Repeated absolute gravity measurements on a dense network at Campi Flegrei – a reliable tool for volcano monitoring

Abstract. Since 1981, relative gravity measurements have routinely carried\nout at the Campi Flegrei caldera, a densely populated area. The gravity\nnetwork also includes two absolute stations periodically measured with a\nlaboratory absolute gravimeter, which does not permit field measurements. At the end of 2014, the Osservatorio Vesuviano, Section of Napoli of the\nIstituto Nazionale di Geofisica e Vulcanologia (INGV-OV), acquired a\nportable absolute gravimeter that allows field operations on outdoor sites.\nTherefore, in 2015 a dense absolute gravity network was established in Campi\nFlegrei. This will permit an advanced approach for volcano monitoring. The\nnet consists of 36 stations, 34 of which located inside the caldera and\nplaced upon or very close to gravity stations belonging to the relative\nnetwork. Five surveys were carried out on June 2015, on February and\nNovember 2017, on October 2018 and on October 2019. The comparison with\nheight changes suggests that significant Δg are partly due to the\nuplift occurred over the same time intervals and mostly to shallow processes\nassociated to the dynamic of the local hydrothermal system. The comparison\nwith the gradients observed during the last large uplift (1982–1984) and the\nfollowing subsidence (1985–2003) confirms this observation. These results\nsuggest that the present activity may be due to a transient or pulsating\nphenomenon as the alternating recharge/discharge of fluids in the surface\nhydrothermal system. Gravity changes detected by absolute measurements are\nin good agreement with those obtained by relative ones, and confirms the\nfeasibility of this methodology for volcano monitoring. Finally, they also\nencourage replacing the relative networks with absolute ones, with all the\nconsequent advantages.