Simultaneous inversion of surface deformation and gravity changes by means of extended bodies with a free geometry: Application to deforming calderas

Changes in gravity and/or surface deformation are often associated with volcanic activity. Usually, bodies with simple geometry (e. g., point sources, prolate or oblate spheroids) are used to model these signals considering anomalous mass and/or pressure variations. We present a new method for the simultaneous, nonlinear inversion of gravity changes and surface deformation using bodies with a free geometry. Assuming simple homogenous elastic conditions, the method determines a general geometrical configuration of pressure and density sources. These sources are described as an aggregate of pressure and density point sources, fitting the whole data set (given some regularity conditions). The approach works in a growth step-by-step process that allows us to build very general geometrical configurations. The methodology is validated against an ellipsoidal body with anomalous pressure and a parallelepiped body with anomalous density, buried in an elastic medium. The simultaneous inversion of deformation and gravity values permits a good reconstruction of the assumed bodies. Finally, the inversion method is applied to the interpretation of gravity, leveling, and interferometric synthetic aperture radar (InSAR) data from the volcanic area of Campi Flegrei (Italy) for the period 1992-2000. For this period, a model with no significant mass change and an extended decreasing pressure region satisfactorily fits the data. The pressure source is located at about similar to 1500 m depth, and it is interpreted as corresponding to the dynamics of the shallow (depth 1-2 km) hydrothermal system confined to the caldera fill materials.