Resident Waves in the Ionosphere Before the M6.1 Dali and M7.3 Qinghai Earthquakes of 21–22 May 2021

Geostationary BeiDou satellites monitor the total electron content (TEC) in the ionosphere over certain locations 24 hr per day without interruption and act as ionosphere-based seismometers. The system detected perturbations in TEC before both the M6.1 Dali and M7.3 Qinghai earthquakes that occurred during the night of 21–22 May 2021. The TEC perturbations reside mainly over an area within a distance of ∼700 km from the epicenters of the earthquakes. The standing waves revealed the persistence of a subsurface wave source before the occurrences of the earthquakes, which differs from the co-seismic ionospheric distributions propagating away from the epicenters. The resident waves in TEC and ground vibrations share a frequency of ∼0.004 Hz, which can be attributed to the resonant coupling between the lithosphere and ionosphere.