Fast characterization of sources of recent Italian earthquakes from macroseismic intensities

Abstract

We analysed the intensity data collected immediately after three strong earthquakes (Mw≥6) that occurred in Italy in the last decade to infer location, orientation and size of the relevant seismogenic faults using the Boxer method. We show that, starting from a few hours after the earthquake, such macroseismic method could have provided a characterization of the seismogenic source comparable to instrumental methods, some of which would have required much more time to provide stable results. Particularly for the 6 April 2009 earthquake, the analysis of macroseismic survey data could have been able to accurately constrain, starting already from the early afternoon of the day of the main shock, location, orientation and size of the area of maximum coseismic displacement, related to the activation of the Paganica-S. Demetrio fault system. For the 20 May 2012 Emilia and the 24 August 2016 Amatrice earthquakes, preliminary macroseismic surveys were carried out within a few days from the main shocks, but their data were made available publicly a month later. For the 20 May 2012 Emilia earthquake, the orientation of the fault could have been correctly determined even if the size would have been underestimated. For the 24 August 2016 Amatrice earthquake, both the orientation and the size could have been correctly assessed. We also compared such macroseismic determinations with those obtained for some historical earthquakes in the same areas. The analysis of the cumulate intensities observed after the shocks of 24 August, 26 and 30 October 2016 indicates an overall Mw 6.8 comparable to that (Mw 6.9) of the largest earthquake occurred in the same section of the Apennines (14 January 1703). Even if the inferred orientations of the 2016 and 1703 boxes are very similar, the epicentres differ of about 20 km indicating a probable distinct fault system for the two earthquakes.