A Procedure for Analyzing Vertical Ground Deformation Anomalies in Active Volcanic Caldera during Unrest Phases

Collapse calderas often experience large up and down movements, which rarely give rise to eruptions. The pattern of vertical deformation has generally an elliptical geometry, very close to a circular one. A simple and powerful method to explore second order anomalies of the volcanic caldera deformation patterns can enlighten important structural features. This method is here presented and applied to the case study of Campi Flegrei caldera ground deformation, representing a typical example of such a quasi-circular geometry, with dominant ground uplift occurring since 1950. Despite a markedly radial dependence of the vertical displacement, with a very constant shape in the time, we explore in the paper the second order anomalies, with respect to a purely radial behavior. To this aim, we have used a procedure based on the polynomial fit of the vertical displacement data, assuming they only depend on the distance from the maximum uplift point (i.e. deformation center); then, we have obtained anomalies map by subtracting from the theoretical deformation so determined the true data. We then obtain very peculiar results, which put in evidence a sharp separation between a less uplifted zone and a more uplifted one than expected, along a NE-SW alignment which approximately define the most seismically active area, in which most of the largest magnitude earthquakes occur. This very peculiar feature is likely to represent a main volcano-tectonic structure in the area, which can be of leading importance for evaluating both the highest seismic hazard area and the most likely zone of possible eruptive vent opening. The results obtained here indicate that the proposed method can be useful to analyze unrest related hazards.