We described the first results of an on-going study of absolute gravity changes after the 17 August 1999 Izmit earthquake in Marmara region. Repeated absolute gravity measurements were carried out six stations with an A10 absolute gravimeter from 2009 to 2011 in the region. A gravimetric calibration baseline (of the range of about 415 milliGal (mGal), 1 mGal=10−5 ms−2) was established in the region for the purposes of the calibration of the relative gravimeters. The absolute gravity measurements, repeated twice a year (October, June), can resolve gravity changes with a precision better than 5 microGal (μGal)/yr interval.

Turkish regional geoid models have been developed by employing a reference earth gravitational model, surface gravity observations and digital terrain models. The gravimetric geoid models provide a ready transformation from ellipsoidal heights to the orthometric heights through the use of GPS/leveling geoid heights determined through the national geodetic networks. The recent gravimetric models for Turkish territory were computed depending on OSU91 (TG-91) and EGM96 (TG-03) earth gravitational models. The release of the Earth Gravitational Model 2008 (EGM08), the collection of new surface gravity observations, the advanced satellite altimetry-derived gravity over the sea, and the availability of the high resolution digital terrain model have encouraged us to compute a new geoid model for Turkey. We used the Remove-Restore procedure based on EGM08 and applied Residual Terrain Model (RTM) reduction of the surface gravity data. Fast Fourier Transformation (FFT) was then used to obtain the residual quasigeoid from the reduced gravity. We restored the individual contributions of EGM08 and RTM to the whole quasi-geoid height (TQG-09). Since the Helmert orthometric height system is adopted in Turkey, the quasi-geoid model (TQG-09) was then converted to the geoid model (TG-09) by making use of Bouguer gravity anomalies and digital terrain model. After all we combined a gravimetric geoid model with GPS/leveling geoid heights in order to obtain a hybrid geoid model (THG-09) (or a transformation surface) to be used in GPS applications. The RMS of the post-fit residuals after the combination was found to be ± 0.95 cm, which represents the internal precision of the final combination. And finally, we tested the hybrid geoid model with GPS/leveling data, which were not used in the combination, to assess the external accuracy. Results show that the external accuracy of the THG-09 model is ± 8.4 cm, a precision previously not achieved in Turkey until this study.

Taking place between North Anatolian and East Anatolian mega shear zones, neotectonics of seismically less active Central Anatolia is often regarded as tectonic escape or extrusion tectonics. Although previous published GPS studies dating back to early 1990’s report coherent rotation, they were mostly focused on seismically more active and more populated Western Anatolia and lack spatial resolution in quantifying second-order tectonic structures within the central area, such as Tuz Golu Fault Zone, Central Anatolia Fault Zone comprising Ecemis Fault and Erciyes Fault, Ezinepazari Fault, related basins and associated processes. However, new dense GPS velocity field of Central Anatolia exhibits systematic local patterns of internal deformation inconsistent with either coherent rotation or translation. Velocity gradients computed along rotation profiles of Central Anatolia show nearly smooth westward increments which can not be explained through a simple rotation/translation of Central Anatolia Basin. Moreover, estimating and removing a rigid-body rotation represented by an Euler Pole computed from sites lying in the middle absorbs the velocity discrepancies between the Eastern and Western part of Central Anatolia down to a few millimetres leaving out systematic residuals. Upon completion of Turkish National Fundamental GPS Network (TNFGN) in 2002, further observations were carried out in Central Anatolia which result in a velocity field of unprecedented spatial density with average inter-station distance of 30-50 km. Also, the data from particular stations of Turkish National Permanent GPS Network (TNPGN) contributed to the analyses. We computed horizontal velocity field with respect to ITRF2000, to Eurasia, and to a computed Anatolia Euler Pole as well. Two distinct models of Anatolia neotectonics, microplate and continuum deformation were tested through rigid-body Euler rotations and strain analysis, respectively. Results show that decomposition of the Eurasia-fixed velocity field into rigid rotations and residuals reveals systematic residuals up to 5 mm/yr with respect to a computed best-fit Euler Pole located at 31.6820N±0.05, 31.6130E±0.02 with 1.3800/Myr ±0.01 rate. Relative velocities computed along rotation paths exhibit westward increasing linear gradients of 0.7 mm to 1.3 mm per 100 km depending on the latitude which is mechanically inconsistent with the assumptions of a coherent transport or rotation due to an extrusion in the east. Moreover, strain analysis results show that up to 100 nanostrain/yr E-W extension rates are observed along approximately N-S striking faults within the region from west of Karliova to Isparta Angle which is another indication of partitioned extensional strain across the Central Anatolia. On the other hand, compressional strains were obtained near eastern branch of Isparta Angle, Tuz Golu and southern Anatolia. In this study, we provide new quantitative results about the fact that deformation in Central Anatolia is not uniform and possible driven by extension through slab pull and/or suction in west-southwest and compression in the south rather than coherent rotation and/or translation/transport of Anatolia driven by an extrusion process in the east.

A new Earth Gravitational Model (EGM08) to degree 2160 has been released to IAG’s EGM Evaluation Group. In this study, we evaluate EGM08 Tide Free Model by using regional gravity, quasi-geoid height and GPS/leveling data. The EGM08-derived quantities are compared with (1) the GPS/leveling quasi-geoid heights, (2) an existing GPS/leveling fitted regional quasi-geoid model (TG03), and (3) the surface gravity anomalies in Turkey. The differences between observed/computed and EGM08-derived quantities are investigated. The mean value and standard deviation of the differences between EGM08 derived and observed quantities are found to be -88.8 cm and 24.2 cm for GPS/leveling height anomalies, 27.1 cm and 75.3 cm for TG03 quasi-geoid heights, and 2.8 mGal and 17.1 mGal for surface gravity anomalies. As Turkish proprietary data were not used in EGM08 computations this work is believed to be an external check for EGM08.