1. S. C. Cande is at the Scripps Institution of Oceanography, La Jolla, CA 92093-0215, USA.
2. C. A. Raymond is at the Jet Propulsion Laboratory, Mail Stop 183-501, California Institute of Technology, Pasadena, CA 91109-8099, USA.
3. J. Stock is at the Seismological Laboratory, Mail Stop 252-21, California Institute of Technology, Pasadena, CA 91125, USA.
4. W. F. Haxby is at Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA.