Browsing by Author "Sahin, Sakir"

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Crustal Poisson's Ratio Tomography and Velocity Modeling Across Tectono-Magmatic Lake Regions of Eastern Anatolia (Turkey): New Geophysical Constraints for Crustal Tectonics
(Pergamon-elsevier Science Ltd, 2019) Toker, Mustafa; Sahin, Sakir
The tomography of the Poisson ratio structures based on the Vp/Vs ratio provides much tighter constraints on the crust and uppermost mantle than Vp or Vs alone. In this study, high-resolution crustal tomography beneath tectono-magmatic lake regions (Lakes Van and Ercek) of Eastern Anatolia (Turkey) was used to estimate Poisson's ratio from the Vp and Vs structures through the joint inversion analyses of the travel times of Vp and Vs using the station records of the permanent network of Kandilli Observatory and Earthquake Research Institute (KOERI, Turkey). The results of Poisson's ratio anomaly reveal distinct and substantial variations in the different tectonic units and show a prominent, continuous and dense high Poisson's ratio anomalies in the range of 0.27 to 0.30 for the lowermost crust/the uppermost mantle beneath the regions of Lakes Van and Ergek and comparably low average Poisson's ratio (similar to 0.23) anomalies for the brittle crust in several local areas. High Poisson's ratios (over similar to 0.29) in the basins of Lakes Van and Ercek and in the vicinity of their boundary faults appear to be due to partial melts. Low Poisson's ratios (similar to 0.23) near seismogenetic faults at similar to 14 km depth beneath the lake regions suggest weaker and more easily deformable crust. The high and low Poisson's ratio structures suggest a strongly fault-controlled and highly heterogeneous crustal architecture beneath the lake regions. High Poisson's ratios at variable depths suggest that the mafic content beneath the lake regions is higher than has ever been expected, thus indicating the existence of intrusive materials of possible mantle origin, interacting with faulted crustal rocks. The regional extent of the high Poisson's ratios (0.27-0.30) of the lowermost crust/the uppermost mantle proposes strong interaction of partial melts with active tectonics in the basins of Lakes Van and Ercek. Key constraints from the estimated Poisson's ratios and V-anomalies indicate the diverse and interactive nature of crustal deformations beneath the lake regions where the underplating of the mantle magma scenario is proposed to exert a key control on tectono-magmatic evolution of the regions of Lakes Van and Ergek and serves as a working hypothesis for further studies.
Upper-To Mid-Crustal Seismic Attenuation Structure Above the Mantle Wedge in East Anatolia, Turkey: Imaging Crustal Scale Segmentation and Differentiation
(Elsevier, 2022) Toker, Mustafa; Sahin, Sakir
Multi-frequency P-and S -wave attenuation tomography models of Lake Van area (East Anatolia) have been obtained by estimating coda-normalized wave spectra of 3027 local earthquakes (2.0 < Mw < 7.1). The 6998 waveforms sampled from surface to a depth of 25 km, and were recorded from 2004 to 2020 at seven broadband, three-component digital seismic stations operated by Kandilli Observatory and Earthquake Research Institute (KOERI). We adopted a two-point ray-bending method to trace rays in a 3-D velocity model. We applied the coda normalization (CN) method to P- and S-wave data sets. We inverted the spectral data with a multiple resolution seismic attenuation (MuRAT) approach to obtain final tomographic models. On average, high (low) attenuation corresponds to low (high) velocity anomalies. The P-and S -wave attenuation contrasts delimit four well-known geological zones. High frequency-short wavelength attenuation contrasts constrain the 5-km-deep zone of interaction between magma and sediments within the Lake Van basin. Low frequency-long wavelength attenuation anomalies mark the central section of Lake Van between depths of 10 km and 20 km. This zone coincides with a rigid stable shear zone overlying a possible weak-ductile lower crust, interpreted as a detachment. Both low (5-15 km depth) and high attenuation (> 20 km) anomalies mark the area of maximum seismic energy release during the Van event. Their contrast highlights the maximum seismogenetic depth above weak-warm, unstable materials. High attenuation in the SE-part of the Lake Van area coincides with a large hydrothermal and/or magmatic folding-overthrusting, interpreted as a suture-metamorphic complex between depths of 10-15 km. Other minor high-attenuation zones deeper than 5 km focus on complex shear zones in the area damaged by the Van event. The paired attenuation structure of the Lake Van area appears linked to multiple tectonic processes of crust-magma interaction better constrains subsurface segmentation structures and differential deformation types at upper-middle crustal depths.