1. Open University School of Environment, Earth and Ecosystem Sciences, , Milton Keynes MK7 6AA, UK
2. University of Bristol School of Earth Science, , Queens Road, Bristol, BS8 1RJ, UK
3. Tbilisi State University Institute of Geophysics, , 1/1M. Alexidze Street, 0171, Tbilisi, Georgia
4. Durham University Department of Earth Sciences, , Durham DH1 3LE, UK