10 th of April 1987 seismic swarm: Correlation with geochemical parameters in Campi Flegrei Caldera (southern Italy)

International audienceA close relationship between geophysical activity (seismicity and ground deformation) and chemical changes in volcanic reservoirs has been proposed several times in active volcanic areas. In Campi Flegrei caldera, especially during the bradyseismic crisis which occurred between 1982-1984, this correlation was never clearly demonstrated because of the high rate of occurrence of earthquakes and the small number of gas samples. After at least two years of both geochemically and geophysically quiescent period, a swarm of 50 earthquakes, felt in the area of the Solfatara crater with 2.0 maximum magnitude, occurred on the 10th of April 1987. At about the same time (before and after), several geochemical parameters showed important changes in concentration. These include water vapour, nitrogen, hydrogen, methane and to a lesser extent hydrogen sulfide in fumarolic gases from Bocca Grande fumarole in the Solfatara crater and the radon content in water wells situated far from the swarm epicentral area. In our opinion, the processes causing the geochemical changes are linked to aseismic creeping mechanisms, which leads to an easier rising of fluids in fumaroles (H20, N2, H2 and CH4) and in the superficial water table (Rn). The subsequent seismicity could be related to consequent local stress accumulation on gas reservoir rocks induced by creeping