1. NORSAR; Gunnar Randers vei 15; N-2007 Kjeller; Norway
2. Géosciences Environnement Toulouse CNRS UMR 5563; Observatoire Midi-Pyrénées (OMP), Université Paul Sabatier; 14 avenue Édouard Belin, 31400 Toulouse; France
3. Department of Geosciences and Program in Applied and Computational Mathematics; Princeton University; Princeton; NJ 08544; USA