ASMI Archivio Storico Macrosismico ItalianoItalian Archive of Historical Earthquake Data

On September 5, 1950 (04:08 GMT), a strong earthquake struck a large part of Central Italy, causing heavy damage in the districts of Teramo, Pescara, L’Aquila and Rieti. This earthquake is considered, in the Italian Seismic Catalogues, as the largest event which occurred in the Gran Sasso Range area. This area is characterized by an infrequent and moderate seismicity. On the contrary several neighboring areas are prone to a high and frequent seismicity. This paper aims to reappraise the 5 September 1950 earthquake and the following seismic period, deepening our knowledge of this event from the macroseismic and seismotectonic points of view. Historical research in public archives allowed us to retrieve many unpublished documents, related to the 1950 earthquake and to the strongest aftershock which occurred on 8 August 1951. Very important documents are in particular the “Genio Civile” folders about damage reconnaissance in the provinces of Teramo and Rieti. Results concern the large increase of intensity points, from 137 to 386 for the 1950 event, and from 33 to 94 for the 1951 event, with respect to the known literature. Maximum intensities are I=VIII for 1950 earthquake and I= VII for 1951. By means of the Boxer code version 3.3 (Gasperini et al., 1999) new epicentral parameters have been calculated from macroseismic data, together with the macroseismic magnitude values (Maw=5.9±0.2, for the 1950 event and Maw=5.2±0.2 for the 1951 aftershock). Finally, a hypothesis about the seismogenic source responsible for the mainshock is proposed. According to the damage distribution, an EW oriented source (strike 91,5±18, length 10,7 km, width 7,4 km) can be hypothesized, located below the Laga Mountains, not related to known tectonic structures of the region. The seismogenic source of the 1950 earthquake could therefore be represented by a blind fault pertaining to a structural level deeper than that of the active normal faults which affects the Apennine extensional domain.