## 2 Data and Methods  
The definition of geomagnetic vertical intensity polarization [2; 9] is shown mathematically as:  
\[Y_{zh}=\left|\frac{Z(\omega)}{H(\omega)}\right| \tag{1}\]  
\[H(\omega)=\sqrt{H_{\mathrm{z}}^{2}(\omega)+H_{\mathrm{y}}^{2}(\omega)} \tag{2}\]where \(Z(\omega)\) is the spectral amplitude value of the geomagnetic vertical component, \(H(\omega)\) is the spectral amplitude value of the full vector of the geomagnetic horizontal component, \(H_{x}(\omega)\) is the north-south spectral value of the geomagnetic horizontal component, \(H_{y}(\omega)\) is the east-west spectral value of the geomagnetic horizontal component, and \(\omega\) is the circular frequency.  
If the instrument frequency response needs to be considered, then  
\[Z^{\prime}(\omega) =G_{Z}(\omega)\cdot Z(\omega) \tag{3}\] \[H^{\prime}{}_{x}(\omega) =G_{H_{x}}(\omega)\cdot H_{x}(\omega)\] (4) \[H^{\prime}{}_{y}(\omega) =G_{H_{y}}(\omega)\cdot H_{y}(\omega) \tag{5}\]  
where \(Z^{\prime}(\omega)\) is the spectral amplitude value of the vertical component of the geomagnetic field observed by the instrument, \(H^{\prime}{}_{x}(\omega)\) is the north-south spectrum of the horizontal component of the geomagnetic field observed by the instrument, \(H^{\prime}{}_{y}(\omega)\) is the east-west spectrum of the horizontal component of the geomagnetic field observed by the instrument, and \(G(\omega)\) is the instrument frequency response curve.  
The fluxgate magnetometer \(G\) value is generally approximated as 1, but induction magnetometers need to be calibrated using the instrument. Induction magnetometers consist of a magnetic sensor and a recorder, so the product of the two is used in practice. In addition, attention should be given to the long-term stability of the \(G\) value.  
The calculation process extracts the high ratio of polarization values to obtain the magnetic field signal from the crust of the primary source. The specific calculation process includes the following steps: first, obtaining the daily polarization value series of stations, fitting the daily polarization value series, and establishing the threshold curve that reflects the annual variation of polarization value; second, removing the daily polarization value below the threshold to obtain the daily polarization value series of high ratio, and using the fitting method to further eliminate the residual annual variation in the daily polarization value series of high ratio; third, the polarization values with higher than 2 times the mean-variance are considered anomalies. To reduce the influence of factors such as the geoelectrical structure, instruments, and the environment on the polarization value, we normalize the polarization value by 2 times the mean squared difference of the one-year window length of each station before drawing the contour of the spatial distribution of the polarization value and then subtract 1 after normalization. The high-value anomalies above 0 are called normalized zero-polarized values. Take the day with the most anomalous stations as the anomaly day and plot the normalized zero polarization values of all stations on the anomaly day. If the conditions of the grouping anomalies are met, the anomaly spatial contour map is drawn after superimposing the multiple anomalies in a group.  
The distribution of fluxgate second sampling observation stations in western China in 2019 is given in Figure 1. Before 2014, the distribution of geomagnetic stations in western China was relatively sparse, and there were large blind monitoring areas in Xinjiang, Tibet, and Qinghai. Since 2015, geomagnetic stations have been rapidly constructed, and the current scale was formed. Therefore, the second sampling observation from the fluxgate magnetometer output of the geomagnetic observation stations west of 110\({}^{\circ}\) E in mainland China from 2015 to 2021 is adopted in this study.