Tide gauge and GPS measurements of crustal motion and sea level rise along the eastern margin of Adria

We use observations from tide gauges and colocated continuous GPS (CGPS) stations to investigate crustal deformation and sea level changes along the eastern margin of the Adriatic Sea. We develop a new method to separate common-mode relative sea level from spatially variable signals. Precise vertical crustal motions determined by CGPS allow us to further separate relative sea level into absolute sea level changes and crustal motions with respect to a local Central Mediterranean-fixed GPS-defined reference frame. From the tide gauge data, we find fairly uniform relative sea level rise along the coast, with mean rate of 0.84 ± 0.04 mm/yr and weighted RMS variation about this mean of 0.2 mm/yr. This rate is a factor of 2-4 lower than estimates for global average sea level rise. In contrast, vertical motion of coastal rocks determined by CGPS vary appreciably from an average of - 1.7 ± 0.4 mm/yr in southern Adria to 0.0 ± 0.4 mm/yr in northern Adria. This difference in crustal motion between the northern and southern regions is independent of our ability to separate sea level from crustal motion, and may be explained by crustal strain associated with an active thrust fault accommodating southern Adria microplate convergence with Eurasia. Enigmatically, the combination of tide gauge and CGPS measurements shows that absolute sea level relative to the GPS-determined reference frame varies by as much as ∼1.8 mm/yr along the Croatian coast in such a way that the relative sea level remains roughly constant. There are several potential explanations for this result deriving from ocean, atmosphere, and solid Earth dynamics.