Browsing by Author "Oreshin, Sergey"

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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