Anomalies on Ionospheric Electron Density Before the 2024 Noto Peninsula Earthquake Using Oblique Ionosondes

In order to investigate possible ionospheric anomalies before a magnitude 7.6 earthquake (EQ) in the Japanese Noto Peninsula on 1 January 2024, we used oblique ionosonde data sounding at Wakkanai and Yamagawa with an assumed one-hop reflection point (ORP) only 169 km away from the EQ epicenter, which was not covered by vertical ionosonde observation. The NmF2 at ORP was analyzed. We found a long-lasting negative anomaly from the preceding day to the EQ occurrence, which was 3.37 standard deviations below the previous 30-day mean. We also found a positive anomaly 2 days before the EQ. In addition, we observed a new type of NmF2 anomaly around midday, which has not yet been reported in the literature. The midday NmF2 was anomalous for 10, 8 days, and a few hours before the EQ, implying possible pre-seismic modifications of the mesospheric meridional neutral wind. On the other hand, we also found NmF2 anomalies 3 and 4 days before a major aftershock on 9 January, revealing that the lithosphere–atmosphere–ionosphere coupling (LAIC) remained active, as seismic activity continued some days after the EQ mainshock. Since both positive and negative anomalies were observed before the EQ, pre-seismic atmospheric gravity wave (AGW) activity is the main agent driving LAIC processes. The present study not only reports pre-seismic anomalies before the 2024 Noto Peninsula EQ but also demonstrates the utility of oblique ionosondes on the ionospheric monitoring over Japan, especially for pre-seismic studies.