Passive fault re-activation and hydrotectonics induced by the October 30, 2016 Mw 6.5 earthquake in central Italy

The Mw 6.5 earthquake occurred on 30th October, 2016 in central Italy is the main shock of the seismic sequence began with the Mw 6.0 earthquake on 24th August 2016 and that is still ongoing. The event – the strongest in central Italy since the 1915 Avezzano earthquake (Mw 7.0) – nucleated along the Mt. Vettore-Bove active normal fault system, a major active extensional tectonic structure of the central Apennines. Several km-long and metre-scale surface faulting has been surveyed all along the main splays and along some antithetic structures of the fault system. Some km to the west, in the Mt. Vettore-Bove fault hanging wall, the Norcia basin occurs. It is bounded by another active normal fault system in the east, parallel to the Mt. Vettore-Bove fault, which caused the 14th January, 1703 seismic event (Mw 6.9). After the October 30 event we surveyed hundreds of metres-long surface fractures (up to 15-20 cm vertical throw) in the Norcia plain, not ascribable to any gravitational process. Four trenches dug across these fractures showed that they corresponded to a synthetic and an antithetic splay of the Norcia fault system that constitute a graben. These splays of the Norcia fault system hence locally re-activated and broke the surface, even if the Norcia fault system was not involved in the seismogenic process of the 2016 seismic events. Moreover, the Torbidone river re-appeared in Norcia after the October event, some springs formed or increased the discharge just few hundreds of metres south-east of the ground ruptures, along the southernmost sector of the graben. Coseismic satellite data (InSAR) revealed that the observed surface fractures along the Norcia fault splays occurred in a zone of deformation (characterised by upward bending and western dislocation) in the Mt. Vettore-Bove fault hanging wall, determined by the coseismic slip. Moreover, the satellite data showed a further localised uplift “on top” of the deformation area, coinciding to where the Torbidone river and springs (re)appeared. Geochemical analyses of the waters evidenced a twofold water contribution, that is, groundwater contained within the alluvial infill of the Norcia basin and a much deeper aquifer related to the major carbonate hydrostructure. The collected data indicate that slip of the Mt. Vettore-Bove fault hanging wall triggered minor re-activation (less than 20 cm) of the splays of the nearby Norcia fault system. Moreover, coseismic slip caused deepwater rising along the passively re-activated, pre- existing Norcia fault system splays. This is the first case ever described where the cause of induced/triggered slip on a fault nearby the earthquake fault has been defined and not associated to the vague process of “sympathetic” rupturing, whose mechanisms have never been deeply investigated or proven.