Acknowledgements
This research has benefited from funding provided by the Italian Presidenza del Consiglio dei Ministri - Dipartimento della Protezione Civile (DPC) within the INGV-DPC 2007-2009 agreement (project S1).
We are grateful to all the persons working on regional and worldwide GPS permanent networks and database maintenance (ASI, EUREF, INOGS, RING, SOPAC and many other institutions, universities and agencies).