Precursory anomalies in the seismo-ionospheric-atmospheric domain associated with offshore earthquakes of magnitude greater than 7.0

Abstract

Research on ionospheric and atmospheric anomalies related to earthquakes has advanced our understanding of lithosphere–atmosphere–ionosphere interactions. Despite this progress, precursor signals are often obscured by solar disturbances such as flares, coronal mass ejections, and geomagnetic storms (i.e. Solar Flares, solar wind speed, interplanetary magnetic field, etc.). This study investigates seismo-ionospheric and atmospheric anomalies associated with two offshore Japanese earthquakes with magnitudes Mw = 7.1 (37.156oN, 144.661oE) and Mw = 7.3 (37.89oN, 143.949oE) using GNSS-derived Total Electron Content (TEC), geomagnetic indices (Kp, Dst, AE), and foF2 data. Anomalies were identified using three statistical techniques: (1) mean ± standard deviation, (2) median ± interquartile range, and (3) wavelet transform-based detection. All methods revealed anomalies 4–10 days before and 3–5 days after the events. To distinguish seismic precursors from solar-induced variations, geomagnetic indices (Kp, Dst, AE, foF2, and quiet-to-storm day ratios) were integrated using both soft (OR) and hard (AND) logic constraints. Furthermore, NOAA/NCEP Outgoing Longwave Radiation (OLR) composite maps showed spatiotemporal alignment with TEC anomalies, supporting the diffusion theory of seismo-ionospheric coupling. The strong consistency across detection methods enhances confidence in these findings. This study offers a robust integrated approach for identifying earthquake-related anomalies under both geomagnetically quiet, improving previously established methodologies.