Investigation of Lower Ionospheric Perturbations Associated with the 7 October 2021 Tokyo Earthquake Using VLF Propagation and LWPC Modelling

Abstract

Very low frequency (VLF) radio wave propagation in the Earth–ionosphere waveguide (EIWG) provides a sensitive diagnostic tool of lower-ionospheric perturbations associated with various geophysical processes. In this study, we investigate possible seismo-ionospheric signatures related to the 7 October 2021 Tokyo-area earthquake (EQ) (Mw ≈ 5.9) using sub-ionospheric VLF observations along the NWC (19.8 kHz, Australia) transmitter to Chofu (Japan) propagation path. The analysis focuses on variations in signal amplitude during the evening terminator period, where ionospheric transitions are most pronounced. A significant shift in the sunset terminator time is observed during pre- and post-seismic time. The recorded VLF data are simulated using Long Wavelength Propagation Capability (LWPC) using the well-known Wait’s two-component exponential electron density model with effective reflection height (h′ ) and sharpness factor (β ). Significant pre-seismic anomalies were identified on 05 October 2021, characterized by a lowering of h′ (~3 km) and an increase in β (~0.11 km⁻¹), accompanied by enhanced fluctuations and broadening of the sunset terminator signature. Post-seismic perturbations on 08 October 2021 exhibited similar shifts with a slightly weaker trend. The inferred electron density profiles reveal enhanced ionization at lower altitudes and steeper gradients during disturbed periods compared to quiet conditions. These results suggest that the D/E-region ionosphere underwent measurable modifications prior to and also after the EQ, consistent with lithosphere–atmosphere–ionosphere coupling (LAIC) mechanisms.