Quaternary normal faults, intramontane basins and seismicity in the Umbria-Marche-Abruzzi Apennine Ridge (Italy): contribution of neotectonic analysis to seismic hazard assessment

In the Umbria-Marche-Abruzzi (UMA) Apennine ridge, measurements of the Quaternary amount of displacement, and the longterm slip-rates related to the extensional active faults, were carried out by geological and geomorphological analyses. This integrated study revealed that most of the major active extensional faults which bound the intramontane Quaternary basins represent the reactivation of pre- and syn-orogenic extensional structures. As a consequence, the total downthrow of the intramontane basin boundary faults must be divided amongst distinct episodes of extension; furthermore, intramontane basin locations can be strongly influenced by pre-Quaternary geological and structural assemblage. The maximum displacement / fault length (D/L) ratio for Quaternary normal faults shows a best fit along a curve which follows the power-law equation D = 0.027 L1.071. We found slip-rates in the order of 0.4 mm/yr for the central Umbria-Marche Apennines (e.g., Colfiorito basin boundary fault), whereas values ranging between 0.5 and 1.2 mm/yr characterize the southern Umbria-Marche and Abruzzi Apennine ridge. These values are consistent with the southward increase of the faults map length, as well as the southward widening of the intramontane Quaternary basins. Furthermore, our data are in agreement with seismological data for the study area which show the magnitude of the recent and historical major earthquakes, ranging from about M 6 to the north up to M 7 to the south.