GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 Discussion and conclusions While the overall tectonic setting of the area is rather well known, the seismotectonics is not as clear. The strongest historical earthquakes are mostly located in the Lunigiana or close to the transfer zone between the Lunigiana and Garfagnana grabens. However, the uncertainty in macroseismic locations does not allow to discriminate whether these events belong to one or the other graben. Given that improving the locations of macroseismic events is a demanding process that also needs additional data, which in turn are not easy to get, knowing the focal solutions of historical earthquakes can shed some light on their attribution to one area. In fact, while the faults that border the grabens have an orientation NNW-SSE, in the transfer zone the distribution of epicentres and the orientation of the planes of the relative focal solutions show a WSW-ENE trend. In such a view, the 1837 event would fall in the transfer zone since the orientation of the nodal planes recall what was expected for events in this area. The KF method does not solve the ambiguity between the results produced by mechanisms that differ by 180 ◦ in the rake angle. In fact, in both cases, it produces the same radiation but with reversed polarities. This ambiguity may only be cleared by additional tectonic/geodynamic (or instrumental) information. In our case, given the location, the depth, the orientation of the nodal planes of the 1837 event which resembles the June, 2013 Mw 5.1 event, we can state that the 1837 mechanism was distensive. References Argnani A., Barbacini G., Bernini M., Camurri F., Ghielmi M., Papani G., Rizzini F., Rogledi S. and Torelli L.; 2003: Gravity tectonics driven by Quaternary uplift in the Northern Apennines: insights from the La Spezia–Reggio Emilia geotransect . Quat. Int. 101–102 , 13–26. Brozzetti, F., Boncio, P., Lavecchia, G. and Pace, B.; 2009: Present activity and seismogenic potential of a low-angle normal fault system (Città di Castello, Italy): constraints from surface geology, seismic reflection data and seismicity . Tectonophysics, 463 , 31–46. Eva, E., Solarino, S. and Boncio, P.; 2014: HypoDD relocated seismicity in northern Apennines (Italy) preceding the 2013 seismic unrest: seismotectonic implications for the Lunigiana-Garfagnana area . Boll. Geof. Teor. Appl., 55(4) , 739–754. Eva E., Pettenati F., Solarino S. and Sirovich L.; 2022: The focal mechanism of the 7 September 1920, Mw 6.5 earthquake: insights into the seismotectonics of the Lunigiana–Garfagnana area, Tuscany, Italy . Geophys. J. Int., 228 , 1465–1477, https://doi.org/10.1093/gji/ggab411. Locati M., Camassi R., Rovida A., Ercolani E., Bernardini F., Castelli V., Caracciolo C.H., Tertulliani A., Rossi A., Azzaro R., D'Amico S., Conte S., and Rocchetti E.; 2016: Database Macrosismico Italiano (DBMI15), versione 1.5 . Istituto Nazionale di Geofisica e Vulcanologia (INGV), https://doi.org/10.6092/INGV.IT-DBMI15.