Crustal structure of the Lunigiana-Garfagnana area (Tuscany, Italy): seismicity, fault-plane solutions, and seismic tomography

The accurate analysis of seismicity and its focal mechanisms and the tomographic inversion of a reliable set of local earthquakes allowed us to make an investigation on the crustal structure of the Lunigiana-Garfagnana area. In 1999 the installation of a seismic network of digital enlarged band stations, to monitor the seismicity of this interesting area, made it possible to investigate the seismo-tectonic features and, upon integration with other data, the 3-D velocity model of this region. The results confirmed and improved what had already been stated in previous works and added new details to the knowledge of this intriguing area. In fact, the distribution of seismicity with depth points out the existence of two seismogenic layers, a shallow one, down to a 35 km depth and a deeper one that begins at the depth of 50 km; in addition, the plane view reveals the presence of a narrow band, running approximately NW-SE, where the seismic activity is less consistent; this feature could be related to the passage from the inner (Tyrrhenian part of the chain) to the outer side of the Apennines. The analysis of a selection of focal solutions suggests that a more complicated situation than just two stress domains (distensive in the inner domain and compressive in the outer one) must exist in this area of the northern Apennines. Evidence of deep compressive mechanisms and the wide spreading of distensive fault-plane solutions suggest that the presence of different stress regimes at different depths is highly likely. Finally, the 3-D velocity model derived from a preliminary tomographic inversion shows the presence of a 20-25 km-thick crust underneath the northern Tyrrhenian Sea and Tuscany which thickens to more than 40 km under the Apenninic chain. Unfortunately, the resolution of tomographic images is very variable and leaves a few question marks on the relationships between deep and shallow structures.