It is not new that the earth's crust in Italy moves continuously under the action of the African and Eurasian continental plates, causing even disastrous earthquakes. Butbeing able to identify the position and extent of the movements linked to a single fault a few kilometers long that breaks during an earthquake, is a result of particular significance for improving knowledge on the seismic hazard of a region.
A GPS station installed near Norcia to study the earthquake (see the photo gallery of INGV activities).
The permanent deformation of the earth's crust caused by the magnitude 6 earthquake which struck the area of the Apennines between Norcia and Amatrice last August 24th was measured not only by satellites with radar techniques, also from GPS stations (Global Positioning System) placed on land in a large region of central Italy. Such stations belong to the Integrated National GPS Networkof the National Institute of Geophysics and Volcanology (INGV), toHigher Institute for Environmental Protection and Research (ISPRA) and the Department of Civil Protection (DPC). They are also presentcornerstones of non-permanent GPS networks, such as INGV's CA-GeoNet and theIGM95 of the Military Geographical Institute. Other GPS data was provided by GNSS networks (Global Navigation Satellite System)of Abruzzo region, Lazio region, ItalyPos,NetGeo, Umbria Region, ASI ed Euref. The stations continuously acquire data on their position thanks to radio signals sent by the constellation of US satellites that have been orbiting the earth 24 hours a day for over 20 years (GPS). The ground displacements recorded in each station were calculated by INGV, analyzing the data with different scientific software (in particular Bernese, Gamit and Gipsy) and subsequently combined to provide a single final result. The displacements were calculated as the difference between the daily positions of the stations in the days before and after the earthquake. In this way the maximum displacements recorded in the individual stations were obtained, including the one located in Amatrice which is the closest to the epicenter of the August 24 shock, with a maximum error of a few millimeters (see figure below and the site of the Integrated National GPS Network for more details on the GPS networks present and cosismic displacement data at the individual stations).
Preliminary analyzes based only on GPS stations active at the time of the earthquake show that it was generated by a fault over 18 km long and inclined by about 50 degrees, which runs in a north-northwest – south-southeast direction and plunges towards the west below the Apennines. The movement of this fault has caused an extension of the Apennine chain of about 3-4 centimeters between the Tyrrhenian and the Adriatic.
GPS data acquired during the August 24 earthquake, as in the case of the last strongest Italian earthquakes (Umbria-Marche in 1997, Molise in 2002 and L'Aquila in 2009), they will allow us to better understand the spatio-temporal evolution of ground deformations measurable on the surface, in the co-seismic and inter-seismic phases, in the vicinity of faults capable of generating strong earthquakes.Joint analysis of GPS data with InSAR spatial data (see the August 30 article “The seismic sequence in central Italy: a first interpretative framework of the INGV”), will allow in the coming days to provide an original and detailed picture of ground deformations and fault characteristics, helping to draw the seismic hazard level of the Apennines in ever greater detail.