1. Department of Oceanic and Atmospheric Sciences, Oceanography Administration Building 104, Oregon State University, Corvallis, OR 97331, USA.
2. Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario, Canada K1A 0Y3
3. U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA.
4. Institute for Geophysics, University of Texas, 8701 North Mopac Boulevard, Austin, TX 78759, USA.