[1] GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
[2] National Geophysical Research Institute, Uppal Road, Hyderabad-500007, India
[3] Fachbereich für Geowissenschaften, Freie Universität Berlin, Berlin, Germany
[4] Institute of the Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, 128810 Moscow, Russia