Lithospheric structure of western Anatolia and the Aegean Sea using GOCE-based gravity field models

Western Anatolia and the Aegean Sea are one of the most active seismic and deformation zones between the Eurasian and African tectonic plates. Due to its tectonic features, there have been severe earthquakes in western Anatolia and the Aegean Sea, both in historical and in the instrumental period. It is important to determine the effect of the stress-load effect of these earthquakes on the lateral elements of western Anatolia and the effect of gravitational loads. The best fitting GOCE gravity fi eld model combined with EGM2008 is used to determine the lithospheric structure of the study area at large scale. The comparison of GOCE gravity fi eld models with ground truth terrestrial gravity data, reducing the omission error of the models using EGM2008 (spatial scales ~100 km down to 10 km) and ERTM2160 models (~10 km to ~250 m), suggests that the GOCE-DIR4 model performs best in the Izmir region, used as an assessment area located in the westernmost part of the western Anatolia region, with a root mean square error of ~8.5 mGal. The free-air gravity anomaly differences between the GOCE-DIR4 model and EGM2008 up to degree and order (d/o) 240 reaches up to 14 mGal indicating the added value of using GOCE models in western Anatolia and the Aegean Sea. Different from previous geophysical studies in the region that used the planar Bouguer gravity anomalies computed from land gravity data alone, in this study, 3D Moho depths and 3D lithosphere-asthenosphere boundary (LAB) depths and effective elastic thickness values are calculated from the spherical Bouguer gravity anomalies both on land and marine areas. The spherical Bouguer anomaly is obtained by subtracting the EARTH2014 topographic/bathymetric model derived gravity effect of the topography from those of the GOCE-DIR4 plus EGM2008 combined global gravity fi eld model. The Moho depth that changes between 25-41 km and the isostatic Moho depth that changes between 19-52 km are computed to investigate the compensation conditions. The LAB depth is found to be between 129-145 km. Another parameter that controls the lithospheric structure, temperature variations of LAB depth are also calculated using empirical equations and found to be between 1309-1316° C. The effective elastic thickness values calculated by LithoFLEX software are in the range of 4 to 20 km in the study area. As a result, the spherical Bouguer anomalies, the Moho depth, the LAB depth and the LAB temperature in western Anatolia determined at 5’ spatial resolution for the fi rst time using a GOCE and EGM2008 combined gravity field model to investigate the geodynamic effect of the Hellenic Arc mechanism in comparison with earthquake distributions.