HypoDD relocated seismicity in northern Apennines (Italy) preceding the 2013 seismic unrest: seismotectonic implications for the Lunigiana-Garfagnana area

We present the results of a study aimed at defining the geometry and kinematics of seismogenic volumes and structures of the Lunigiana-Garfagnana region (northwestern Apennines) as depicted by background seismicity recorded before the seismic crisis of 2013. In this analysis we profited from earthquakes located with the high precision algorithm HypoDD and the availability of a large set of focal mechanisms. The obtained data set of well-located hypocentres allowed us to define some previouslyunknown, or only poorly-defined, geometric characteristics. We also confirmed, with a finer detail, some already-known first order features such as the presence of two NW-SE-trending zones of seismicity, west and east of the Apennine water divide, separated by a low seismicity corridor. The main findings of this study are: 1) most of the seismicity of the western zone is located in the Lunigiana graben, north-NW of the Apuane Alps; 2) at depth, the Lunigiana seismicity deepens to the east parallel to the top of the basement, which in turn coincides with an extensional detachment (~30° E-dipping); and 3) the Lunigiana seismicity terminates southwards with a dense cluster of epicentres oriented nearly E-W, parallel to the transfer fault zone that delimits the Apuane Alps to the north; south of this cluster, a strong reduction of seismicity is observed and the locations are shifted to the eastern sector. These findings might help in interpreting the seismotectonics of the 1481, 1837, 1920 and 1995 earthquakes, all located within the E-W-trending cluster at the southern termination of the Lunigiana seismicity.