A first GPS measurement of vertical seafloor displacement in the Campi Flegrei caldera (Italy)

Abstract

This study shows how the GPS technique can be utilized for seafloor displacement measurements and improved the survey control infrastructure in Campi Flegrei caldera, two thirds of which is submerged under the sea. In the Gulf of Pozzuoli, about 2.5 km from the coast where the sea depth is 97 m, a continuous GPS station (CFB1) has been installed since the end of 2011 on the top of a elastic-beacon buoy, rigidly connected by a steel cable to the ballast on the sea bottom. We investigate the use of GPS data to estimate the vertical displacement of the seafloor under the buoy. The GPS data were processed in kinematic mode and the vertical component of the measure- ments was corrected for the errors due to the horizontal motion of the buoy induced by wind and sea currents. We report here the results for approximately 17 months of continuous GPS data acquisition, and we show, for the first time, a measure of vertical displacement of the seabed in the Gulf of Pozzuoli. From January 2012 to May 2013, the seafloor uplifted by about 3–4 cm. The similarity of the pattern of the CFB1 time-series compared to the permanent GPS stations of the NeVoCGPS network located onshore is remarkable, evaluation of the Pearson's correlation coefficient between these stations and CFB1 indicates that the stations are measuring the same phe- nomenon. This result is important, because all models of evolution of bradyseism in the Campi Flegrei caldera are based on the interpretation of measures only on the emerged part of the caldera, without use of any measures to date in the Gulf of Pozzuoli. The methodology shown in this paper is reliable over time and economical, compared to other systems of measurement of marine geodesy. The major limitation is the depth of the sea, confining this technique to the shallow water, up to 100 m depth. However, a large part of the submerged Campi Flegrei caldera is shallower than 100 m, so geodetic monitoring by means of GPS buoys at several sites in the Gulf of Pozzuoli would allow to extend interpretative models to the entire caldera, submerged and emerged.