## 4 Case Analysis of Concepcion Earthquake
### Ionospheric Structure Anomaly before Earthquake
Therefore, the 15-day investigation showed a unique regional enhancement on February 21 and February 25 near the Concepcion epicenter. The large-scale Nmrf2 abnormal and sporadic ionosphere scintillation in other periods, however, had no such mesoscale local effect as an important distinction of \"source\" type perturbations from other anomalies. The next section discusses the disturbance of electron density in the F2 layer and other ionosphere heights near the earthquake area on February 21 and February 25. In recent years, among the Mw 8+ earthquakes in the South American plate, the ionosphere anomaly usually occurs within 8 days before the earthquake [7]. There is an obvious correlation between different ionosphere parameters [39]. For instance, several ionosphere parameters (GPS-TEC, ion+, electronic temperature K, etc.,) anomalies in the south of the epicenter six days before the events were detected. Near the same geological fault, the ionosphere anomaly drifts from the south-central area to the epicenter north before the Chilean earthquake (April 1, 2014, Mw 8.2) have been previously recorded [7]. At 12-18 UT, February 21, when seismo-ionosphere effects occurred, both disturbed and quiet intervals were divided. Figure 8 shows that nine COSMIC occultation events occurred in the disturbance interval (14-18 UT). Except for the northeast \(\delta\) of \(<\)\(\delta_{threshold}\), the occultation points in other directions all had obvious disturbance characteristics. In this interval, the strongest Nmrf2 anomaly area of the F2 layer appeared in the southeast of the epicenter (\(\delta>0.5\)). Unfortunately, in the quiet interval (12-14 UT), only five COSMIC occultation events appeared in the south of the epicenter. Our deep exploration of the northern ionosphere disturbance characteristics was hindered. The importance of the spatial distribution and sparsity of the COSMIC occultation events is self-evident. During the disturbance interval, the ionosphere's abnormal state in other directions as well as the central and southern parts of the epicenter needs to be further confirmed. Figure 9 shows 10 COSMIC occultation events near the epicenter from 06:00 to 12:00 on February 25. According to the same definitions in Figure 8, six and four COSMIC occultation observation points were captured, respectively, in the disturbance (07-12 UT) and the quiet interval (06-07 UT). During the disturbance interval, the COSMIC occultation event distribution presented large-scale atmospheric oscillations in the F2 layer. A sharp energy exchange of lithosphere-atmosphere may have occurred for up to 5 h over the ionosphere. With the intense disturbance (\(\delta>70\%\)) at 10:32 UT and 11:54 UT, the ionosphere's reaction reached its peak when it diffused around the atmosphere. By COSMIC RO profiles, the change in electron density before some shocks (e.g., the Tohoku event [40]) could be recorded in a response to violent fluctuations. Because the ionosphere may still vibrate even during quiet conditions, a reasonable strategy was adopted to estimate the limitation of \"normal\" fluctuations. Figure 10a shows that a slight fluctuation in the ionospheric profiles was a common phenomenon (\(-2.75\)-\(2.93\times 10^{3}\) el\(\cdot\)cm\({}^{-3}\)). Most of them were within a certain range, and other outliers were in an irregular fluctuation. Figure 8: From 12:00–18:00 on 21 February 2010, the disturbance intensity (\(\delta\)) of the Nmf2 observations. The intense Nmf2 disturbance is distinguished by the depth of color, especially for the dark grey or black patches indicating severe traveling ionosphere disturbances (TIDs) in the F layer. The subgraph on the upper left is the reaction amplification near the epicenter. The triangles and dots in this map represent the occultation event distribution of Period 1 (14–18 UT) and Period 2 (12–14 UT) near the epicenter, respectively. The focal sphere in both graphs represents the epicenter. Figure 9: From 06:00–12:00 on 25 February 2010, the disturbance intensity (\(\delta\)) of the Nmf2 observations.