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Significant changes in amplitude and frequency characteristics were observed in the magnetotelluric (MT) time series recorded during Bhuj earthquake (∼7.6 Mw), at a site ∼350 km from the epicenter. The telluric and magnetic signals recorded in the frequency range (10−1-101 Hz) of MT spectrum show considerable variations in their spectral characteristics during the earthquake event compared to the data recorded before and after the earthquake. The spectral analysis brings out sharp changes in amplitude of low-frequency signals during the earthquake as compared to the typical flat spectrum observed before and after the earthquake. The wavelet analysis of the electric and magnetic field data reveals two different spectral regimes; (1) the flat spectrum related to the natural MT signals, and (2) localized, high amplitude signals (in time and frequency) related to the onset of main shock. Three more high amplitude events are noted in the wavelet spectrum, after the main shock event, and can be speculated to be associated with the after-shock events. The MT impedance estimates clearly show scattered apparent resistivity and phase values during the earthquake suggesting that the high amplitude electric and magnetic signals were not related by an MT transfer function. The MT impedance estimates made before and after the earthquake are strikingly normal and smooth. The Pearson’s correlation coefficients between the orthogonal electric and magnetic fields show a drastic drop for the data measured during earthquake, while the MT fields recorded before and after the earthquake are well correlated. The observed MT signals during the seismic activity do not show any external geomagnetic origin and may be attributed to co-seismic EM phenomena. The probable mechanisms responsible for the co-seismic EM phenomena could be electro-kinetic and seismic dynamo effects.

Co-seismic EM signals in magnetotelluric measurement—a case study during Bhuj earthquake (26th January 2001), India