1. Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185 U.S.A., clikoch@sandia.gov
2. Also at University of Arizona Gould Simpson, 1040 E 4th Street, Tucson, Arizona 85721 U.S.A.
3. Roy M. Huffington Department of Earth Sciences, Southern Methodist University, P.O. Box 750395, Dallas, Texas 75275‐0395 U.S.A.