1. Research Associate, Dept. of Civil and Environmental Engineering, Portland State Univ., Portland, OR 97201.
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Portland State Univ., Portland, OR 97201 (corresponding author).
3. Geophysical Consultant, MeerCAT Geophysics and Honorary Senior Lecturer, School for Geosciences, Univ. of Witwatersrand, Johannesburg 2000, South Africa.