Determination of the time dependent microgravity area changes occurring as a result of the august 17, 1999 İzmit earthquake in the Marmara Region.

The objective of this study is the measurement and analysis of the time dependent microgravity area changes occurring as a result of the August 17, 1999 İzmit Earthquake in the Marmara Region.The microgravity data for this purpose were obtained by time dependent campaign measurements accomplished between the years 2006-2011 within the scope of the TUBITAK, MRC, Earth and Marine Sciences projects ?TÜRDEP - multi-disciplinary earthquake research in high risk geostrategic regions of Turkey with different tectonic regimes and investigation of vertical displacements of the earth?s crust in the Marmara Region by absolute gravity and GPS?.Round trip measurement technique was employed in order to control the drift errors. Instrumental and environmental effects that may be included in the measurement values are corrected and reduced according to the microgravity area theory and the scope of this study. After the correction and reduction operations, scaling is also performed in order to associate measurements performed at different days. Correction, reduction and scaling operations are accomplished using the GRAVAP software. Microgravity time series were created for each measurement location from the microgravity area values determined during the campaign.Weighted least squares regression was applied to the time series data obtained and annual micro gravity velocity values were obtained. It was decided to analyze the obtained microgravity data by viscoelastic modeling. For this purpose, a numerical model was created to compute the changes in the microgravity area and the geoid, constituting the source of time dependent deformations occurring on the earth. By the numerical modeling approach, it is significant to determine the suitable modeling parameters at the optimum time with the required accuracy. For this purpose, reference analytic solutions were compared against the numerical modeling solutions. The results showed that the numerical model is accurate and reliable.Modeling operation was primarily done in the Marmara Region based on microgravity area changes and tests were performed in order to determine the viscosity values of the low crust of ductile structure and the upper mantle. After the tests, the viscosity value was determined as approximately 5x1018 Pa.s for semi-infinite viscoelastic medium. After the modeling operation, the harmony was investigated between the model and the measured microgravity area changes in the Eastern Marmara and the Western Marmara regions. After this investigation, microgravity changes could be modeled in high harmony in the Eastern Marmara region and the events could be identified inducing the microgravity changes. Long term changes could be modeled in the western Marmara region but the amplitude values obtained from the model were lower than the measured data. It is expected that the amplitude difference between the model and the measured data in the Western Marmara region is either a result of insufficient modeling using only the deformations occurring after the 1999 earthquake or the lateral changes in the crustal structure of this region.