*3.2. Method for PEIA*

Because ionospheric variation is affected by many factors, such as solar activity, earth rotation, geomagnetic conditions, and so on, it has a robust diurnal periodicity. The 15-day moving median (MM) method is a quartile-based statistical analysis technique that is used to express the dispersion of data in a powerful statistical technique proposed by Liu [19,20]. This technique is often used to check data anomalies and identify and strengthen periodic signals. Thus, abnormal disturbances in the TEC time series can be extracted using a quartile-based statistical technique. TEC data that were not disturbed by solar activity and geomagnetic anomalies within 15 days before the earthquake were used. The median (*Q2*), upper (*Q1*), and lower quartiles (*Q3*), and the interquartile range (*IQR*) were calculated. To further improve the reliability of abnormal signals, the constraint method of 1.5 × *IQR* is adopted [21]. For instance, in order to generate MM, UB and LB for the 16th day, the TEC values for the first 15 days were utilized. Similarly, 15 days of TEC data from between the 2nd and 16th day were used to generate bounds for the 17th day. If more than one-third of the data (e.g., eight hours are anomalous in a day) were greater or lesser than the UBs and LBs in a day, this day was taken as anomalous [20].

$$\begin{array}{l} TEC\_{up} = Q\_2 + 1.5 \cdot IQR \\ TEC\_{low} = Q\_2 - 1.5 \cdot IQR \\ IQR = Q\_3 - Q\_1 \end{array} \tag{2}$$

where *TEC* is the median of the first 15 days at the same time, *IQR* is the quartile of the first 15 days, and equals the *IQR* multiplied by 1.5 to delimit the upper and lower thresholds. Likewise, *TECup* and *TEClow* are the upper and lower confidence limits of the *TEC*, respectively, and if the observed *TEC* falls outside of either of the respective bounds, it is declared that a lower or upper abnormal signal is detected, respectively. If the observed value exceeds the upper threshold, a positive disturbance of the *TEC* is considered. Meanwhile, when the variations in *TEC* are below the lower bound, it may be a negative PEIA. The TEC exceptions were calculated as follows:

$$\text{Add}\,TEC = \begin{cases} \,TEC - TEC\_{up} \text{ while } TEC > TEC\_{up} \\ 0 \text{ while } TEC\_{low} \lesstext{, } TEC \lessapprox TEC\_{up} \\ \,TEC - TEC\_{low} \text{ while } TEC < TEC\_{low} \end{cases} \tag{3}$$
