<論文・報告>巨大地震の発生と電離圏全電子数異常の関係

2011年東北地方太平洋沖地震(M9.0)は, 東日本大震災を引き起こし, 西南日本では今後数十年内に南海トラフ巨大地震が発生し, 西日本大震災が引き起こされると危惧されている. こういった巨大地震の発生予測は, 現状では困難とされているが, 日置(1)により, 東北地方太平洋沖地震発生40分前に電離層全電子数(TEC)異常が生じたと報告されている. TEC異常は様々な要因で発生するため, 地震発生前のTEC異常については地震発生との関連について, 依然議論が続いている. そこで, 本研究では, 日置(1)が指摘したGNSS観測点と衛星の一つのペアについて, 地震前後の20日間, および2011年を通して各月の11日のTEC変動を調べた. 結果として, 地震発生前を除いて, 同様なTEC異常は観測されなかった. しかしながら, 観測点・衛星ペア数や観測期間が限られているため, 巨大地震発生とTEC異常の関連について, 確定的なことは言えず, 今後更に検討すべき課題と考えられる.
The 2011 Tohoku earthquake (M9.0) caused the great East Japan disaster. In southwest Japan, the Nankai earthquake is expected to occur in the next several decades to produce the great West Japan disaster. It is thought that the prediction of such large earthquakes is difficult at present. However, Heki(1) reported the existence of a pre-seismic TEC positive anomaly around 40 minutes prior to the occurrence of the 2011 Tohoku earthquake. Since the TEC anomaly is caused by several factors, a pre-seismic TEC anomaly and its relationship to the occurrence of great earthquakes have been discussed. In this study, for the pair of GNSS station and the satellite where Heki reported the pre-seismic TEC anomaly, I analyzed TEC variations for 20 days before and after the Tohoku earthquake and on the 11th day of each month throughout 2011. As a result, I did not recognize any TEC anomalies, except on the day when the Tohoku earthquake occurred. However, the limited number of GNSS stations and satellite pairs and the analyzed days prevented me from concluding the existence of pre-seismic TEC anomalies prior to great earthquakes. For confirmation, it would be necessary to investigate much more data for longer periods.