# Monitoring of Ionospheric Anomalies Using GNSS Observations to Detect Earthquake Precursors
## 3 Results  
### Test of the Model Using the 2015 Nepal Mw 7.8 Earthquake  
The earthquake occurred in Nepal on 25 April 2015, and some correlated coseismic ionospheric disturbances have already been discussed by many authors in the literature [65; 66; 67]; in this work, we investigated the ionosphere using the permanent GNSS network shown in Figure 2.  
The network consists of 74 stations, 37 of which are double-constellation ones (i.e., GPS + Glonass). The stations are distributed in Asia, Africa, Australia, Europe, and the Antarctic region; some of them belong to networks in the Nepal region. Their data are available from the IGS [68; 69; 70], SONEL [71], UNAVCO [72], and CDDIS [73; 74] databases. In order to increase the number of IPPs, we processed both GPS and Glonass data. We analyzed data from Day of Year (DOY) 81 to 115 of 2015 considering a cutoff angle of 12\({}^{\circ}\) and a ionospheric layer height of 450 km. The position of the center of the cap has a longitude of 94\({}^{\circ}\) and a latitude of 0\({}^{\circ}\), while the cap amplitude is 108\({}^{\circ}\). Finally, data were processed every 300 s.  
Figure 1: The methodology adopted in this work. Figure 3a shows the VTEC series computed at the epicenter point and compared with the VTEC gathered from GIMs. It is possible to see a few negative values due to (i) the fact that no constraints for VTEC values were introduced in the model and (ii) the absence of stations in the Indian Ocean. The mean difference between the computed VTEC values and those of the GIMs is about 20-25%. Possible reasons for such differences are (i) the fact that the network is local and rather small (there is a limited number of stations) if compared with the global one used to produce GIMs and (ii) that the order and the degree of the spherical expansion are different from those used for GIMs. We computed the SNR starting from the estimated VTEC rate values and their standard deviation. In Figure 4a, the VTEC rate SNR is superimposed on its 4-hour moving average: the SNR seems slightly increased in the period between DOYs 95 and 115 if compared with that of the previous 15 days. In Figures 3 and 4, the VTEC and VTEC rate SNR are also combined with the 24h map of their values (Figure 3b and Figure 4b, respectively) and finally compared to the plot of the geomagnetic activity index Kp (this index measures the energy of the solar wind impacting the Earth, and it is used by the space weather services in Near Real-Time [75]) reported in Figures 3c and 4c. All of these values were computed in the epicenter of the earthquake. Data on global geomagnetic activity were downloaded from the GFZ German Research Centre for Geosciences [76]. Figure 2: Distribution of the permanent GNSS stations used to study the Nepal earthquake. The cap edge, its center, and the projection of the epicenter are also shown. As a result, the VTEC seems to be influenced by the solar activity around DOYs 100-101. In this period, the values estimated through the proposed model and those deduced from GIMs are lower than the values of the other days. The same seems true for the low value of DOY 111. Furthermore, the high values of the VTEC rate SNR for DOYs from 101 to 107 seem to be correlated with the increased solar activity as shown by the Kp index, whereas those from DOYs 96 to 99 do not seem to be correlated with the solar activity. The analysis of the local VTEC and the local TEC rate SNR at the epicenter point hence does not give us any apparently useful information about ionospheric anomalies caused by the earthquake. Figure 3: VTEC time series, 24 h VTEC, and Kp index for an overall comparison (all the values were computed at the epicenter of the Nepal 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.