Seismic shaking recorded simultaneously on TEC and PPP: a case study of the Hualian earthquake

An M7.4 magnitude earthquake struck Hualien, Taiwan, on April 3, 2024, at 07:58:12 local time. We collected data from ground-based Global Navigation Satellite System (GNSS) receivers near the epicenter and retrieved coseismic displacements using precise point positioning (PPP) and ionospheric total electron content (TEC) from dual-frequency phase observations received from geostationary Earth orbit (GEO) satellites. The TEC data reveal not only coseismic ionospheric disturbances occurring ~10 minutes after the earthquake, but also disturbances occurring ~1 minute post-event, which are time-synchronized with the coseismic displacements. This type of TEC disturbance is consistently observed across three stations. We projected the displacement in two directions: the line-of-sight (LOS) between the satellite and receiver, and the normal plane to the LOS. The trends in TEC are consistent with the displacements projected onto the LOS normal plane, with a magnitude relationship of approximately 0.05 TECU increase in TEC per 10 cm increase in displacement. In contrast, displacements projected along the LOS direction do not show the same trends in TEC. Therefore, we confirm that LOS movement caused by seismic shaking of GNSS receivers affects the calculation of GNSS-based GEO-TEC.