Schumann resonance anomalies associated with two (M~7) Tohoku offshore earthquakes in the spring of 2021

Abstract

ABSTRACT. We describe the Schumann resonance (SR) anomalies associated with two earthquakes (EQs) observed in Japan in the spring of 2021. SR is the global electromagnetic phenomenon observed in the ELF (extremely low frequency) band, and its resonance peaks are observed in the power spectra on natural radio noise at frequencies of 8, 14, 20, Hz, etc. The natural source of ELF radiation is the global lightning activity occurring in the Earth-ionosphere cavity. The anomalies were observed for the first time in Japan for the EQs in Taiwan when the distance between the observatory and the EQ epicenter was a few Mm (1 Mm = 1000 km). Recently, a new SR anomaly was addressed, related to nearby (a few hundred km) EQs (Hayakawa et al., 2019, 2020a,b). This paper presents the SR anomalies observed in the vicinity of Nagoya-city for two relatively close (~500 km) successive EQs with magnitude around 7 that occurred offshore the Tohoku area in Japan. The anomaly is characterized by the noticeable simultaneous increase or decrease in the amplitudes of three SR modes. This SR unusual behavior was observed prior to and after each of the two EQs that occurred in February and March, 2021. Observational data were interpreted in the model of seismogenic perturbations of the lower ionospheric conductivity profile. Model computations imply the full-wave solution of the ELF electromagnetic problem in the form of the Riccati equation and the 2D (two dimensional) telegraph equations. We show that observed disturbances in the SR power spectra might be attributed to two types of seismogenic modifications in the lower ionospheric profile: the compression or the expansion of the vertical profiles of mesospheric conductivity over the EQ epicenter.