1. U.S. Geological Survey, Denver Federal Center, PO Box 25046, MS 966, Denver, Colorado 80225, cbworden@usgs.gov
2. Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305‐4020
3. Stanford University, Stanford, California 94305‐4020
4. Also at Department of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand.