Discerning Double Coseismic Travelling Ionospheric Disturbances Following the April 2024 Hualien Earthquake, from GNSS TEC

Abstract

The 2nd April 2024 Hualien earthquake of M7.4 set off coseismic travelling ionospheric disturbances (CTIDs), which were captured and analysed using differential slant total electron content (dSTEC) as recorded by the dense Global Navigation Satellite System (GNSS) network of Taiwan. The CTIDs showed strong southward directivity. A sharp prominent peak, followed by a rather subdued peak was observed ~11-14 mins and ~16-22 mins after the earthquake, respectively. This was more evident from the hodochrone diagrams which revealed CTIDs with dual velocity in the range of AWs – with a peak velocity of ~0.78 km/s and 0.485 km/s for the faster and slower one, respectively. To explain this double AW signature, we assumed a second source for the generation of the slow AW pulse as the fault rupture had propagated NNE of the epicentre and numerically synthesized the AW signatures. The synthetic curves from the two sources showed azimuthal dependency with respect to amplitude and time lag. The wave propagation and CTID waveforms are numerically modelled for the GNSS sites situated south of the equator, which showed significant consistency regarding amplitude and time delay when compared to the observed dSTEC.