Geomagnetic and Ionospheric Effects of Two Consecutive Strong Earthquakes in Morocco on September 8, 2023

Abstract—

The geophysical effects of a strong seismic event as two earthquakes with magnitudes of 6.8 and 4.9 occurred on September 8, 2023, in Morocco at close times 22:11 and 22:30 UTC with an epicentral distance between the foci of ~4 km were considered. The data of a number of INTERMAGNET observatories and the magnetic recording results obtained at the Mikhnevo Geophysical Observatory of Sadovsky Institute of Geosphere Dynamics of Russian Academy of Sciences, were used. In the absence of considerable global disturbances of the Earth’s magnetic field, earthquakes were accompanied by a series of three positive bay-shaped geomagnetic variations with a maximum amplitude from ~1 to ~10 nT, following each other after ~60 min. The maxima of the induced magnetic field variations were observed almost synchronously at a distance from ~800 to ~10000 km. The magnetic effect delay time relative to the main shock of the first earthquake was ~70 min. Taking into account the almost planetary nature and high synchronicity of the magnetic field disturbances caused over a significant range of distances, as well as the time delays corresponding in order of magnitude to the travel time of the seismic signal of a distance multiple of the Earth’s dimension, it is suggested that the magnetic effect of the seismic event in question was caused by a global source such as an excited geodynamo. The ionospheric effect of the seismic event under consideration is reported as variations in the critical frequency f₀F2 calculated from the data obtained by the ground-based sounding station of the del Ebre Observatory.