1. F. M. Phillips, M. G. Zreda, M. A. Plummer, Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA.
2. L. V. Benson, U.S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA.
3. D. Elmore and P. Sharma, Physics Department, Purdue University, West Lafayette, IN 47907, USA.