Seismicity driven by rapid fault cementation in tuffs under hydrothermal conditions of the Campi Flegrei caldera (Italy)

Volcanic calderas, including Campi Flegrei (Italy), are characterized by intense shallow seismicity (<4 km depth, magnitude <4), commonly associated with hydrothermal fluid circulation. In these settings, seismogenic volumes constitute highly reactive systems where pressurized fluids, elevated temperatures, and mineral reactions interact to modulate fault strength, wall rock and fault zone stiffness, and slip behavior (i.e., from aseismic creep to seismic slip). Despite the dense monitoring network at Campi Flegrei—one of the most active and densely populated volcanic–geothermal systems worldwide—the mechanical and chemo-textural behavior of shallow faults under hydrothermal conditions and its implications for the seismic cycle remain poorly constrained.

Here, we investigate the coupled mechanical, mineralogical, geochemical, and microstructural evolution of experimental faults composed of Neapolitan Yellow Tuff, a highly reactive pyroclastic rock representative of the shallow (<1 km) intra-caldera faulted volume at Campi Flegrei. We performed fourteen hydrothermal rotary-shear experiments at constant slip velocity (10 µm s⁻¹), systematically varying temperature (T = 23–400 °C), effective normal stress (σₙ^eff = 5–30 MPa), and pore-fluid pressure (P_f = 5–30 MPa) to reproduce liquid, vapor, and supercritical water conditions expected within the upper ~2 km of the caldera. Slip stability was assessed from the occurrence of stick–slip events (laboratory earthquakes), and associated stress drops, while friction coefficients and apparent fault stiffness were retrieved from stick-slip cycles. Mechanical observations were complemented by post-mortem mineralogical, geochemical, and microstructural analyses using X-Ray Diffraction, X-Ray Fluorescence, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, and Energy Dispersive System microanalysis (XRPD, XRF, FTIR, SEM–EDS).

At room temperature, deformation is dominated by stable or slow slips associated with distributed grain-size reduction and limited induration. With increasing temperature, thermally activated mineral reactions alter fault rheology (and behavior). Between 300 and 400 °C, zeolite dehydration, clay dehydroxylation, volcanic glass dissolution, and rapid secondary mineral precipitation promote pervasive cementation and pore-space sealing, producing a dense, welded fault fabric. These processes strengthen grain-to-grain contacts, increase friction coefficients (from ~0.67 at room temperature to ~0.84 at 400 °C), and significantly enhance fault stiffness (from ~2.5 GPa/m at 23 °C up to ~10 GPa/m at 400 °C), leading to strongly unstable, earthquake-like slip with laboratory stress drops of up to ~25 MPa. Increasing effective normal stress further amplifies frictional instabilities through compaction, strain localization, and strengthening of grain contact junctions. In contrast, vapor-dominated conditions at ≥300 °C inhibit cementation, resulting in smaller stress drops while maintaining unstable fault slip behavior.

Our results demonstrate that hydrothermal fluid–rock interactions can rapidly shift shallow volcanic faults across slip modes by modifying fault fabric, stiffness, and strength. Temperature-driven mineral breakdown and pore-space sealing play a fundamental but often overlooked role in the shallow seismicity at Campi Flegrei, as well as in similar tuffaceous geothermal reservoirs, with important implications for fault mechanics, seismic hazard, and volcanic dynamics across small-to-long term time scales.