Doppler Frequency Shifts Associated With the 18 September 2022 M6.8 Taitung Earthquake Observed by Local CW‐HF Doppler Sounding Systems in Taiwan

Doppler frequency shifts (DFSs) recorded by local continuous wave‐high frequency (CW‐HF) Doppler sounding systems have been used to study seismo‐traveling ionospheric disturbances (STIDs) triggered by seismic waves of M ≥ 5.5 earthquakes in the Taiwan vicinity during 2000–2022. We observed the first significant STID in DFSs induced by the 18 September 2022 M6.8 Taitung earthquake. To investigate this significant STID in more detail, DFSs of five sounding frequencies recorded by a network of nine receiving stations of the CW‐HF Doppler sounding systems, together with the total electron content (TEC) derived by 35 ground‐based GPS receivers, ionograms, and seismograms, are examined. Results show that horizontal speeds of the significant STID in DFSs of the 6.1 MHz sounding frequency and in TECs are 1.4 and 1.3 km/s, respectively. Upward propagation with a speed of sound of 0.72 km/s is estimated through DFSs of the five sounding frequencies, where the reflection heights are about 160–225 km altitude. We further examine DFSs associated with the top nine earthquakes in the Taiwan vicinity during 2022–2023. Results show that for near field observations, in addition to the earthquake magnitude, the total earthquake intensity plays an important role in triggering significant STIDs.