Lithological control on multiple surface ruptures during the 2016–2017 Amatrice-Norcia seismic sequence

Abstract

On August 24th 2016, a Mw 6.0 earthquake started the Amatrice - Norcia (Central Italy) seismic sequence, generated by the extensional tectonics along the Apennines, that had its apex with the Mw 6.5 October 30th mainshock. As a unique documented case reported in Italy, complex surface faulting occurred during both earthquakes along the Mt. Vettore fault. Multiple surface faulting was accompanied at depth by the development of a km-scale normal fault-propagation fold. This fold was characterized by breakthrough and by surface rupture within thick carbonatic layers only in the central and north-western area (Mt. Vettore). On the contrary, the fault remained blind where flexural slip was active in sandy-silty turbiditic deposits in the south-eastern area (Mt. Gorzano). We explain the different faulting behaviour with the occurrence of more rigid and competent lithologies in areas characterized by breakthrough and with the occurrence of weaker lithologies in areas characterized by blind faulting. Overall, the entire seismic sequence appears as a gradual gravitational adjustment of the hangingwall block, slipping along a NW-trending and 80 km long fault system. In particular, the following crustal blocks, partially overlapping and with different length (30, 40 and 22 km, respectively), progressively collapsed during the sequence: the Amatrice sector during the August 24th 2016, Mw 6.0 event, the Norcia-Visso sector during the October 26th 2016, Mw 5.9 and the 2016 October 30th Mw 6.5 event, and the Campotosto Lake sector during the four January 18th 2017, M>5 events. The progressive involvement of these three rock volumes, during the seismic sequence is here explained by the occurrence of a low angle detachment that limited the maximum potential depth of the mainshocks and consequently the dimensions of involved rock volumes, therefore limiting the magnitudes of the mainshocks.