Constraining the Source of Unrest at Campi Flegrei (Italy) Using Geodetic And Micro-Gravity Data.

Campi Flegrei is an active volcanic caldera, 12-14 km in diameter, immediately west of Naples, Southern Italy. Geological and historical records indicate that Campi Flegrei has been the site of intense uplift and subsidence phenomena. A historical period of uplift has occurred within Campi Flegrei from mid-1982 to 1984. Peak surface deformation reached about 1.8 m and the caldera experienced significant seismic activity. Since January 1985, the caldera floor is slowly sinking. Several mechanisms have been proposed to explain the caldera unrest: hot fluid migration, intense magma degassing, or the intrusion of a magma body followed by fluid removal. Combined geodesy and gravity measurements can be used to infer the density of the intrusive fluids, and can better constrain the deformation source. Given the significant density difference between silicate melts ( ∼2500 kg/m³) and hydrothermal fluids ( ∼1000 kg/m³), it is reasonable to use density estimates from gravity to distinguish between these two possible sources of caldera unrest. We determine the location and geometry of the inflation source inverting leveling and trilateration measurements collected between 1980 and 1983, assuming a vertical prolate ellipsoidal source, a spherical source and a penny shape crack source. The two sources that best fit the geodetic data are a penny shape crack (χ²=64, 4.2 km deep beneath the town of Pozzuoli) or a point source (χ²=70, 3.1 km deep beneath the town of Pozzuoli). To resolve the density of the intrusion, we invert leveling and micro-gravity data from 01/1982 to 06/1984. Preliminary results, using a spherical source, indicate an inflation source 3.0 km deep, with a volume change of 0.066 km³ and a density of 2350 kg/m³. On the other hand, a point source 3.1 km deep beneath the town of Pozzuoli fits geodetic and gravity measurements from the 1989-91 subsidence period (volume decrease = 0.005 km³), but provides an unrealistic estimate of the body density. Future work will include the use of sill-like sources to improve the density estimates.