High-resolution seismic catalogue of the Tuscan Magmatic Province, Italy with evidence of fluid-driven seismicity

Summary Understanding the mechanisms controlling microseismicity in fluid-rich areas is key for assessing seismic hazards and the interaction between tectonics, magmatism, and geothermal systems. The Tuscan Magmatic Province, straddling the Tyrrhenian coast of southern Tuscany and northern Latium in Italy, is an ideal place to study these interactions. This region features high heat flow, extensive hydrothermal fluid circulation and several geothermal systems. However, the role of these fluids in controlling the microseismicity in the area is not fully understood. To shed light on this topic, we deployed a broadband seismic network that integrated the permanent regional networks from September 2020 to September 2021. Using state-of-the-art machine learning detection and probabilistic location methods, we detected and located 1,944 high-quality earthquakes with moment magnitudes ranging from Mw −0.2 to 2.8. By detecting approximately four times more events than reported by the regional catalogue, this new and high-resolution earthquake catalogue for the Southern Tuscany region (Italy) provides a more refined characterization of seismicity within Southern Tuscany’s unique geological framework. Our study reveals strong spatial clustering of seismicity along a NW-SE striking strip, parallel to the Apennines belt, and terminating in correspondence of the Monte Amiata geothermal system. West of this alignment, an additional cluster is associated with the Larderello-Travale geothermal system. Focal mechanisms indicate a coexistence of strike-slip and extensional style of deformation. A most prominent sequence consists of more than 500 events aligned along a NW-SE normal fault and exhibits clear upward hypocenter migration. The estimated velocities of the seismic front are in the order of 200-600 m/h. The calculated hydraulic diffusivities (10-15 m2/s) suggest fluid diffusion as the primary driver of the swarm. This is in agreement with the seismic sequences departing from magmatic bodies. To explain the source of the seismic swarm we propose a valve-type mechanism in which an initial tectonic failure phase progressively opens the fault system, enabling fluid advection that sustains subsequent seismic activity. The spatial correlation between seismicity and low-velocity crustal zones further supports the role of deep fluid circulation. These results show that microseismicity around geothermal systems in Southern Tuscany is shaped by the combined effect of tectonic stress loading and transient fluid overpressure, with implications for seismic hazard assessment and geothermal resource management.