1. Dep. of Mathematics and Physics; Univ. of Roma Tre; Via della Vasca Navale 84-00146 Rome Italy
2. School of Engineering, Univ. of Guelph; 50 Stone Road East Guelph Ontario N1G 2W1 Canada
3. Dep. of Earth and Environmental Sciences; Univ. of Waterloo; 200 University Avenue West Waterloo Ontario N2L 3G1 Canada