Preliminary analysis of lower ionospheric perturbations using VLF/LF transmitter signal associated with 2024 M7.5 Noto Peninsula earthquake in Japan

We report on the lower ionospheric anomalies observed around the occurrence time of the Noto Peninsula Earthquake (M7.5) on January 1, 2024. A case study was conducted based on the continuous time series data of the electric field amplitude acquired by the VLF/LF-transmitter signal-receiving network operated by the University of Electro-Communications in Japan. We used the nighttime fluctuation method to analyze the data and derived the daily changes in (1) trend (the average of the nighttime average amplitude fluctuations) and (2) dispersion (the variance of the nighttime average amplitude fluctuations). As a result, a decrease in trend was observed in 4 of the 8 transmission-reception paths 6 to 9 days before the earthquake. An increase in dispersion was also observed in conjunction with the decrease in trend. These characteristics indicate a short-term lower ionosphere anomaly before earthquakes. For the 2 paths close to the epicenter, a remarkable increase in dispersion was observed one day before the earthquake. This anomaly shows the imminent precursor of the earthquake that contains clear oscillations in electric amplitude in the period of 30 – 240 minutes, implying the LAI coupling due to AGW. The simultaneous occurrence of propagation paths also indicates the spatial extent of the earthquake preparation zone.