[1] Univ Grenoble 1, Inst Sci Terre, F-38041 Grenoble, France
[2] CNRS, F-38041 Grenoble, France
[3] Tohoku Univ, Grad Sch Sci, Dept Geophys, Sendai, Miyagi 9808578, Japan
[4] CNRS, Inst Phys Globe, F-75238 Paris 05, France
[5] MIT, Dept Earth Atmospher & Planetary Sci, Cambridge, MA 02139 USA