1. Professor, Dept. of Civil and Environmental Engineering, Colorado State Univ., Fort Collins, CO 80523 (corresponding author).
2. Graduate Student, Dept. of Civil and Environmental Engineering, Colorado State Univ., Fort Collins, CO 80523.
3. Assistant Professor, Dept. of Civil Engineering and Engineering Mechanics, Univ. of Arizona, Tucson, AZ 85755.
4. Associate Professor, Dept. of Physics, Colorado State Univ., Fort Collins, CO 80523.