The Ground Deformation History of the Neapolitan Volcanic Area (Campi Flegrei Caldera, Somma–Vesuvius Volcano, and Ischia Island) from 20 Years of Continuous GPS Observations (2000–2019)

The Neapolitan volcanic area includes three active and high-risk volcanoes: Campi Flegrei caldera, Somma–Vesuvius, and Ischia island. The Campi Flegrei volcanic area is a typical example of a resurgent caldera, characterized by intense uplift periods followed by subsidence phases (bradyseism). After about 21 years of subsidence following the 1982–1984 unrest, a new inflation period started in 2005 and, with increasing rates over time, is ongoing. The overall uplift from 2005 to December 2019 is about 65 cm. This paper provides the history of the recent Campi Flegrei caldera unrest and an overview of the ground deformation patterns of the Somma–Vesuvius and Ischia volcanoes from continuous GPS observations. In the 2000–2019 time span, the GPS time series allowed the continuous and accurate tracking of ground and seafloor deformation of the whole volcanic area. With the aim of improving the research on volcano dynamics and hazard assessment, the full dataset of the GPS time series from the Neapolitan volcanic area from January 2000 to December 2019 is presented and made available to the scientific community.

Study of Surface Emissions of 220Rn (Thoron) at Two Sites in the Campi Flegrei Caldera (Italy) during Volcanic Unrest in the Period 2011–2017

The study concerns the analysis of 220Rn (thoron) recorded in the surface soil in two sites of the Campi Flegrei caldera (Naples, Southern Italy) characterized by phases of volcanic unrest in the seven-year period 1 July 2011–31 December 2017. Thoron comes only from the most surface layer, so the characteristics of its time series are strictly connected to the shallow phenomena, which can also act at a distance from the measuring point in these particular areas. Since we measured 220Rn in parallel with 222Rn (radon), we found that by using the same analysis applied to radon, we obtained interesting information. While knowing the limits of this radioisotope well, we highlight only the particular characteristics of the emissions of thoron in the surface soil. Here, we show that it also shows some clear features found in the radon signal, such as anomalies and signal trends. Consequently, we provide good evidence that, in spite of the very short life of 220Rn compared to 222Rn, both are related to the carrier effect of CO2, which has significantly increased in the last few years within the caldera. The hydrothermal alterations, induced by the increase in temperature and pressure of the caldera system, occur in the surface soils and significantly influence thoron’s power of exhalation from the surface layer. The effects on the surface thoron are reflected in both sites, but with less intensity, the same behavior of 222Rn following the increasing movements and fluctuations of the geophysical and geochemical parameters (CO2 flux, fumarolic tremor, background seismicity, soil deformation). An overall linear correlation was found between the 222−220Rn signals, indicating the effect of the CO2 vector. The overall results represent a significant step forward in the use and interpretation of the thoron signal.

Numerical Modelling of Injection-induced Seismicity at Campi Flegrei Caldera, Southern Italy

<p>Modelling volcanic processes at active volcanoes often requires a multidisciplinary approach, which adequately describes the complex and ever-dynamic nature of volcanic unrests. Campi Flegrei caldera (southern Italy) is an ideal laboratory where numerical modelling of injection-induced seismicity could be tested to match the observed seismicity. In the current study, thermal-hydraulic-mechanical (THM) effects of hot-water (fluid) injections were investigated to ascertain whether the observed seismicity (past and ongoing seismic swarms) could be quantitatively reproduced and modelled in isothermal or non-isothermal approximations. Fluid-flow modelling was carried out using a coupled TOUGHREACT-FLAC<sup>3D</sup> approach to simulate THM effects of fluid injections in a capped reservoir, where the sealing formation serves as a geological interface between supercritical reservoir and fractured shallow layers of the caldera. Results from previous seismic, deformation, tomographic and rock physics studies were used to constrain the model for realistic volcano modelling. The results indicated that fluid injections generated overpressure beneath the caprock and subjected it to different stress regimes at its top and bottom, and this prompted deformation. Thus, caprock deformation, triggered by injection-induced basal compressional forces and top extensional fractures, is a critical factor determining the required timing for pressure build-up and fault reactivation, and magnitudes of seismicity. Higher fluid injection rates and temperature contrasts, heterogeneity due to fault and its contrast with the host rock, and caprock hydraulic properties were among the identified secondary factors modulating fault reactivation and seismicity. Simulation results revealed that seismicity can be better modelled in isothermal (HM) approximations. A comparative study of the THM-modelled seismicity and 4-month-long (August 5<sup>th</sup> to December 5<sup>th</sup>, 2019) seismic monitoring data recorded at the Osservatorio Vesuviano showed that our model reproduced the magnitudes and depths (~2.5 Ms within 2 km) at the onset of the ongoing unrests on October 5<sup>th</sup>, 2019. However, the model could not adequately reproduce the highest magnitude (3.3 Ms at 2.57 km) seismicity on April 26<sup>th</sup>, 2020 observed since 1984 major unrests.</p><p> </p>

Offshore ground movements in the Campi Flegrei caldera during the last ~12 ka

<p>Appraisal of morphodepositional markers tied to ancient sea-levels in high-resolution seismic profiles together with geo-archaeological markers along the coast of the Pozzuoli Bay provided insights into the vertical deformation of the submerged part of the Campi Flegrei caldera (Southern Italy).</p><p>The collapse of the central part of the Campi Flegrei caldera is associated with the eruption of the Neapolitan Yellow Tuff (NYT) at ~15 ka. The NYT caldera collapse was followed by central dome resurgence associated with alternations of fast uplift and subsidence displacements that accompanied with discrete phases of intra-caldera volcanic activity. Previously, the evolution of ground movement in the Campi Flegrei caldera has been reconstructed using marine deposits uplifted onland or archaeological evidence and historical accounts and thus offers a mainly 2D appraisal of the deformation pattern. However, a complete reconstruction of post-collapse deformation suffers of the limitation that nearly two-thirds of the caldera are submerged beneath the Pozzuoli Bay.</p><p>We contribute to fill this gap by providing a reconstruction of offshore and coastal deformation through estimation of the vertical displacement of morphodepositional markers in high-resolution seismic reflection profiles and geoarchaeological markers directly surveyed at shallow depths. Our interpretation reveals the occurrence of different sediment stacking pattern whose provides evidence of rapid and oscillating ground movements. Whereas the offshore morphodepositional markers provide displacement information for the last ~12 ka, for the last ~2 ka our interpretation is supported by ancient Roman sea-level indicators. The multi-dataset analysis has allowed disentangling the signal related to the post-caldera dynamics from a broader deformation signal that affects this part of the extensional margin of the Apennines.</p><p>The integration of offshore data in the study of past episodes of ground deformation, by yielding a more complete picture of the ground motions associated to the post-collapse evolution of the Campi Flegrei caldera, bears a significant contribution for a 3D reconstruction of this high-risk resurgence caldera. Besides, the multidisciplinary approach presented here can be relevant for investigations of other calderas spanning the sea-land transition.</p>

Aseismic strain episodes at Campi Flegrei Caldera, Italy

Since 2004 a research project has been developed to monitor subsurface deformation of Italian volcanoes using borehole strainmeters and long-baseline tiltmeters. Six Sacks-Evertson dilatometers were installed around Campi Flegrei caldera and Vesuvius during 2004–2005 (Scarpa et al., 2007), and in 2008 these instruments were supplemented by two arrays of 28–280 m long water-tube tiltmeters in underground tunnels. Relevant strainmeter and tiltmeter data have been collected and analysed from the instruments installed near Campi Flegrei caldera during the recent unrest episodes. In the period 2004–2005 strain, tilt and GPS data from Campi Flegrei indicate the onset of surface deformation that accompanied a low rate of vertical displacement that continued to 2006, corresponding to an increase of CO2 emission. This strain episode preceded caldera microseismic activity by a few months, as was observed also during a significant inflation episode in 1982. Other transient strain episodes occurred in October 2006, which were accompanied by a swarm of VT (Volcano-Tectonic) and LP (Long Period) events, in 2009, at the time of renewed gas emission activity at Solfatara, and again in March 2010, several minutes before a seismic swarm. The time scale of these transient strain events ranges from some hours to several days, putting tight constraints on the origin of ground uplifts at Campi Flegrei. Their location is compatible with a source inferred from long term deformation signals, located about 4 km beneath Pozzuoli. A proposed mechanism for these aseismic strain episodes is that they are associated with magma growth in reservoirs with occasional pressure relief associated with the leakage of gas.

Tracking Episodes of Seismicity and Gas Transport in Campi Flegrei Caldera Through Seismic, Geophysical, and Geochemical Measurements

This article presents findings from two episodes of seismicity and gas emission that occurred on 7 October 2015 and 6 December 2019 in Campi Flegrei caldera. This caldera has been affected by long-term unrest since 2004. The 6 December 2019 episode, consisting of a swarm of 38 earthquakes (maximum duration magnitude 3.1, the largest between 1984 and March 2020), occurred at the end of a one month period characterized by an increase in the ground uplift rate from 0.19±0.01 to 0.72±0.05 mm/day. A sudden increase in the fumarolic tremor amplitude, which is a proxy of gas emission-related parameters recorded at Solfatara–Pisciarelli hydrothermal area (e.g., CO2 air concentration), was observed during the seismicity episode. The uplift rate decreased immediately after the swarm (0.10±0.01 mm/day), whereas the fumarolic tremor amplitude remained higher than that observed prior to the swarm. Through analyzing the time series of uplift recorded in Pozzuoli (central area of the caldera) from differential measurements on tide gauges, we were able to identify the 2015 episode. This episode was characterized by increasing uplift rates that culminated in a seismic swarm of 33 earthquakes on 7 October, which was followed by decreasing uplift rates. We computed double-difference locations of earthquakes from the two swarms and found that they located along a conduit-like path, coinciding with a high-resistivity contrast zone, previously identified by audiomagnetotelluric measurements. The focal mechanisms of the major earthquakes of both swarms indicate fault planes radial with respect to the maximum uplift area. These phenomena can be interpreted as episodes of the volcanic and (or) hydrothermal system pressurization that culminate in an injection of fluids along the conduit-like path, which behaves as a valve that allows fluid discharge and the temporary depressurization of the source region.

Four Years of Continuous Seafloor Displacement Measurements in the Campi Flegrei Caldera

We present 4 years of continuous seafloor deformation measurements carried out in the Campi Flegrei caldera (Southern Italy), one of the most hazardous and populated volcanic areas in the world. The seafloor sector of the caldera has been monitored since early 2016 by the MEDUSA marine research infrastructure, consisting of four instrumented buoys installed where sea depth is less than 100 m. Each MEDUSA buoy is equipped with a cabled, seafloor module with geophysical and oceanographic sensors and a subaerial GPS station providing seafloor deformation and other environmental measures. Since April 2016, the GPS vertical displacements at the four buoys show a continuous uplift of the seafloor with cumulative measured uplift ranging between 8 and 20 cm. Despite the data being affected by environmental noise associated with sea and meteorological conditions, the horizontal GPS displacements on the buoys show a trend coherent with a radial deformation pattern. We use jointly the GPS horizontal and vertical velocities of seafloor and on-land deformations for modeling the volcanic source, finding that a spherical source fits best the GPS data. The geodetic data produced by MEDUSA has now been integrated with the data flow of other monitoring networks deployed on land at Campi Flegrei.