[1] Seismic data recorded in the 2-30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and $160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1:1:2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio n = 1/3, a value appropriate for hot rock. Both imaged cracks dip $60°to the northwest and strike northeast-southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are $200 m 3 for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit.

We have analyzed a multiparametric data set of seismological, geodetic and geochemical data recorded at Campi Flegrei caldera since 1982. We focus here on the period 1989–2010 that followed the last bradyseismic crisis of 1982–1984. Since then, there have been at least five repeated minor episodes of ground uplift accompanied by seismicity. We have reanalyzed old paper and digital seismic data sets dating back to 1982. The paper recordings show evidence of long‐period events in January 1982 and March 1989, and we have digitized some of these significant waveforms. Furthermore, the revision of digital seismograms dating back to 1994 shows a significant swarm of long‐period events in August 1994. Volcano‐tectonic and long‐period events hypocenters have been relocated in a three‐dimensional velocity model. Statistical analysis of volcano‐tectonic seismicity shows many similarities and few differences between 1982–1984 and the following period 1989–2010. Long‐period waveforms have been analyzed using spectral analysis, which shows a grouping into three macrofamilies. Similarities in the seismic signature of episodes of minor uplift suggest that they originate from the injection of fluids into the deep part of a geothermal reservoir (about 2.5 km depth) and in its transfer toward a shallower part (about 0.75 km depth). Most of the observed geophysical signals are related to this second phase. The evidence consists of spatial and temporal connections between the ground deformation, long‐period and volcano‐tectonic seismicity and changes in the geochemical parameters of fumaroles. In this study we focused our analysis on two uplift episodes observed in 2000 and 2006. The joint inversion of Differential Synthetic Aperture Radar (DInSAR) and tiltmeter data show that during these periods the ground deformation was generated by at least two distinct sources located at different depths, with the shallower activated in the later stages of the uplift episodes. Our interpretation of the recent dynamics of Campi Flegrei is that the deep part of the geothermal reservoir inflates in response to mass and heat input from a magmatic source. When the pressure exceeds a threshold, fluids starts to migrate into the shallower part. During this transfer, long‐period sources are activated in response to the fluid motion. The gradual diffusion of fluids in the surrounding rocks lowers the resistance of a pervasive fracture system generating shallow microseismicity. Finally, fluids reach the surface, which gives a distinct geochemical signature to the overlying fumaroles.

We found the first evidence, in the last 30 years, of a renewed magmatic activity at Campi Flegrei caldera from January 2012 to June 2013. The ground deformation, observed through satellite interferometry and GPS measurements, have been interpreted as the effect of the intrusion at shallow depth (3090 ± 138 m) of 0.0042 ± 0.0002 km3 of magma within a sill. This interrupts about 28 years of dominant hydrothermal activity and occurs in the context of an unrest phase which began in 2005 and within a more general ground uplift that goes on since 1950. This discovery has implications on the evaluation of the volcanic risk and in the volcanic surveillance of this densely populated area.

Two paroxysmal explosions occurred at Stromboli volcano in the Summer 2019, the first of which, on July 3, caused one fatality and some injuries. Within the 56 days between the two paroxysmal explosions, effusive activity from vents located in the summit area of the volcano occurred. No significant changes in routinely monitored parameters were detected before the paroxysmal explosions. However, we have calculated the polarization and the fractal dimension time series of the seismic signals from November 15, 2018 to September 15, 2019 and we have recognized variations that preceded the paroxysmal activity. In addition, we have defined a new parameter, based on RSAM estimation, related to the Very Long Period events, called VLP size, by means of which we have noticed significant variations through the whole month preceding the paroxysm of July 3. In the short term, we have analyzed the signals of a borehole strainmeter installed on the island, obtaining automatic triggers 10 minutes and 7.5 minutes before the July 3 and the August 28 paroxysms, respectively. The results of this study highlight mid-term seismic precursors of paroxysmal activity and provide valuable evidence for the development of an early warning system for paroxysmal explosions based on strainmeter measurements. Stromboli (Aeolian Archipelago, Italy) is an open conduit volcano with persistent explosive activity. It is located in the Mediterranean Sea, not far from the coasts of Sicily and Calabria (Fig. 1). The persistent explosive Strombolian activity consists of several hundred of moderate-intensity events per day. Typical Strombolian explosions eject pyroclastic fragments at the height of some tens of meters, which fall a short distance from the eruptive vent. Explosions occur in numerous eruptive vents located in the summit area of the volcano that can change over time both in number and position. However, the eruptive vents can be grouped into three areas (Fig. 1), northeast (NE), central (C) and southwest (SW), and are distributed along the dominant structural direction (NE-SW) of a graben-like collapsed area at the top of the volcanic edifice 1-3. Major explosions 4,5 eject pyroclastic material over a hundred meters high, which can fall outside the crater terrace in the area visited by tourists. The frequency of these phenomena varies in time, with an average of 2 events per year 5-7. Paroxysms, violent explosions that produce eruptive columns more than 3 km high and are often accompanied by pyroclastic flows, can also occur at Stromboli 8-13. Ballistic blocks associated with these explosions can reach up to 2 m in diameter. Strombolian paroxysms are rare and their occurrence frequency varies over time.

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