Relative earthquake location of low-energy volcanic seismicity at Campi Flegrei

This work intends to contribute to the comprehension of the Campi Flegrei caldera (CFc) unrest, through the relative relocation of the diffuse seismicity recorded during the 1982-84 unrest and after its reactivation in 2005.

The CFc is one of the best monitored volcanic areas in the world, with a multi-parametric network of observing stations operating in the area. The shallow structure of the caldera, between 1 and 3 km, is a high-temperature hydrothermal system formed by a sequence of volcanoclastic, tuffs, lava, and marine deposits. The temperature gradient measured in deep boreholes, down to a depth of about 3 km, exceeds 150°C/km. A zone of pressurized gas and sill intrusion is possibly located at 3-4 km. A long-term magma reservoir is hypothesized in the deep structure (7-9 km), persistently supplying CO₂ to the surface (observable for the continuity of gas emission to the fumaroles of Solfatara-Pisciarelli as well).

While the unrest of 1982-84 has been generally associated with magma injection, a mechanism of fluid pressurization and heating of the CFc hydrothermal system is thought to be the primary forcing of ground deformation and shallow seismicity of the ongoing unrest. However, the mechanisms that control the interaction between the rising of fluids from deeper volumes and the seismicity within and below the hydrothermal system are still debated.

In this work we use the arrival times of located seismic events to perform a double-difference relative relocation of earthquakes that occurred in the years 1982-84 and 2005-2022. Moreover, by using calibration laws for magnitude scales to infer the moment magnitude M_w from available catalogs of duration magnitude M_d, we estimate the spatial distribution of the cumulative seismic energy released during the two considered time spans.

The final distributions of hypocenters and radiated seismic energy, and their spatio-temporal evolution, suggest constraints for the identification of preferential pathways of rising fluids and for the imaging of structural barriers. These results can be interpreted jointly in light of previous works and available tomographic models for the definition of possible scenarios of unrest evolution.