In the present paper, we analyse ground tilt and seismicity at Campi Flegrei caldera and Ischia Island, two volcanic areas located in the south of Italy. These areas have been well studied for many years from a petrological, volcanological and geophysical view point. Moreover, due to the high seismic and volcanic risk for the populations living there, they are continuously monitored by networks of geophysical and geochemical sensors. We summarize the most important results that we obtained so far, concerning the observations of relationships between seismic activity and ground tilt anomalies, focusing on the time interval 2015–2018. First, we present a detailed description of the tiltmeter and seismic networks in both the investigated areas, as well as their development and improvement over time that has enabled high quality data collection. From the joint analysis of the seismic and borehole tiltmeter signals, we often notice concurrence between tilt pattern variations and the occurrence of seismicity. Moreover, the major tilt anomalies appear to be linked with the rate and energy of volcano-tectonic earthquakes, as well as with exogenous phenomena like solid Earth tides and hydrological cycles. The analysis that we present has potential applicability to other volcanic systems. Our findings show how the joint use tilt and seismic data can contribute to better understanding of the dynamics of volcanoes.

We analyse tiltmeter time series recorded from April 2015 to March 2019 at three borehole instruments installed at Campi Flegrei caldera (Italy). We evaluate the crustal response in terms of ground tilting to external excitations of medium/long-period tidal constituents by applying a polarization analysis. The azimuths of the tilt vectors show well-defined polarization directions and the ground tilting planes oscillate with the periodicity of the corresponding tidal constituents. For two of the three tiltmeters, the average ground oscillation pattern related to the monthly Mm and fortnightly Mf constituents show seasonal variations, which can be ascribed to rainfall-induced tilting. In addition, for the same two instruments, a clear seasonal amplitude modulation of the diurnal S1 constituent appears in the time series, revealing the occurrence of site thermoelastic effects. The results indicate that the tidal tilting is mainly controlled by the local stress field distribution and rheology; in addition, seasonal exogenous factors like rainfalls play a role in modulating the ground deformation.

Improvement and development of the tiltmetric monitoring networks of Neapolitan volcanoes

One of the strategies to detect the precursors of an eruption is to define the background dynamical state of a volcano for a prompt recognition of deviations from the basic condition. Mt. Vesuvius (Italy), currently in a quiescent state, is one of the most monitored volcanoes in the world, inciting multidisciplinary advanced studies. Hence an understanding of the links among the different monitored parameters is mandatory. In recent decades the joint analyses of ground tilt and seismicity have added to the understanding of the volcano’s activity. In this paper, we outline the first steps towards a comprehension of the link between Mt. Vesuvius earthquakes and co-seismic ground tilt, after excluding the contribution of other external forces acting on the ground, such as tides, landslides or exceptional meteorological phenomena. We used the seismicity with a duration magnitude ≥ 2.0 recorded at Mt. Vesuvius in the period 2018–2020 to estimate the source parameters and to calculate the associated static displacement. Then, we compared the ground inclination retrieved from the estimated seismic deformation with the long-term ground motion trend measured by tiltmeters. We found that in most cases the two vectors have a comparable size and direction.