1. LMTG; Observatoire Midi-Pyrénées CNRS; 14 Avenue Edouard Belin,; F-31400; Toulouse; France
2. Department of Geophysical Sciences; University of Chicago; 5734 South Ellis Avenue,; Chicago; Illinois; 60637; USA
3. LST; Ecole Normale Supérieure; F-69364; Lyon Cedex 07; France
4. Institut de Physique du Globe de Paris; 4 Place Jussieu, Case 89,; F-75252; Paris Cedex 05; France
5. Mathematics and Computer Science Division; Argonne National Laboratory; 60439; 9700 South Cass Avenue,; Argonne; Illinois; USA