Since early 2021, peculiar Volcano-Tectonic (VT) seismic sequences with very short inter-event times have become increasingly frequent and evident in the Campi Flegrei caldera (Italy), which has been experiencing a long-term unrest since 2005. During the same period the ground deformation (uplift), seismicity and gas emission that characterize the current unrest showed an acceleration. Within this type of seismic sequence, we identify burst-like swarms, characterized by inter-event times so short that they are often not easily recognizable. Here we show that these sequences are located in an area encompassing the main hydrothermal field and a zone affected by a geodetic anomaly (a region that uplifts less rapidly than the surroundings), which became evident in 2021. This type of seismicity has been associated with phreatic explosions and critical phases of unrest in other volcanoes and currently characterizes the state of activity of Campi Flegrei caldera.

Calderas are depressions formed by some of the largest volcanic eruptions. Their long-lived inter-eruptive periods are occasionally interrupted by phases of unrest, in which escalating seismicity, ground deformation and gas emissions raise concerns of potential volcano reawakening. However, interpretation of such physico-chemical signals is complicated by few examples of monitored unrest that culminated into eruption and by our fragmentary understanding of the drivers and timescales of caldera reactivation. Here we show that multi-decadal gas observations at the restless Campi Flegrei caldera in Italy record an unprecedented increase in isotopically light sulfur release from fumaroles since 2018. We then use hydrothermal gas equilibria and numerical simulations of magmatic degassing to propose that such a change in sulfur emissions results from decompression-driven degassing of mafic magma at ≥6 km depth, along with some extent of sulfur remobilization from hydrothermal minerals. Examination of a global dataset indicates that, despite the diversity in eruptive behaviour and tectonic setting, increasing sulfur output may be a common process during unrest escalation at calderas generally. Hence, our observations and models of sulfur behaviour may inform interpretations of unrest and hazard assessment at reawakening calderas and hydrothermal active volcanoes worldwide.

Nyiragongo volcano is known for its active lava lake and for socioeconomic issues arising from future possible eruptive events having major impacts on the community living in the Virunga region. The 2020 field expedition inside the summit crater has allowed the collection of unprecedented field observations to state on the current activity. Since the February 2016 intracrater event, the crater floor level has been rising much faster than during the 2010–2016 period. The current activity is reminiscent of the 1970–1972 and 1994–1995 periods preceding the lava lake drainage events in 1977 and 2002. Numerical simulations, successfully validated with data over the past 30 years, show that the rising of the crater floor could slow down in the next months/years and reach a critical equilibrium. Based on the past eruptive history and on the current activity, a flank eruption in the March 2024 to November 2027 interval could be a possible scenario.