Anomalies of Schumann resonances as observed near Nagoya associated with two huge (M∼7) Tohoku offshore earthquakes in 2021

Abstract

Anomalies in Schumann resonance (SR) (or global electromagnetic ELF (extremely low frequency) resonance due to global lightning activity in the Earth-ionosphere cavity) attract lot of attention in possible association with earthquakes (EQs). The first anomalies were observed in Japan for EQs in Taiwan when the distance between observatory and the EQ epicenter was a few Mm (1 Mm = 1000 km). Recently, Hayakawa et al. (2019, 2020a, b) have suggested a new SR anomaly related to the nearby (a few hundred km) EQs unlike the previous distant EQs. This paper presents the SR anomalies observed in the vicinity of Nagoya-city for two relatively close (∼1000 km) successive EQs of the magnitude around 7 that occurred offshore the Tohoku prefecture in Japan. The anomaly is characterized by the enhancement in amplitudes of SR modes, and this abnormal behavior was detected before and after each of the two EQs. Observational data were interpreted using the seismogenic perturbations of the lower ionospheric conductivity. Model computations imply the full wave solution of ELF electromagnetic problem in form of Riccati equation and the 2D telegraph equations. We show that observed anomalous alterations in the SR spectra might be attributed to two types of seismogenic modifications: the compression or the expansion of the vertical profiles of mesosphere conductivity over the EQ epicenter.