The Campi Flegrei Caldera is a large volcanic complex located in the densely populated Bay of Naples, Italy. Since 2005, the ground there has been uplifting, accompanied by earthquakes of increasing magnitude. The current differential uplift reached ~1.4 m, while the earthquake with the maximum magnitude, Md 4.6, occurred only a few weeks ago, on 30-06-2025. Seismicity and ground deformation are attributed to a degassing magma source and the subsequent interactions between magmatic and hydrothermal fluids, but the details and geometry of the seismogenic structures are still a matter of debate.
Now, an international team composed by researchers from the University of Pisa (Italy), the Istituto Nazionale di Geofisica e Vulcanologia (INGV), and the GFZ Helmholtz Centre for Geosciences (Germany) has investigated several volcano-tectonic earthquakes at Campi Flegrei, occurring over the last 10 years. The seismological study, using advanced techniques forearthquake source characterization , reconstructs with unprecedented detail the geometry of the different seismogenic structures activated by the ground inflation.
The study also provides the first evidence of very long period (VLP) signals at Campi Flegrei, reporting more than a dozen of VLPs, with durations of the order of ~60-120 seconds and a dominant frequency of ~0.11 Hz since 2018. The VLPs occur beneath the CO2 emissions at Solfatara, in response to volcano-tectonic earthquakes.
VLP source components and resonance processes are sometimes observed at other volcanoes, but they were never recognized before at Campi Flegrei. “We interpret the VLP signals as the resonance of one or more fluid-filled, crack-like structures, connecting the deformation source at depth with the fumarole locations at the surface. This oscillating, gas-filled structure offers a path for degassing, with a mutual interaction between fluid ascent, shallow fracturing and resonance processes,” says Giacomo Rapagnani, first author of the study and PhD student at the University of Pisa, funded by a INGV scholarship.
What does this mean? “Interestingly, the VLP waveforms and spectra have been extremely similar in recent years. This suggests that the geometry and conditions of the resonating source have remained quite stable, and we are not seeing clear indications of substantial changes in the physical properties of the fluids which sustain the resonance,” says Simone Cesca, second author and scientist at the GFZ, section Physics of Earthquakes and Volcanoes.
“This observation underscores the relevance of the obtained results for tracking the dynamic evolution of the volcano, though further investigation is needed to better constrain the physical processes that govern the system”, says Gilberto Saccorotti, from INGV.
“This study highlights how the development and application of sophisticated techniques for the analysis of seismological data are essential to better understand complex geophysical processes, such as earthquakes and volcanic eruptions. Only by pushing the limits of our ability to analyze large volumes of heterogeneous data can we improve our understanding of these phenomena and more effectively mitigate their related risks.” says Francesco Grigoli, Professor of Geophysics at the University of Pisa.
The study is published in the journal Nature Communications Earth and Environment.
Original study:
Rapagnani, G., Cesca, S., Saccorotti, G., Petersen, G., Dahm, T., Bianco, F., Grigoli, F. , 2025. Coupled earthquakes and resonance processes during the uplift of Campi Flegrei caldera. Commun Earth Environ 6, 607 (2025).
https://doi.org/10.1038/s43247-025-02604-7
