2011年3月11日05:46:23 UT，日本東北近太平洋地區發生一芮氏規模9.0的逆衝型地震。此地震造成地表強烈的錯動並引發毀滅性的海嘯。而地震與海嘯垂直震盪與大氣產生同振，此同振以聲重力波(Acoustic-gravity wave; AGW)的形式傳至電離層與游離氣體作用，引發移行電離層擾動現象(Seismo-traveling ionospheric disturbance，簡稱 STID)。利用臺灣與日本之都卜勒電離層探測網觀測地震與海嘯後之移行電離層擾動現象，都卜勒頻譜顯示出明顯的短週期與長週期都卜勒頻移擾動。研究結果顯示此為雷利波與海嘯引發之移行電離層擾動現象，其垂直傳播速度為666 m/s；水平傳播速度約為2.76 km/s與218 m/s。同時使用Hilbert-Huang Transform (HHT)分析高頻都卜勒電離層探測系統與地震儀測得之擾動訊號，估算由地震引發的特徵週期約為15-45秒與180-270秒；海嘯之特徵週期為20-60分鐘。由於海嘯重力波斜向傳播之特性，使得電離層海嘯訊號到達測站時間較海面上海嘯提早約一個小時左右。最後應用圓圈法、波束聚集法與射線追蹤法鎖定移行電離層擾動源頭，驗證此電離層擾動現象起因於日本東北大地震及後續的海嘯。 A megathrust earthquake of magnitude 9.0 occurred near the east coast of Honshu (Tohoku area), Japan on 11 March 2011, 05:46:23 UT, producing a strong vertical earth surface motion and inducing devastating tsunamis. The vertical shock created mechanical disturbance (acoustic gravity waves, AGWs) in the neutral atmosphere, which propagated into the ionosphere and interacted with the ionized gas (hereafter, seismo- traveling ionospheric disturbance; STID). A network of HF Doppler Sounding System in Taiwan and Japan are used to study the STID after the Tohoku earthquake. Results show that the clear STIDs consisting of a package of short-period and long-period Doppler shifts are excited by Rayleigh waves and tsunami waves with a vertical speed of 666m/s; horizontal speed of 2.76 km/s and 214 m/s, respectively. Hilbert-Huang Transform (HHT) is applied to analysis the oscillation signals of ionosphere of HF Doppler and seismometer triggered by Rayleigh waves, the characteristic periods of seismic waves are around 15-45 and 180-270 seconds; the characteristic periods of tsunami waves are 20-60 minutes in the ionosphere. Due to the oblique propagate of gravity wave of tsunami, the arrival time of tsunami wave in the ionosphere approximately one hour ahead of the tsunami in the ocean. Finally, searching the epicenter and the origin of tsunami with the circle method, beam-forming and ray-tracing technique confirms that the observed STIDs are induced by the Rayleigh wave and tsunami waves of the Tohoku earthquake.
