## 3 Results
### Case Studies  
Before a more quantitative, global statistical analysis is made, some case studies are presented to visually check the results of the technique proposed. To do that, videos were created showing the daily \(\Delta S_{4}\) overlapped with the earthquakes. These videos are provided as supplementary materials. Video S1: anomaly_s4_180 days_fullCoverage.m4v) shows the full region covered by CYGNSS for the whole period covered in the study, and Video S2: case_study.m4v show the period and region corresponding to the case study detailed here. As in the previous figures, in the videos, earthquakes are kept in each frame for six days before they occur until six days after, so it is possible to visually check whether there is an increasing \(S_{4}\) anomalous deviation before the occurrence of an earthquake, and if it stays for some time after the earthquake happens.  
A case in which some positive variations in the \(S_{4}\) index are distributed around an earthquake cluster occurred at the beginning of July 2019, in the oceanic region of Vanuatu's archipelago. Vanuatu is a group of islands of volcanic origin that formed after the subduction of the Australian plate under the Pacific plate, which makes this region suffer from large recursive earthquakes [32]. Some of the largest earthquakes from the cluster mentioned are shown in Figure 10, indicating their magnitudes, UTC times, and depths. Please note that all of them are located eastward of the fault, where the subduction takes place. The circular curves around the earthquakes indicate the shakemap Modified  
Figure 8: \(S_{4}\) variations map computed from the seven days prior to \(16\) March 2019, after subtracting the preceding \(60\) day average over all oceanic regions between \(40^{\circ}\)S and \(40^{\circ}\)N latitudes. Blue stars indicate the earthquakes’ epicenters happening from six days before to six days after.  
Figure 9: \(S_{4}\) variations map for one of the minimum coverage days, from \(-40^{\circ}\) to around \(-5^{\circ}\) in average.  
Mercalli Intensity (MMI) contours, which are defined from I to X measuring the intensity of the effects produced on the surface.  
During the last few days of June and the first week of July, several positive variations in the ionospheric scintillation values were found in this region. Moreover, the anomalies appear after a period of stabilization in the scintillation activity, indicated by close-to-zero \(\Delta S_{4}\) values in most of the area.  
Figure 11a shows the map of the region around the Vanuatu archipelago for 30 June, a few days before the earthquakes shown in the previous image. Figure 11b, shows the map for 1 July, the day of the strongest earthquake (M6.0) in this cluster, and an increase is observed in the closest pixels to this earthquake, and also in the Southern New Caledonia region. Figure 11c corresponds to 4 July, and the peaks in \(S_{4}\) variations are now clear in the whole area, and that can also be related to other earthquakes happening in 6 July in the South. It is important to remark that \(\Delta S_{4}\) is computed from the average of the previous 7 days, so, variations are shown for the same day \(D\) are produced before it, even if the day \(D\) is a few days after a particular earthquake.  
Finally, Figure 11d shows 7 July, in which the positive values in the \(S_{4}\) deviations persist although the northern region is starting to miss data due to the oscillation in the region covered shown before.