# Seismic Reflection Analysis of AETA Electromagnetic Signals
## 4 Results
### Comparison with Principal Component Analysis (PCA) Method  
PCA is a method widely used in the field of data mining and data analysis. It has the advantages of separating different signals and effectively identifying relatively weak signals and can be used for pre-earthquake electromagnetic disturbance analysis and total electron content (TEC) anomaly detection [25; 26; 27]. Therefore, we compared the results of the anomaly analysis of the Juzhaigou earthquake with those of the LAE using the PCA method. The results of the PCA are shown in Figure 11.  
Figure 11 shows the thermogram of the abnormal values of each monitoring station from 1 August 2017 to 31 August 2017. The abscissa and the ordinate represent date and time, respectively. It can be seen from the figure that 5 days before the earthquake a continuous anomaly band with an increasing level of abnormality appeared at the JZG monitoring station at \(5\sim 6\) o'clock, and that the value of the abnormal points exceeded two in the 2 days before the earthquake, which was significantly higher than the other historical abnormal points in the background. The outlier point appeared at \(21\sim 22\) o'clock on the day of the earthquake, which means that the anomaly degree of the electromagnetic data at this time is greater than that at other times of the day. After the earthquake, the anomaly degree weakened generally, and the abnormal band appeared again at \(5\sim 6\) o'clock and lasted 15 days. However, other monitoring stations near the seismic epicenter have a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & **Station** & **Abnormal Points** & **Outliers over 0.7** & **Total Points** & **Epicenter Distance** \\ \hline
1 & JZG & 16 & 0 & 1152 & 40 km \\
2 & SP & 17 & 2 & 1152 & 64 km \\
3 & PWX & 12 & 0 & 1152 & 111 km \\
4 & QCX & 6 & 0 & 1152 & 147 km \\
5 & MX & 45 & 1 & 1152 & 167 km \\
6 & WC & 12 & 0 & 1152 & 192 km \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Station abnormal points before the Juzhaigou earthquake. Figure 9: A detailed fitted curve for one day. (**a**) The fitting curve without anomalies; (**b**) the fitting curve with anomalies. relatively small degree of anomaly, and their thermograms do not constitute a continuous band anomaly. In short, compared with the PCA method, the LAE can be used to observe the anomaly time and amplitude of the station more intuitively and conveniently. At the same time, in terms of the ability to identify seismic anomalies, the LAE method is robust. For example, for stations such as SP, it can capture anomalies not found by the PCA method. Therefore, the LAE is more suitable for anomaly analysis of AETA data before earthquakes. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Station** & **Installation** & **Location** & **Epicenter Distance** \\ \hline — & Epicenter & 12 December 2017 & 28.34\({}^{\circ}\) N, 104.90\({}^{\circ}\) E & 0 \\  
WZ & 1 August 2018 & 29.58\({}^{\circ}\) N, 103.43\({}^{\circ}\) E & 198 km \\ \hline \hline \end{tabular}
\end{table} 
Table 3: AETA Station within 200 km of the Yibin epicenter. Figure 10: The abnormal values in PSFR, GAX and MBXB stations before the Yibin earthquake.