**3. Methods**

### *3.1. Bilinear Interpolation*

To determine the TEC of the earthquake center, we used the adjacent four grid points to interpolate the TEC using the bilinear interpolation method. Figure 2 shows the ionospheric grid interpolation diagram.

**Figure 2.** Schematic diagram of the ionospheric grid interpolation algorithm.

According to the Lagrange interpolant, the ionospheric VTEC value of the interpolation point was obtained. The formula is as follows:

$$\begin{array}{c} T = \frac{T\_1}{(X\_2 - X\_1)(Y\_1 - Y\_3)}(X - X\_1)(Y - Y\_3) + \frac{T\_2}{(X\_2 - X\_1)(Y\_1 - Y\_3)}(X\_2 - X)(Y - Y\_3) \\ + \frac{T\_3}{(X\_4 - X\_3)(Y\_4 - Y\_2)}(X - X\_3)(Y\_2 - Y) + \frac{T\_4}{(X\_4 - X\_3)(Y\_4 - Y\_2)}(X\_4 - X)(Y\_2 - Y) \end{array} \tag{1}$$

where *X* and *Y* are the coordinates of the undetermined interpolation point, the TEC is represented with *T*, and the grid coordinates are *Xi* and *Yi*, respectively.
