This is an electronic version of an article published in Geophysical Journal International: complete citation information for the final version of the paper, as published in the print edition of Geophysical Journal International, is available on the Blackwell Science Synergy online delivery service, accessible via the journal's Website at: http://www.blacksci.co.uk/GJI, or http://www.blackwell-synergy.com.
GJI1610881.pdf


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