Seismic methods to investigate the interplay between magmatism and tectonics in the Northern Apennines hinterland, Tuscan Magmatic province, Italy

The Northern Apennines hinterland is normally referred to as a back-arc area affected by magmatism since the late Neogene, with intrusive and extrusive magmatic rocks being scattered through the southern Tuscany and the Tuscan Archipelago. This setting makes this geothermal area one of the most important worldwide.  The very high geothermal gradient of the Tuscan Magmatic Province (up to 200–1000 mW/m2 in the central part of the Larderello-Travale geothermal field) fuels vigorous fluid flow that promotes a widespread geothermal activity. The plumbing magmatic systems progressively migrated eastwards. In Tuscany, the Elba Island is proposed to be the ancient (and now exhumed) analogue of the Larderello-Travale geothermal system where Pliocene to Pleistocene granites have been found at about 3 km depth. The occurrence of such magmatic bodies links to one of the most debated structures of the region, namely the K-horizon seismic reflector. Early authors suggest that this level may represent the transition between ductile and brittle geological units. Other authors propose that the k-horizon may represent a level where the magmatic brines released by the cooling plutons are stored at super-critical conditions.

To understand the spatial and temporal relationship between tectonics and eastwards migration of magmatism, we deployed a seismic network composed of 30 broadband sensors that complemented the permanent INGV network from September 2020 to August 2021. We used QuakeMigrate to build the seismic catalogue and identified 915 events. The seismic activity is well spread across the region and occurs in swarm sequences in areas marked by higher geothermal gradients, particulary in the Larderello-Travale geothermal field. We investigate the seismic clusters and compare them against the locations obtained with the permanent monitoring network. Our results significantly reduce the magnitude of completeness for the Northern Appennines hinterland and provide new insights into the tectonics of the region. This study will be continued with a newly funded project that will use ambient noise methods acquired by nodal networks to provide high-resolution tomographic inversions of the geothermal systems of the Tuscan Magmatic Province.