Characterisation of Coseismic Ionospheric Perturbations Associated with the 20 April 2026 Mw 7.4 Miyako, Japan Earthquake

On 20 April 2026 at 07:52:58 UTC, an Mw 7.4 earthquake struck offshore Miyako, northeastern Japan. We investigated the resulting coseismic ionospheric disturbances (CIDs) using 1 Hz multi-frequency GNSS observations and ionosonde measurements. Slant total electron content derived from L1–L2, L1–L5, and L2–L5 combinations was detrended using a Savitzky–Golay filter and band-pass filtered at 2–10 min. Directional hodochrone analysis showed strong azimuthal anisotropy, with the observed perturbations propagating at apparent horizontal velocities of 0.41–1.92 km s−1 across four sectors, substantially below typical Rayleigh-wave-associated CID velocities. The observed timing and waveform characteristics are consistent with vertically excited acoustic disturbances sampled obliquely by the GNSS links. Independent ionosonde observations at Wakkanai, Icheon, and Okinawa demonstrated Multiple Cusp Signatures in the bottomside F-region within 12–17 min, with decreasing cusp sharpness toward lower latitudes. Geomagnetic and solar indices indicate relatively quiet conditions during the observations, reducing the likelihood of contamination by disturbances driven by space weather. The results enrich our understanding on coseismic ionospheric disturbances in the Mw 7–7.5 range, showing that moderate subduction earthquakes can produce direction-dependent ionospheric signatures, with apparent velocities lower than typical Rayleigh-wave-associated CID velocities, detectable through combined GNSS-TEC and ionosonde monitoring.