## 3 Results and Discussion
### Ionospheric Disturbance during Earthquake  
Besides the displacement, the earthquake also would cause ionospheric disturbances. Therefore, the VTEC variation during the earthquake is analyzed in this section. The ionospheric VTEC from SF-PPP will be used to interpolate the VTEC above the epicenter. Thus, the accuracy of VTEC interpolation highly relies on the geographical distribution of GPS observation stations around the earthquake location. According to Figure 2, the GPS stations utilized in the Tohoku-Oki earthquake present an abnormal geographical distribution. Therefore, only the ionospheric disturbance in the Lushan earthquake is analyzed in detail. Generally, the direct phenomenon of the seismic-ionospheric coupling is atmospheric oscillation. When an earthquake happens, gravity waves will be generated, and propagated to the ionosphere, causing ionospheric anomaly disturbances [54]. While pursuing the origin of atmospheric oscillation excitation, there are three possibilities [55]: (1) the block structure produces an undulating motion during the earthquake preparation; (2) the heating of greenhouse gas released near the active fault causes abnormal thermal radiation in the near-surface atmosphere [56]; (3) the inert gas is released near the active fault [57]. Moreover, according to the works of [PERSON] [20], [PERSON] [19], and [PERSON] [18], earthquake acoustic waves can also cause perturbations in the ionosphere. The earthquake energy is released in the shape of seismic waves. We adopt GPS data from 22 CMONOC stations in Sichuan Province to investigate ionospheric anomalies during the Lushan earthquake. The distribution of those stations is shown in Figure 5. Solar and geomagnetic activities and variations in weather bring a strong disturbance effect on the ionosphere. Therefore, it is necessary to eliminate these effects before identifying whether an ionospheric anomaly is engendered by the earthquake. According to [PERSON] et al. [58], during the Days Of Year (DOY) of the 90-120 period, solar activity was at a low level ([[http://www.sepc.ac.cn/](http://www.sepc.ac.cn/)]([http://www.sepc.ac.cn/](http://www.sepc.ac.cn/)), accessed on 27 August 2019) and weather conditions were normal, except for the DOY 101, 102, and 112. The geomagnetic indices were extracted on the website [[http://wds.kugi.kyoto-u.ac.jp/](http://wds.kugi.kyoto-u.ac.jp/)]([http://wds.kugi.kyoto-u.ac.jp/](http://wds.kugi.kyoto-u.ac.jp/)), accessed on 27 August 2019. According to Dst and Kp, from April 13 th to 27 th (DOY 103-117), there were geomagnetic disturbances on the DOY 114-116, and the geomagnetic field was low on the other days. The IDW method mentioned in the SF-PPP-based regional VTEC model part is utilized to analyze the distribution of VTEC near the epicenter before and after the earthquake. The VTEC data on DOY 105, 106, 109, and 110 (the day earthquake happened) are applied for spatial interpolation. The seismic-ionosphere effect and the spatial distribution/variation of ionospheric anomaly calculated by SF-PPP are presented in Figure 8. In addition, the time series of VTEC on DOYs 105, 106, and 110 are shown in Figure 9. It is obvious that the VTEC on DOY 110 is lower than DOYs 105 and 106. In contrast, the corresponding solutions from GIM data are shown in Figure 10. According to Figure 8, compared to DOY 105 and 106 before the earthquake, the VTEC near the earthquake epicenter decreased on DOY 110. However, the location of the VTEC anomaly area was inconsistent with the vertical projection of the earthquake epicenter. Such an area appeared significantly in the south of the epicenter, while the VTEC was relatively lower above the earthquake zone. Meanwhile, there were no reports of obvious or approximate VTEC anomalies in other parts of the whole world before and after the Lushan earthquake [58]. These pieces of evidence further prove that the ionospheric anomaly was associated with the Lushan earthquake.