Seismotectonic analysis of a complex fault system in Italy: the "Garfagnana-North" (Northern Tuscany) line

Abstract

We present the results obtained combining different techniques to determine the seismotectonic character of the Garfagnana region (northern Tuscany). There, the existence of a rather complex fault system is acknowledged and somewhat mapped, but apart from the geological evidences, very little is known about its extension with depth and the regime. The seismic potential of the system is also well known. The area was characterized, in the past, by destructive earthquakes; in particular a major event (Ms=6.4) struck the Lunigiana-Garfagnana area in September 1920, but many others have been reported. Therefore, the seismicity is under constant monitoring by the national seismic network (RSNC – National Central Seismic Network) and a pool of local stations, belonging to a regional network (RSLG – Regional Seimic network of Lunigiana and Garfagnana). These additional stations account for the lower magnitude seismicity. Such a concentration of seismic stations, and the consequent availability of several seismograms, makes likely to record and localize earthquakes down to a very low magnitude threshold (inferior to Ml = 2.0) with extremely narrow hypocentral parameter errors . Making use of the resulting databases, several analyses were conducted to determine the shape, size, extension with depth of the fault and the associated seismicity. The methodology consists in seismic tomography (1D and 3D velocity models), precise location algorithms NonLinLoc and HypoDD (very constrained and reliable locations) and computation of focal mechanisms (fault orientation and source), all combined with the constraints provided by the geology. The main findings of the study are that the concentration of the recent seismic activity is close to the likely location of the most relevant historical events. In particular the earthquakes are distributed along a plane in the range 0 – 20 km depth dipping 30° NE. All focal mechanisms show a transtensive character.