Volcanic structures investigation through SAR and seismic interferometric methods: The 2011–2013 Campi Flegrei unrest episode

Observations from satellites provide high-resolution images of ground deformation allowing to infer deformation
sources by developing advanced modeling of magma ascent and intrusion processes. Nevertheless, such models
can be strongly biased without a precise model of the internal structure of the volcano. In this study, we jointly
exploited two interferometric techniques to interpret the 2011–2013 unrest at Campi Flegrei caldera (CFc). The
first is the Interferometric Synthetic Aperture Radar (InSAR) technique, which provides highly-resolved spatial
and temporal images of ground deformation. The second is the Ambient Noise Tomography (ANT), which images
subsurface structures, providing the constraints necessary to infer the depth of the shallow source at CFc (between
0.8 and 1.2 km). We applied for the first time a tool to delineate the deformation source boundaries from
the observed deformation maps: the Total Horizontal Derivative (THD) technique. The THD processes the vertical
component of the ground deformation field detected through InSAR applied to COSMO-SkyMed data. The
patterns retrieved by applying the THD technique show consistent spatial correlations with (1) the seismic
group-velocity maps achieved through the ANT and (2) the distribution of the earthquakes nucleated during the
unrest at ~1 km. High-velocity anomalies, the retrieved geometrical features of the deformation field, and the
spatial distribution of seismicity coincide with extinct volcanic vents in the eastern part of the caldera (Solfatara/
Pisciarelli and Astroni). Such a coincidence hints at a significant role of the extinct plumbing system in either
constraining or channeling the eastward propagation of magmatic fluids. Here, we demonstrated that a joint
analysis of the InSAR patterns, seismic structures, and seismicity allows us to model in space and time the
characteristics and nature of the shallow deformation source at CFc. Using published literature, we show that the
effects of structural heterogeneities at shallow depths may have a more significant early-stage impact on the
evolution of the surface displacement signals than deeper magmatic sources: these secondary structural effects
may produce local amplification in the deformation records which can be mistakenly interpreted as early signals
of impending eruptions. The achieved results are particularly relevant for the understanding of the origin of
deformation signal at volcanoes where magma propagation within sills is expected, as at CFc.

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