# Monitoring of Ionospheric Anomalies Using GNSS Observations to Detect Earthquake Precursors
## 3 Results
### Model Test Using the 2023 Turkey Mu 7.8 Earthquake  
The second case examined in this work was the Mw 7.8 earthquake that occurred on 6 February 2023, in the southwestern region of Turkey, near the Turkey-Syria border. The network of GNSS stations used to investigate the presence of ionospheric anomalies is shown in Figure 7. The network consists of 55 IGS stations; 52 of them are double-constellation (GPS+Glonass), and the remaining 3 track only GPS signals (data retrieved from IGS, Sonal, UNAVCO, and CDDIS databases). In our analyses, we considered the period from DOYs 1 to 37 of 2023. A cutoff angle of 12\({}^{\circ}\) and a ionospheric layer height of 450 km were used. The position of the center of the cap has a longitude of 37\({}^{\circ}\) and a latitude of 37\({}^{\circ}\), while the cap amplitude is 82\({}^{\circ}\). Finally, data were processed every 300 s.  
Figure 5: Maps of DTEC at 08 UT: (**a**) from GIMs; (**b**) from the proposed model. Figure 8a shows the VTEC series computed at the epicenter point and compares it with the VTEC retrieved from GIMs. Also, in this case, it is possible to see a few negative values caused by the same factors discussed above. There are a small number of GNSS stations available within the central-northern region of Africa. The differences are slightly smaller than those reported in Section 3.1 and are around 15% here. The VTEC rate SNR, along with its own 4-hour moving average, is shown in Figure 9a. In this image, there are periods characterized by a higher noise level (as the moving average seems to report). In Figures 8 and 9, the VTEC and VTEC rate SNR are also combined with the 24h map of their values (Figure 8b and Figure 9b, respectively) for each DOY (once again, computed in the epicenter of the earthquake) and finally compared with the plot of the geomagnetic activity index Kp reported in Figures 8c and 9c. Here, we can notice that (i) the high VTEC values in DOYs 15 and 18 correspond to a time span during which Kp values indicate an increased solar activity and (ii) for DOY 37, when the earthquake occurred, the VTEC value is high, especially in the central hours of the day, if compared with the days before. In the image of the 24h VTEC rate SNR, there are no visible effects of solar activity. In any case, the analysis of the local VTEC and the local VTEC rate SNR at the epicenter point does not produce any apparently substantial and definitive information about ionospheric anomalies. Figure 6: Maps of VTEC rate SNR: (**a**) at 03 UT; (**b**) at 08 UT. Figure 8: VTEC time series, 24 h VTEC, and Kp index for an overall comparison (all the values were computed at the epicenter of the Turkey earthquake): (**a**) VTEC time series of values estimated through the proposed model (blue dots) vs. GIM values (red line). The earthquake time is highlighted in green. Labels refer to midnight. (**b**) 24 h VTEC per DOY. Labels refer to midday. (**c**) The geomagnetic activity index Kp. Labels refer to midnight. Figure 7: Distribution of the permanent GNSS stations used to study the Turkey earthquake. The cap edge, its center, and the projection of the epicenter are also shown. Figure 10a shows the DTEC maps derived from GIMs at 18 UT in which an anomaly with a duration of approximately 2 h is visible on DOY 29. Other anomalies, at the same hour, start from DOY 34, leading to the high DTEC values of the earthquake day (DOY 37). The DTEC derived using the proposed model (see Figure 10b) does not show the anomaly on DOY 29. An area of low DTEC is present over the epicenter site on DOY 31 (in this case with a duration of approximately 2 h), and from DOYs 34 to 37 high DTEC values characterize the same area. In this case, the maps of the VTEC rate SNR are not useful for investigating ionospheric anomalies. Conversely, by applying the idea of comparing the VTEC rate SNR with the mean of the previous 15 days, some interesting series appear at 17 UT (hence, one hour before) starting from DOYs 32 to 37.