[U-15] The 2024 Noto Peninsula Earthquake (1:J)

[U15-P77] What do we know from traveling ionospheric disturbances triggered by the 2024 Noto earthquake measured with two dense GNSS observation networks in Japan?

*Atsuki Shinbori¹, Yuichi Otsuka¹, Takuya Sori³, Michi Nishioka², Septi Perwitasari² (1.Institute for Space-Earth Environment Research (ISEE), Nagoya University, 2.National Institute of Information and Communications Technology, 3.Research Institute for Sustainable Humanosphere (RISH), Kyoto University)

Keywords:Dense GNSS observation network, Noto earthquakeuake, Ionosphere, Traveling ionospheric disturbances, Total electron content, Atmospheric waves

The Earth’s ionosphere is formed through the partial ionization of the upper atmosphere by solar extreme ultraviolet (EUV) radiation and precipitation of energetic particles from the magnetosphere. The electron density variation in the ionosphere is caused by both solar activity and lower atmospheric disturbances associated with metrological phenomena, earthquakes, tsunamis, and volcanic eruptions. It has been well-known that traveling ionospheric disturbances (TIDs) with a concentric wave structure appear approximately 8 minutes after the onset of large earthquakes and volcanic eruptions [e.g., Hebert et al., 2020; Shinbori et al., 2022]. The generation mechanism of TIDs is a neutral oscillation in the thermosphere driven by acoustic waves, which are generated by the vertical motion of lower atmosphere related to earthquakes and volcanic eruptions. In a case of the 2024 Noto earthquake with a magnitude of 7.6, such TIDs appeared with a concentric wave structure over Japan in the two-dimensional maps of detrended total electron content (TEC) obtained from two dense global navigation satellite system (GNSS) observation networks (GEONET and SoftBank) in Japan. In this study, we elucidate the temporal and spatial evolution of TIDs and its peculiarity after the onset of the 2024 Noto earthquake by analyzing GNSS-TEC data with high spatial resolution of 0.02 degrees x 0.02 degrees in geographical latitude and longitude. In the present analysis, we used the 15-min high-pass filtered (detrended) TEC data and subtracted the 3-min running median value from the detrended TEC to eliminate the background TIDs with a much slower propagation velocity than the TIDs related to the earthquake. As a result, the TIDs with a concentric wave structure appeared around the epicenter of the earthquake approximately 8.5 minutes after the onset of the earthquake and the wave structure expanded radially with time. The amplitude of the southward propagating TIDs was much larger than that of the northward propagating TIDs. Considering an inclination of magnetic field lines over Japan, an angle between the wave vector of the acoustic wave propagating southward and magnetic field line becomes small. In this case, charged particles in the ionosphere can more easily move along the magnetic field line on the south side. Therefore, the amplitude of the TEC variations becomes larger on the south side of the epicenter. The initial excursion of the detrended TEC variations occurred almost along the fault planes that moved up and down due to earthquakes, and the TEC amplitude was spatially inhomogeneous. This result suggests that the TEC variations are driven by the acoustic waves generated at several points along the fault planes. From the distance-time plot of the detrended TEC, the propagation speed of the TIDs was estimated approximately 1,000 m/s. This propagation speed is almost consistent with that of the acoustic wave propagating in the thermosphere at an altitude of 550 km. Because the propagation speed of TIDs is faster than that of tsunami, we can obtain information of the occurrence of tsunami from ionospheric observations in advance.

Acknowledgments
The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science".