Constraints on vertical crustal motion for long baselines in the central Mediterranean region using continuous GPS

GPS-determination of vertical crustal velocity relative to a globally defined reference frame is limited by the accuracy of the reference frame origin and scale. However, the precision of relative vertical rates at regional-distance scales remains largely unexplored. We investigated the precision of regional-scale vertical velocity estimates using data from seven continuously operating GPS stations located in the central Mediterranean. The stations form a network of aperture ∼ 500 km and each has been in operation for at least 5 yr. We used published constraints on recent crustal uplift inferred from late Quaternary shoreline data to aid in the definition of a local vertical reference frame for GPS rates to facilitate an assessment of GPS vertical precision. We explored different methods of defining the local reference frame. The simplest method involves the estimation of a single scalar translation parameter. Adjusting the reference frame by this simple method yields agreement between CGPS and Holocene shoreline data sets at the level of 0.2 to 0.4 mm/yr. Relative rates determined using CGPS are also consistent with tide gauge records and sea level determined by satellite altimetry at the level of about 0.2 mm/yr. These results suggest that regional-scale relative velocity precision well below the mm/yr-level is now possible using continuous GPS.