Rome, 3 November 2016
The earthquake of 30 October in Central Italy produced at least 15 km of fault scarp between the towns of Arquata del Tronto and Ussita, at the intersection of the fault plane responsible for the earthquake and the topographic surface. This cosismic displacement (that is, caused by the earthquake) is common for earthquakes with a magnitude close to or greater than 6 and represents the continuation towards the surface of the rupture and sliding that occurred on the deep fault.
View of the western slope of Mount Vettore where two cosismic fault scarps produced by the event of 30 October can be seen, one higher up along the main fault plane and one further down along a minor fault.
Already after the earthquake of 24 August some escarpments had been observed on the side of Mount Vettore, but they were much more limited (see portion of the fault system highlighted in green on the map - figure below), as well as those marked further north which extend up to Cupi and caused by the earthquake of 26 October (see portion of the fault system highlighted in orange on the map).
Map of active faults (in red) known in the area of the seismic sequence that began on August 24th. The stars in different colors indicate the location of the three main events of the sequence (August 24 M 6.0, October 26 M 5.9, October 30 M 6.5). The colored bands indicate the sectors of the fault system along which cosismic ruptures were produced during the event indicated with the same color (August 24 in green, October 26 in orange, October 30 in pink).
The 30 October fault scarps (see portion of the fault system highlighted in pink on the map) are very evident and appear as a step in the topography of variable size between 20 and 70 cm, their location along the geological fault, together with their geometry and entity of the deformation are entirely consistent with the movement that occurred in depth which reached peaks of more than 2 m which produced the lowering of the western sector with respect to the eastern one. Similar subsidences have also been measured by processing satellite data and all together these observations, carried out on the earth's surface, allow us to understand what happened in depth and therefore to characterize the earthquake and its seismogenic fault.
Cosismic breaks are not randomly located. These occur in correspondence with active geological faults which, in the case of this sequence, form the Vettore-Porche-Bove system already known to Italian geologists. In fact, large earthquakes repeatedly break the same faults and extensional ones cause the lowering and relative lifting of the two portions of crust separated by the fault. The repetition of successive earthquakes along the same faults leads to the accumulation of the deformations of each earthquake which is the basis of the growth of the mountains and the expansion of the basins (eg Mt. Vettore-Piana di Castelluccio). The earthquake is therefore one of the main driving forces in the evolution of the landscape of this beautiful sector of the central Apennines.
Also during the earthquake of 23 November in Irpinia, fault escarpments were produced for about 40 km between Lioni and Sant'Angelo dei Lombardi, with escarpments up to 120 cm high.
In red the trace of the fault scarp produced by the Irpinia earthquake of 23 November 1980 (Mw 6.8 according to the CPTI15).
Fault escarpment of the 1980 Irpinia earthquake on Mount Carpineta, here the vertical rejection reached even 120 cm.
Fault scarp from the 1980 Irpinia earthquake across the Piana di San Gregorio Magno, at the southern termination of the rupture where the vertical outflow was 20-40 cm.
The size of the escarpment and, in particular, its length and height are proportional to the magnitude of the earthquake. The graph shows that for a magnitude 6.5 one can expect the formation of scarps about twenty km long and 40 cm high on average, in agreement with what was observed for the October 30 earthquake.
Empirical relationships linking the earthquake magnitude with the length of the surface fault and with the height of the mean slope (dislocation). The red star is the earthquake of October 30, 2016 (M 6.5) and the blue one is related to the earthquake of 1980 (M 6.8).