1. Institute of Geophysics and Tectonics, School of Earth and Environment, University of LeedsLeeds LS2 9JT, UK (e-mail: butler@earth.leeds.ac.uk)
2. Amerada Hess, The Adelphi Building1-11 John Adam Street, London WC2N 6AG, UK
3. Ries-Coward Associates Ltd70 Grosvenor Road, Caversham, Reading RG4 5ES, UK