Magnetic Field and Electron Density Anomalies from Swarm Satellites Preceding the Major Earthquakes of the 2016-2017 Amatrice-Norcia (Central Italy) Seismic Sequence

A systematic inspection of the magnetic field and electron density, recorded by Swarm three-satellite constellation over the seismic region hit by the 2016-2017 Amatrice-Norcia (Central Italy) seismic sequence, has allowed us to identify some possible precursory anomalies, when disturbed periods of the geomagnetic conditions are properly taken into account and/or avoided. This paper aims at studying and interpreting the electromagnetic phenomena occurred before and during the 2016-2017 Amatrice-Norcia (Central Italy) seismic sequence, in order to look for any possible evidence of precursory anomalies. Results show magnetic field and electron density anomalies of four tracks that precede the major earthquakes of the seismic sequence. After an inspection of the geomagnetic conditions, a Swarm Charlie track, acquired on 20/08/2016 that precedes by 3.2 days the beginning of the whole seismic sequence, remains unexplainable with the normal geomagnetic disturbance phenomena of the Earth's magnetic field. Furthermore, we carry out a blind study of possible relationship between abnormal magnetic field signals detected by Swarm satellites during geomagnetic quiet conditions and major seismic events from about 4 months before the start of the seismic sequence until about the first 8 months from the seismic sequence (i.e. a total of one year of analysed data). We find a very interesting increase of such anomalies starting about 40 days before the beginning of the seismic sequence. It coincides and follows surface and atmospheric alterations, resulting in a temporal sequence of anomalies from Earth's surface up to ionosphere, supporting the possibility of lithosphere-atmosphere-ionosphere coupling models.