Space-based GNSS radio signals to investigate ionospheric plasma changes preceding the 2016 Al Hoceima-Morocco earthquake, Mw=6.4

Abstract

This paper examines abrupt variations in ionospheric electron density preceding the Al Hoceima earthquake (January 25th, 2016, Mw = 6.4, Northern Morocco). The observed anomalous behavior in the F2-ionospheric region, at about 350 km altitude, raises the possibility of a connection with impending seismic activity of moderate-to-great magnitude, supporting the hypothesis that Total Electron Content (TEC) variations could serve as potential earthquake precursors. For this purpose, we have exploited one of the main ionospheric keys, TEC, through a network of over one hundred dual-frequency Global Positioning System/Global Navigation Satellite Systems (GPS/GNSS) receivers. Through calculation algorithms based on spherical harmonic analysis of GPS/GNSS Observation-Navigation data, we were able to produce local ionospheric maps to restore the GPS-TEC signal and investigate potential ionospheric disturbances associated with this shallow-focus and strong earthquake. Since the ionospheric TEC is a function of the variability and dynamics of the Earth’s ionosphere, mainly time-space and solar-geomagnetic activities, we had to consider each of these disturbing factors separately. In fact, the seismic zone of Al Hoceima (Morocco, North Africa), at about 35°N latitude, belongs to the region of low geographical latitudes. In such regions the variations of the ionospheric layer are slight. Moreover, during the earthquake preparation period, space weather conditions exhibited a calm state characterized by low solar activity and the absence of geomagnetic storms. The adequate effects of these physical conditions allow us, through wavelet transform, to emerge solely signatures of earthquake-related ionospheric disturbances. Based on the seismo-ionospheric combination model, we have highlighted some ionospheric electron density irregularities that decreased abnormally, near the epicenter several days prior to the 2016 Al Hoceima main event. From these findings, we can state that such research provides a promising approach for predicting earthquakes through large fluctuations in the ionized shell of the Earth’s atmosphere, thereby offering the prospect of a life-saving seismic alarm.