1. Berkeley Seismological Laboratory, 209 McCone Hall, Berkeley, CA 94720, USA.
2. Department of Geology, University of Maryland, College Park, MD 20742, USA.
3. Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France.
4. Institut de Physique du Globe de Paris, 1 rue Jussieu, 752382 Paris Cedex 05, France.