1. Institute of the Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya 10, 128810 Moscow, Russia.
2. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany.
3. Freie Universität Berlin, Geophysik, Malteser Strasse 74-100, 12249 Berlin, Germany.