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The tectonic regime of Eastern Anatolia is determined by the Arabia-Eurasia continent-continent collision. Several dynamic models have been proposed to characterize the collision zone and its geodynamic structure. In this study, change in crustal thickness has been investigated using gravity, magnetic and topographic data of the region. In the first stage, two-dimensional low-pass filter and upward analytical continuation techniques were applied to the Bouguer gravity data of the region to investigate the behavior of the regional gravity anomalies. Next the moving window power spectrum method was used, and changes in the probable structural depths from 38 to 52 km were determined. The changes in crustal thickness where free air gravity and magnetic data have inversely correlated and the type of the anomaly resources were investigated applying the Euler deconvolution method to Bouguer gravity data. The obtained depth values are consistent with the results obtained using the power spectrum method. It was determined that the types of anomaly resources are different in the west and east of the 40°E longitude. Finally, using the obtained findings from this study and seismic velocity models proposed for this region by previous studies, a probable two-dimensional crust model was constituted.

This paper presents a probable isostatic model of the East Anatolian Region, which lies in a belt of significant plate movements. Probable locations of the horizontal and vertical discontinuities in the crust structure were determined using the normalized full gradient (NFG) method. For the purpose of explaining the mechanism that supports topography corresponding to the crust thickness in the region, calculations of effective elastic thickness (T e ) were carried out initially by utilizing admittance and misfit functions. According to these results, the effective elastic thickness value obtained was less than the crust thickness, even though the isostatic model does not conform with the Airy model. Consequently, it was assumed that there could be problems beneath the crust. Hence, the NFG method was applied on the Bouguer gravity data of the region in order to investigate probable discontinuities in the crust structure. According to the NFG results, vertical structural transitions were observed at a depth ranging between 10 and 30 km, which begin immediately north of the Bitlis Zagros Suture Zone (BZSZ) and continue in a northerly direction. The relationship between the effective elastic thickness (T e ; 13 km in average as determined in the last stage), and the seismogenic zone in the region was investigated. If the T e value happens to be less then the crustal thickness, then one can say that there are problems in the crustal structure of the region similar to Eastern Anatolia. Indeed, when NFG results of the study area are examined, numerous vertical and horizontal discontinuities in the crust can be observed. These discontinuities, which correspond to low Bouguer gravity anomalies and shallow focal depth-earthquakes, are probably the source of the factors which rule the tectonic mechanism of the region.

The bottom of the magnetized crust determined from the spectral analysis of magnetic anomalies is generally interpreted as the level of the Curie point isotherm. The spectral analysis method to estimate the depth extent of magnetic sources was applied to the magnetic anomalies of Eastern Anatolia and compared with the tectonic regime and heat flow data in the region. The Curie point depth of eastern Anatolia ranges from 6 to 24 km. The computed Curie depths and the heat flow values derived by using Curie depths are consistent with the geological and the seismological findings of the region. Shallow Curie point depths and high heat flow values are seen between the Bitlis Suture Zone and the pontides. At the same time, this region might contain a thinner crust than expected.

Some test studies were performed for comparison of two travel-time inversion schemes for tomographic evaluation of crosshole ground-penetrating radar (GPR) data. The first scheme was a linearized inversion based on Tikhonov regularization (Method 1). In this scheme, ray tracing was not a part of the inversion algorithm and the Jacobian matrix was calculated by numerical differentiation. Travel-time calculations were performed by a finite-difference eikonal equation solver. Model velocity fields were updated by matrix inversion techniques using iterative conjugate gradient solvers. The inversion process was stabilized by a smoothness-constrained regularization. The second scheme was based on a ray tracing algorithm (Method 2) and velocities were updated by a simultaneous iterative reconstruction technique (SIRT) using both straight- and curved-ray approximations. The test studies included synthetic travel-time data sets generated from the models with various velocity distributions. Broyden’s update was implemented within Method 1 to expedite the calculation of the Jacobian matrix, and this greatly improved the computational performance. In the tests, the effect of the regularization parameter on the models from Method 1 was examined. Also, how the straight-ray and curved-ray assumptions affected the solutions from Method 2 was illustrated. The effect of the initial velocity distribution on the resulting tomograms was exemplified by the solutions from both Method 1 and Method 2. The velocity tomograms from Method 1 were characterized by smaller travel-time residuals, Euclidean distances and lower errors in the velocity of cells. Also, the convergence rates of the solutions from Method 1 were faster than those from Method 2. Method 1 better imaged the zones with the high velocity contrast than Method 2, and both methods produced similar velocity distributions within the zones with low velocity contrast. Overall, Method 1 yielded better solutions compared to Method 2, and the curved-ray inversion generated relatively better results than the straight-ray inversion.