Seismicity in central Tuscany, Italy: Insights from a regional seismic deployment

Seismic activity in Tuscany, Italy, is driven by the interplay between complex tectonics and fluid flow processes. Fluid-driven seismic sequences are typically associated with high-enthalpy geothermal regions, such as the Larderello-Travale Geothermal Field (LTGF). To better understand the regional tectonic setting, we derive a detailed catalogue of earthquake hypocenters and magnitudes from a seismic network consisting of 30 permanent seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and 30 temporary stations deployed in Tuscany in the framework of a specific acquisition survey  (TEMPEST), during a period of one year (from September 2020 to September 2021).

We applied an automated processing routine including a machine‐learning (ML) phase picker, PhaseNet, and the Gaussian Mixture Model Association (GAMMA) algorithm, a sequential earthquake association and location workflow. We initially obtained nearly 1 million P-phases and 2 million S-phases, yielding around 5k detected events. We then located the events with NonLinLoc and applied quality metrics to filter out potential false detections (22%) and recognize the high-quality solutions, which represented 30% of the initial 5k locations. The high-quality catalogue has been relocated with the Double-Difference software scrtdDD to better constrain earthquake clusters and to improve the robustness of  the subsequent analysis of the seismic sources in the region.

We identified a five-day-long sequence of 203 earthquakes with magnitudes Mw ranging from 0.4 to 2.0, oriented NW-SE, north of Monte Amiata (Southern Tuscany). We observe a temporal upward migration of the events between 10 to 5 km depth towards SE. We have obtained seven high-quality focal mechanisms from first-motion polarities by applying a Bayesian approach (Mtfit). Our results suggest the occurrence of a seismic swarm with predominant normal faulting mechanisms and average dipping angles of 50° NE. Our solutions could be associated with a normal fault system that is compatible with the local geology.