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.

• Ground vibrations and total electron content (TEC) perturbations share a frequency of ∼4 mHz • Geostationary BeiDou satellites detected TEC wave prior to large earthquakes • TEC perturbations resided persistently around the epicenters

Supporting Information:
Supporting Information may be found in the online version of this article.
Scientists study the LAI (Lithosphere, Atmosphere, and Ionosphere) coupling utilizing multiple geophysical parameters before earthquakes (Hayakawa, 2015(Hayakawa, , 2016. Acoustic-gravity waves originate near the Earth's surface and propagate upward into the ionosphere that is considered to be a potential channel in the LAI coupling before earthquakes (Hayakawa, 2011;Liu et al., 2016). However, the seismo-LAI coupling via the acoustic-gravity channel is generally referred to thermal anomalies due to that obvious ground vibrations are barely observed before earthquakes. Bedford et al. (2020) and  observed that ground vibrations persist in a large-scale area for a long time before earthquakes. Meanwhile, Chen, Sun, et al. (2020) reported that the frequencies of the ground vibrations vary from low (∼10 −4 Hz) to high (∼10 −3 Hz) through statistical methods. The ground vibrations share the frequency with the acoustic-gravity waves that drive us to investigate the seismo-LAI coupling via the acoustic-gravity channel. In this study, the TEC from the geostationary BeiDou satellites are utilized for mitigating influence due to orbiting satellites. Ground vibrations from the broadband seismometers are compared with the TEC to examine their relationship associated with two major earthquakes in China during the 21-22 May 2021.

Results and Discussion
Fluctuations in TEC have been observed by the novel instrumental system named "Monitoring Vibrations and Perturbations in Lithosphere, Atmosphere, and Ionosphere" (MVP-LAI; 29.6°N, 103.9°E;  http://geostation.top/) during the night since mid-May 2021 ( Figure 2). The TECs from the geostationary BeiDou satellites reach their maxima near noontime and exhibit highly perturbed in the night time. To examine the perturbations in detail, the moving average with a 60-min temporal window was removed from the raw TEC time series to mitigate the variations in local time caused by dynamo and photochemical processes (Davies, 1990). The perturbations with amplitudes of ∼0.2 TECU occurred in both the day and night before May. The pronounced perturbations (∼1 TECU amplitude) persisted for ∼40 days after 18 May and attenuated gradually in late June ( Figure 2). The middle-scale traveling ionospheric disturbances (MSTIDs) in the GPS TECs prefer to occur over China in the night time of summer Ding et al., 2011). Typically, the MSTIDs with velocity of ∼100 m/s to ∼200 m/s propagate in various directions in day and night (Cheng et al., 2021;Otsuka et al., 2021). The amplitude of the MSTIDs observed by GPS is less than 1 TECU that is smaller than that observed by the Geostationary BeiDou satellites at the MVP-LAI system after 18 May ( Figure 2). Atmospheric gravity waves could cause the daytime MSTIDs, and electro-dynamical forces, such as the Perkins instability, could cause the nighttime MSTIDs . The MSTIDs mainly propagate in the southwest direction from high to low latitudes in nighttime (Otsuka et al., 2009;Saito et al., 1998). On the other hand, ionospheric plasma bubbles can cause large TEC changes in night time. However, according to the GPS TEC and ROCSAT in-situ observations, ionospheric plasma bubbles should be weak over Asia in the summer under the low solar activity condition (Su et al., 2006;Sun et al., 2015)  the Dali and Qinghai earthquakes, respectively. The occurrence of the two earthquakes accompanied by the pronounced TEC perturbations in night time.
A sufficient quantity of the TEC data observed by the GNSS network covered the period of the two earthquakes, which benefits to investigate if or how the perturbations are related to the two earthquakes. A total of 170 groundbased GNSS receivers that are operated by the Crustal Movement Observation Network of China and receive electromagnetic signals from the five geostationary BeiDou satellites were utilized to retrieve continuous TEC data ( Figure 1). These retrieved TEC data were then utilized to investigate the pronounced fluctuations at the pierce points at an altitude of 350 km in the spatiotemporal domain (Liu et al., 1996). TEC perturbations with an amplitude of >1 TECU distributed mainly over an area south of the epicentre of the Dali earthquake in the post-sunset hours of 21 May 2021, nearly 1.5 hr before the earthquake occurrence (Figures 3a and 3b; also, see Figures S1-S2 in Supporting Information S1 and Movie S1). The perturbations reside over an area almost within a distance of ∼700 km from the epicentre of the earthquake (marked by the rectangle in Figure 3c). Moreover, the co-seismic perturbations in TEC propagated away from the epicentre at a velocity of ∼150 m/s (Figure 3c). The co-seismic perturbations can be seen on the map in Figures 3d and 3e near the epicentre of the Dali earthquake.
However, perturbations with amplitudes greater than 1 TECU appeared around the time of the Qinghai earthquake (Figures 3d and 3e; also, see Figures S1-S2 in Supporting Information S1 and Movie S1). They were distributed mainly within a distance of ∼850 km from its epicentre (marked by the rectangle in Figure 3f). The perturbations appearing more than ∼900 km beyond the epicentre after 24:00 local time maybe the middle-scale traveling ionospheric disturbances) propagating from higher latitudes (Cheng et al., 2021;Otsuka et al., 2021). The waves resided over the Qinghai earthquake from 20:30-23:30 local time (marked by the rectangle in Figure 3f). In short, the perturbations in TEC resided over the areas within ∼700 km of the epicenters of the two earthquakes before their occurrences even though they exhibit distinct characteristics. The persistence of ionospheric perturbations over a particular location suggests a wave source beneath the perturbations (Chou et al., 2017;Sun et al., 2019). Typically, MSTIDs propagate in the southwest direction with velocity of ∼100 to ∼200 m/s from higher latitude in nighttime (e.g., Otsuka et al., 2021). Bubbles usually occur at low latitudes and propagate eastward with phase velocity of ∼100 m/s (Haase et al., 2011;Saito et al., 2008). However, the TEC waves as shown in Figure 3 (also see Figures S1-S2 in Supporting Information S1 and Movie S1) persisted mainly over a specific location. Accordingly, the pronounced TEC perturbations recorded by the geostationary BeiDou satellites during the study period are unlikely the signature of the typical MSTIDs or bubbles.
The persistence of ionospheric perturbations can be contributed from variations of the atmospheric boundary layer. Pulinets and Davidenko (2018) observed the positive anomaly in TEC formed at nighttime (i.e., between sunset and sunrise) emerging within a few days before strong earthquakes. The positive anomaly is referred to regulations of the height distribution of cluster ions due to the formation in the ionosphere bound with the diurnal dynamics of the atmospheric boundary layer (Pulinets & Davidenko, 2018). Alternatively, the perturbations can be dominated by ground vibrations in the lithosphere. A previous study  utilized the multiple instruments to verify vibrations and/or motions in the lithosphere, triggering changes in the TEC in the ionosphere. Long-term crustal vibrations can be found in regions where earthquakes are forthcoming; these have been detected by multiple instruments including broadband seismometers, ground-based GNSS receivers, and magnetometers distributed over a wide spatial area . These long-term crustal vibrations exhibit variable frequency characteristics at frequencies of ∼5 × 10 −4 Hz tending to ∼5 × 10 −3 Hz along the approaches of forthcoming earthquakes due to variations in areas with increased seismicity (Chen, Sun, et al., 2020). Continuous seismic data from a broadband seismometer in the MVP-LAI system show the variable frequency characteristics associated with the two earthquakes ( Figure 4a). Enhancements in the power spectrum density from the ground vibrations were observed at a frequency of ∼0.003 Hz in mid-April 2021; the frequencies of such enhancements tended to be high over time and reached ∼0.005 Hz a few days before the occurrence of the two earthquakes ( Figure 4a). The enhancements with frequencies varying from 0.003 to 0.005 Hz shown in Figure 4a confirmed the existence of the phenomenon of the variable frequency of ground vibrations before major earthquakes, as reported in previous studies (e.g., , Chen, Sun, et al., 2020. However, enhancements in the TEC appear mainly in a frequency band between 0.004 and 0.006 Hz (Figure 4b).
Enhancements with frequencies varying from ∼0.004 to ∼0.005 Hz can also be observed during 30 April to 20 May. Previous studies (Chou et al., 2020;Dautermann et al., 2009) have reported that tsunamis, volcanic eruptions, and Rayleigh waves in the lithosphere can trigger such variations in the TEC. Observations and numerical simulations indicate that ground vibrations and the TEC share a frequency of ∼0.004 Hz (Chen, Saito, et al., 2011;Dautermann et al., 2009;Matsumura et al., 2012;Saito et al., 2011) due to the resonant coupling in the lithosphere, atmosphere, and ionosphere. The evolution of similar frequencies in the ground velocity and the waves persistently reside over the epicenters before the two earthquakes that suggest the TEC resident waves due to the resonant LAI coupling. Notably, the anomalous frequency band observed in this study is close to it associated with acoustic waves. The factor of the acoustic waves can be entirely excluded after an examination of ground vibrations leading to the TEC anomalies.

Conclusion
Observations of the TEC made by geostationary BeiDou satellites suggest that the resonant coupling caused perturbations in the TEC before the M6.1 Dali and the M7.3 southern Qinghai earthquakes. Perturbations with amplitudes >∼1 TECU resided persistently over the epicenters of these earthquakes owing to the resonant coupling. Ground vibrations with variable frequencies close to 0.005 Hz are promising candidates for the sources of the TEC perturbations. Perturbations in the TEC associated with resident waves due to the resonance at a frequency of ∼0.004 Hz were clearly identified as occurring both after the earthquakes as previous studies (Dautermann et al., 2009;Saito et al., 2011) had shown-and before them. The results suggest that the TECs recorded by the BeiDou satellites can function as space-based seismometers detecting perturbations from the subsurface. The resonant coupling differs from the electric field dynamo, which is one of the major candidates for ionospheric earthquake precursors recommended in previous studies (Sun et al., 2019 and references therein). Atmospheric gravity waves that propagate from near the Earth's surface and reach and break around the dynamo region can be examined further (Liu et al., 2009Oyama et al., 2016;Sun et al., 2011). and the Sichuan earthquake Agency-Research Team of GNSS based on geodetic tectonophysics and mantle-crust dynamics in the Chuan-Dian region (Grant no. 201803), Fundamental Research Funds for the central Universities of China (Grant no. JZ2021HGPB0058). Meanwhile, this work was also supported by the Center for Astronautical Physics and Engineering (CAPE) from the Featured Area Research Center program within the framework of Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The authors would like to thank the reviewers for their comments that help improve this paper.