 1-D shear-wave velocity structures throughout the region by using an interstation method (slant stacking technique). The linearized least squares algorithm was used to obtain the 1-D shear-velocity model that best fit the observed phase velocity dispersion curve. The normalized statistical resolution matrix was calculated to measure the reliability of the solution. Inversion results revealed that the solution quality of the upper crust is weak due to the high resolution lengths. The average shear-wave velocities in the lower crust scale down to approximately 3.5 km/s. It was inferred that this low-velocity zone shown in the lower crust may be associated with widespread volcanism. Final 2-D S-wave velocity models obtained from the inversion revealed that the crust-mantle boundary is similar to 42 km, and shear velocities vary from 3.6 to 4.2 km/s. Furthermore, the upper mantle (similar to 45-70 km) velocities are slower than globally suggested models (e. g., IASP91), and this is possibly related to shallow hot asthenospheric material.
Crustal S-Wave Structure Beneath Eastern Black Sea Region Revealed by Rayleigh-Wave Group Velocities
(Pergamon-elsevier Science Ltd, 2016) Cinar, Hakan; Alkan, Hamdi
In this study, the crustal S-wave structure beneath the Eastern Black Sea Region (including the Eastern Black Sea Basin (EBSB) and Eastern Pontides (EP)) has been revealed using inversion of single-station, fundamental-mode Rayleigh-wave group velocities in the period range of 4-40 seconds. We used digital broadband recordings of 13 regional earthquakes that recently occurred in the easternmost EBSB recorded at stations of the Kandilli Observatory and Earthquake Research Institute (KOERI). The average group-velocity-dispersion curves were generated from 26 paths for the EBSB, and 16 paths for the EP, and they were inverted to determine the average 1-D shear-wave structure of the region. We have created a pseudo-section, roughly depicting the crustal structure of the region based on the group velocity inversion results of all station-earthquake paths. The thickness of the sedimentary layer reaches 12 km in the center of EBSB (V-S= 2.5-3.1 km/s) and decreases 4 km in the EP. There is a thin sedimentary layer in the EP (V-S = 2.7 km/s). A consolidated thin crust that exists in the EBSB possesses a high seismic velocity (V-S = 3.8 km/s). While a thin (similar to 26 km) and transitional crust exists beneath the EBSB, a thick (about 42 km) continental crust exists beneath the EP where the Conrad is clearly seen at about a 24 km depth. Thick continental crust in the EP region is clearly distinguished from a gradational velocity change (V-S = 3.4-3.8 km/s). The Moho dips approximately southwards, and the V-S velocity (4.25-4.15 km/s) beneath the Moho discontinuity decreases from the EBSB to the EP in the N-S direction. This may be an indication of a southward subduction. (c) 2015 Elsevier Ltd. All rights reserved.
Investigation of the Crustal and Upper-Mantle Structure of the Eastern Pontides Orogenic Belt (Ne, Turkey): a Receiver-Function Study
(Springer, 2019) Alkan, Hamdi; Cinar, Hakan; Oreshin, Sergey; Vinnik, Lev
In this study, we use teleseismic P and S receiver functions (i.e., S-to-P and P-to-S converted signals) and their joint inversions to determine seismic discontinuities in the crust and upper mantle. Eight permanent broadband stations from the KOERI (Kandilli Observatory and Earthquake Research Institute) that are distributed along the eastern Pontides orogenic belt (EPOB), NE, Turkey, comprise our database. Inversion is performed by using a simulated annealing technique with and without travel time residuals. Our inversion results reveal the Moho depth, a high S velocity lid, a low-velocity zone, and the underlying upper mantle layer. The studied area is divided into two regions based on the station locations: (a) a northern region and (b) a southern region. The inversion results from the northern area produce crustal models that indicate that the uppermost crust is represented by a low P and S wave velocity (Vp=5.0km/s and Vs=2.8km/s). These velocities are clear evidence of (1) the sedimentary and volcanic rocks that widely crop out in the region and (2) a thinner uppermost crust, whereas the velocities of the southern region (Vp=6.0km/s and Vs=3.1km/s) indicate a thicker uppermost crust (7km). Our calculated Vp/Vs velocity ratio in the lower crust is approximately 1.90 and 1.80 in the northern and southern regions, respectively. These ratios are generally attributed to mafic rocks. Beneath the northern stations, the crustal thicknesses are 30, 33, 37, and 40km from east to west, while the depths of the Moho are 46, 42, 39, and 44km beneath the southern stations. Some velocity histograms show a transition from the high S velocity mantle lid to the low-velocity zone, which is known as the lithosphere-asthenosphere boundary. A representative value of the boundary's depth is around 83km in the north and 88km in the south. The hypothesis of some researchers regarding the existence of southward subduction beneath the eastern Pontides orogenic belt during the Late Mesozoic-Cenozoic is supported by our 2-D and 3-D velocity-depth models.
Lake Van (Southeastern Turkey) Experiment: Receiver Function Analyses of Lithospheric Structure From Teleseismic Observations
(Springer Basel Ag, 2020) Alkan, Hamdi; Cinar, Hakan; Oreshin, Sergey
The P and S wave receiver functions and their joint inversions are used to study the lithosphere beneath the Lake Van region using approximately 600 teleseismic earthquake data (M-w >= 5.8) at different azimuths collected from 10 permanent broadband stations operated by Kandilli Observatory and Earthquake Research Institute and Disaster and Emergency Management Authority in the region. The dataset is taken from the European Integrated Data Archive. The simulated annealing method is used for the joint inversion of P-wave and S-wave receiver functions, since the P410s phase cannot be detected reliably in the depth stacks. This may be due to the low olivine content and high basalt content at this depth. The inversion process is therefore performed without travel time residuals. The crustal thickness is observed at nearly 45 km in all the velocity models obtained from the inversion. The relatively low-velocity layer (V-s = 3.4 km/s) at depths of the middle crust may be associated with volcanic centers near Nemrut, in the west of the region. Additionally, another low-velocity layer (V-s = similar to 3.0 km/s) is observed in the upper crust around the Suphan Volcano. Also, V-p/V-s and Poisson's ratios are calculated for the study area. Their high values (V-p/V-s >= 1.85, and sigma >= 0.285) correspond to the partial melting of the lower crust in the region. According to velocity models obtained from the inversion results, the most important outcome is that the average variance of the Moho discontinuity (similar to 45 km) and lithosphere-asthenosphere boundary (similar to 90 km) under the seismograph stations can be interpreted as a likely part of the Arabian oceanic plate in relation to the collision zone.
Lake Van (Southeastern Turkey) Experiment: Receiver Function Analyses of Lithospheric Structure From Teleseismic Observations (Vol 13, Pg 700, 2020)
(Springer Basel Ag, 2020) Alkan, Hamdi; Cinar, Hakan; Oreshin, Sergey
The Lithospheric Structure Underneath the Circum Black Sea: Teleseismic Receiver Functions and Rayleigh Wave Phase Velocity Analysis
(Pergamon-elsevier Science Ltd, 2021) Alkan, Hamdi; Cinar, Hakan
In this paper, the lithospheric structure underneath the Circum Black Sea is investigated on the basis of teleseismic receiver functions and Rayleigh wave phase velocity analyses. A joint inversion of P- and S-wave receiver functions are performed by using an iterative algorithm, similar to the simulated annealing method. The array method is employed to obtain the Rayleigh wave inter-station phase velocity dispersion curves. The dataset recorded during 2010-2020 by sixteen broadband stations, operated by several seismological agencies, includes more than 450 events (Mw >= 5.8). The Moho depth increases northward from 35 km to 40 km along both coastlines. Following the velocity models, it is suggested that the crust structure beneath the stations is the continental type. The lithosphere-asthenosphere boundary ranges from similar to 90 km to similar to 120 km from the east coast of the East Black Sea basin, while it is low-sloping (from similar to 100 km to similar to 93 km) for the west coast of the West Black Sea basin, and it is nearly 90 km along with the Pontides. The average P-wave and S-wave velocities inferred from the models indicate the continental lithosphere around the Black Sea coastal region. According to these results, tectonically, it points out that the southward subduction exists in the eastern part of the East Black Sea region beneath the Pontides, while it is not observed in the westernmost Black Sea region.