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 ∼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.

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