1. Department of Earth Sciences; Durham University; Durham; UK
2. Department of Geography and Geology; University of Copenhagen; Denmark
3. Department of Earth Science and Engineering; Imperial College; London; SW7 2AZ; UK
4. U.S. Geological Survey; Menlo Park; California; 94025; USA
5. Department of Geophysics; Colorado School of Mines; Golden; Colorado; 80401; USA
6. Department of Physics of the Earth; Sankt-Petersburg State University; Sankt-Petersburg; Russia