On the seafloor horizontal displacement from cGPS and compass data in the Campi Flegrei caldera

Seafloor deformation monitoring is now performed in the marine sector of the Campi Flegrei volcanic area. MEDUSA infrastructure consists of 4 buoys at depths of 40-96m equipped with cGPS receivers, accelerometers and magnetic compasses to monitor buoy status and a seafloor module with a bottom pressure recorder. We study the seafloor deformation in the caldera. Previously we show that cGPS onland network and MEDUSA timeseries for the years 2017-2020 are in agreement with the deformation predicted by a Mogi model describing the observed deformation of an active volcano. Only for buoy A data differ significantly from model, at 6.9sigma and 23.7sigma for the horizontal speed (v) and direction. We devised a new method to reconstruct the sea bottom displacement including cGPS and compass data. The method, applied to buoy A and validated also on C, uses compass data to correct cGPS positions accounting for pole inclination. Including systematic errors, the internal consistency, within 3sigma (2sigma) for the speed (angle), between the results derived for different maximum inclinations of the buoy pole up to 3.5deg shows that the method allows to significantly reduce the impact of the pole inclination which can alter the estimation. We find good convergence of the velocity and deformation angle for increasing values of the buoy pole inclination. We found v=3.521+-0.039(stat)+-0.352(syst)cm/yr and an angle -115.159+-0.670(stat)+-7.630(syst)deg. The relative impact of potential systematics (statistical) effects increases (decreases) with cutoff. Our analysis gives v consistent with Mogi at 5.2sigma(stat) or 0.5sigma(stat and syst), and a deformation angle consistent at 4.3sigma(stat) or at 0.3sigma. The module of the vectorial difference between v from the data and Mogi diminishes by a factor 7.65+-1.23(stat) or +-5.78(stat+syst) compared with previous work. Potential improvements are discussed.