1. Geosciences Rennes, UMR 6118, CNRS, Université de Rennes 1; Rennes France
2. Now at Los Alamos National Laboratory; Los Alamos New Mexico United States
3. Department of Civil and Environmental Engineering, Massachusetts Institute of Technology; Cambridge Massachusetts USA
4. IPR, Université de Rennes 1-CNRS, UMR 6251; Rennes France