[S-SS06] Earthquake prediction and forecast

[SSS06-P09] Re-examination of TEC anomalies in the ionosphere before large earthquakes

*Kai Koyama¹, Yoshihiro Kaneko¹ (1.Kyoto University)

Keywords:Predictability of large earthquakes, TEC fluctuations, ionosphere

Since the 2011 Mw9.0 Tohoku earthquake, the possibility of short-term earthquake forecasting based on total electron content (TEC) ionospheric monitoring of the Global Navigation Satellite System (GNSS) has been proposed by Heki (2011) and others. Heki (2011) reported that GNSS data for the 2011 Tohoku earthquake and other large earthquakes exhibit positive TEC anomalies 10 to 80 minutes before the earthquake origin times. In contrast, other studies (e.g., Eisenbeis and Occhipinti, 2021) argued that the positive TEC anomalies reported by Heki (2011) and others are artificial signals arising from their data analysis methods. In this study, we re-examine the presence of "pre-seismic TEC anomaly" before several large earthquakes including the Tohoku earthquake by analyzing data from many GNSS station-satellite pairs comprehensively. To remove noise specific to each station, we apply the stacking of the TEC data over a large area according to the latitude and longitude and remove the moving average. In the case of the Tohoku earthquake, we find that pre-seismic signals pointed out by Heki (2011) appear first in the northeast (Hokkaido) region and later in the southwest (Kyushu) region, which is inconsistent with the expected pre-seismic TEC anomaly spreading outward from the epicentral region of the Tohoku earthquake. This is also confirmed by our analysis using Akaike's Information Criterion (AIC) and animations showing TEC fluctuations over Japan. These results suggest that the pre-seismic anomaly was likely unrelated to the earthquake itself, but was linked to large-scale traveling ionospheric disturbances (LSTID), a phenomenon in which fluctuations in electron density propagate through the ionosphere. Our results further suggest that similar phenomena may explain pre-seismic TEC signals observed for other large earthquakes. The results of this study would help resolve the ongoing debate on whether pre-seismic TEC signals exit or not.