1. GFZ German Research Centre for Geosciences, Potsdam, Germany. E-mail: gesap@gfz-potsdam.de
2. Institute of Geosciences, University of Potsdam, Potsdam, Germany
3. National Observatory of Athens, Institute of Geodynamics, Athens 11810, Greece
4. Ruhr University of Bochum, Institute of Geology, Mineralogy and Geophysics, Germany