3GRAINS: 3D Gravity Interpretation Software and its application to density modeling of the hellenic subduction zone

3GRAINS is a program, which allows interactive modeling of a density structure with respect to given gravity anomalies. It is written in standard C++C++ with the use of Qt Library. It has been compiled on the Linux platform, but it can be also compiled under the Windows environment. The program uses rectangular prisms to calculate the gravity effect of a model. Blocks used in interpretation can have different size; it is useful to model shallow layers with higher resolution than the deep ones. Gravity stations used in modeling can have arbitrary distribution. The software uses the altitude of the station to compute its anomaly. The gravity effect of each block for each station is calculated once and saved together with the modeled structure. A major objective was to reduce the amount of memory used by the program. Thanks to the applied compression algorithm, high-resolution density structures can be accommodated by an average modern computer. The absolute accuracy of modeled anomalies depends on scaling of the problem. The relative error does not exceed 0.001. A very important characteristic of the software is a simplicity in creating and changing of the models by means of the graphical user interface. The three-dimensional density structure can be browsed in 2D cross-sections on one horizontal and two perpendicular vertical planes. The program offers a number of export options. Numerical models as well as different cross-sections and layer boundaries can be saved to a file for further visualization of the results.As an example a density model of the Hellenic Subduction Zone is presented. The model reflects a very complicated tectonic situation of the region and includes main tectonic units: seawater, sediments, crust and upper mantle. Horizontal density distributions within each unit allow recognition of different types of the lithosphere.