1. Department of Civil Engineering, University of Colorado Denver, Campus Box 113, P.O. Box 173364, Denver, Colorado 80217-3364, USA
2. Earth Sciences Division, Lawrence Berkeley National Laboratory, Mail Stop 74R316C, 1 Cyclotron Road, Berkeley, California 94720, USA
3. Department of Electrical Engineering, University of Colorado Denver, Campus Box 110, P.O. Box 173364, Denver, Colorado 80217-3364, USA