Electromagnetic (EM) signals from a seismogenic zone a few weeks to a few days before an earthquake are considered as a promising attribute in earthquake precursory studies. EM perturbations, termed as the coseismic electromagnetic perturbations, also occur within the source zone at the onset of an earthquake and at recording sites during the passage of seismic waves. In the present study, we have analyzed the coseismic electromagnetic perturbations of the M 6.4 Nepal earthquake of 2023-11-03 and its main aftershock (M 5.6) of 2023-11-06, recorded at eight long-period magnetotelluric (LMT) sites installed in profile mode in the Ganga Basin about 150 to 250 km south-west of the earthquake epicenter. The time series were detrended, bandpass filtered, and rotated in the radial and transverse directions using back-azimuth. These time series mimic seismograms and show EM inductions corresponding to the arrival of the P, S and surface waves. Amplification of the surface wave at sites in the middle sector of the profile covering the Sharda depression having thick sedimentary succession and decrease in the amplitude at the southernmost site at the edge of the depression highlight the role of geological heterogeneities in controlling the EM induction. The presence of two very low amplitude consistent peaks at most sites, spread over a profile length of about 120 km, preceding the earthquake by 70 s and 43 s, respectively, is enigmatic. These signals arrive at most sites almost simultaneously and have peak-to-peak amplitude in the range of -0.07 to + 0.11 µV/m and −0.04 to + 0.07 µV/m, respectively, at fourth site. This is probably the first reporting of such preseismic electromagnetic signals in EM time series. We infer that these signals are possibly linked to the fast propagating EM waves generated during the final stage of the earthquake source zone preparation just before the initiation of the rupture. These signals need to be explored in the future for understanding of the causative physical processes. The results also reveal on average about 5 times reduction in the amplitude of the surface wave-induced electric fields with the drop in the earthquake magnitude from 6.4 to 5.6.
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