1. Postdoctoral Fellow, Dept. of Engineering and Geology, Univ. of Chieti Pescara, 65127 Pescara, Italy (corresponding author).
2. Professor, Dept. of Engineering and Geology, Univ. of Chieti Pescara, 65127 Pescara, Italy.
3. Professor, Dept. of Structural Engineering, Univ. of California, San Diego, La Jolla, CA 92093.