# Detection of Geomagnetic Signals as Precursors to Some Earthquakes in China
###### Abstract  
Extracting weak seismic-magnetic signals from strong electromagnetic backgrounds has always been an important part of seismic-electromagnetic research. Based on the data of \(D/H/Z\) components measured through a three-axis ring-core-type fluxgate magnetometer in GLM (Geermu), YG (Jiayuguan), and KSH (Kashi) observatories, we analyzed geomagnetic signals as precursors to some earthquakes in China using the polarization method. The most effective main frequency of seismic-electromagnetic emission was determined according to the skin effect. The results showed that only 5 out of 37 earthquakes were found to have probable signals of seismomagnetic anomalies before them. Further research revealed that the significant enhancements of polarization ratios of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) (\(Y_{\rm ZH}\), the spectral power ratio of \(Z\) to \(H\) and \(Y_{\rm ZG}\), the spectral power ratio of \(Z\) to \(G\)) occurred five days prior to the Jinta earthquake (\(M_{\rm s}\) 5.4) and continued for about two months. The polarization ratio of \(Y_{\rm ZH}\) reached an abnormally high value about one month before some earthquakes (\(M_{\rm s}\) 7.4, \(M_{\rm s}\) 6.7, \(M_{\rm s}\) 6.7) near the KSH station and lasted for about one week to half a month, then returned to the normal value. Similarly, some enhancements of polarization ratios of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) occurred two months before the Haixi earthquake (\(M_{\rm s}\) 5.0) and lasted for about one month. Analysis results showed that the enhancements prior to earthquake events may be closely related to the Jinta earthquake and a series of earthquakes near the KSH station; however, the high value of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) in GLM station had nothing to do with the Haixi earthquake.  
polarization analysis; earthquake precursor; geomagnetic anomaly +
Footnote †: journal: Applied Sciences  
and identification of weak signals from a strong geomagnetic background field. Therefore, [PERSON] et al. developed the polarization method and found that the spectral power ratio of the vertical magnetic field (\(Z\)) to horizontal magnetic field (\(H\)) anomalously increased within the frequency range of 0.01 Hz prior to an earthquake on 8 August 1993 [12]. Polarization analysis has since been widely applied in seismic-electromagnetic studies [13; 14; 15; 16; 17; 18; 19]. In order to prevent the phenomenon that the amplitude of horizontal component (\(H\)) is small when the geomagnetic activity is weak but the polarization value increases abnormally, [PERSON] and [PERSON] et al. eliminated the geomagnetic data of the horizontal component with small amplitude during periods of weaker geomagnetic activity and developed an improved polarization method [5; 20]. They applied the improved polarization method to the ULF data observed in China and found some significant precursory effects for three earthquakes. Nevertheless, it was demonstrated that this special phenomenon should be large-scale or even global [21; 22; 23], and it can be identified by multiple-station comparisons.  
In this study, we applied the original polarization method to analyze geomagnetic data observed at the JYG, GLM, and KSH stations in China to identify potential precursory signals to earthquake events.