Seismicity, seismotectonics and historical earthquakes of the Northwestern Apennines, Italy: A reappraisal

The inner NW Apennines includes the Lunigiana region, which has been shaken by several strong (M > 6.0) earthquakes in the last millenium until the most recent 2013 Mw 5.1, Fivizzano earthquake. In this study we profit from more than 30 years of seismicity and an updated map of the faults to perform a seismotectonic analysis. In order to enlarge our study to the strongest events, we also compute the focal parameters of the 1834 and 1837 earthquakes applying the KF technique to macroseismic intensities. Our findings show that most of the seismicity in the study region is organized in clusters. The southern cluster is oriented nearly E-W and marks the border between the Lunigiana and Garfagnana grabens, NW of the Alpi Apuane. The northern cluster is less limited in size and number of events, and this masks the predominant apenninic NW-NNW orientation. NE of the Lunigiana graben, seismic events align along an apenninic direction as well. On the external side, earthquakes are almost perpendicular to the chain in an NEE-SWW orientation. Cross sections show the existence of three domains along the NE-SW direction. While the external and internal domains are well defined, the domain in between may represent the area in which switching from shortening to extension is ongoing. The processes occurring in this domain entail energies that are at times unleashed as considerable earthquakes, including the notable seismic event of 1834.