Vertical crustal movements from differential tide gauge observations and satellite altimetry in southern Italy

Our goal is to determine vertical crustal movement rates from tide gauge and satellite altimetry measurements. Tide gauges measure sea level, but as they are fixed to the crust, they sense both sea surface height variations and vertical crustal movements. The differential sea level rates of sufficiently nearby stations are a good means to determine differential crustal movement rates, when sea level height variations can be assumed to be homogeneous. Satellite altimetric measurements determine sea surface height variations directly and can be used to separate the crustal signal from the sea surface height variations in tide gauge measurements. The correction of the tide gauge sea level rates for the sea surface height contribution requires collocation of the satellite pass and the tide gauge station. We show that even if this is not the case, the satellite altimetric observations enable correction of differential tide gauge rates for the effects of sea surface rate inhomogeneities. We apply the methodology to an area of broad scientific interest, due to its high seismic risk and its location as standpoint for a proposed major bridge connecting Sicily to the Italian mainland. We find that the Southern Calabria and the eastern Sicily tide gauges have a deficit in sea level increase of 1-2 mm/yr with respect to the north western Sicilian tide gauge. The satellite altimetric observations show that this differential movement must be caused by a tectonic component, because the sea surface rates are higher offshore eastern Sicily compared to offshore western Sicily. The satellite altimetric rates show that the sea surface rates are inhomogeneous in the Mediterranean and have larger amplitudes as we move away from the coast than immediately offshore. Our technique can be applied to any part of the world where tide gauge observations are available, because satellite altimetric observations are global.