# Detection of Geomagnetic Signals as Precursors to Some Earthquakes in China
## 3 Polarization Results and Earthquakes
### Polarization Results of Haixi Earthquake  
On 5 June 2013 at 00:43 UT an \(M_{\rm s}\) 5.0 earthquake occurred in the Haixi region, China. The focal depth of this earthquake was 4 km and the closest station (about 155 km) was GLM observatory in western China (Figure 1c). According to the skin effect (Equation (3)), the possible seismic-electromagnetic emission frequency originating from this earthquake source was approximately 0.1 Hz. The polarization ratios of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) of the geomagnetic data at approximately 0.1 Hz in GLM station were thus obtained. Figure 3a,b show the temporal evolution of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) in the GLM observatory during 2013 through five-day moving average. Figure 2: Polarization ratios of \(Y_{\rm ZH}\) at (**a**) JYG, (**c**) CNH observatories and \(Y_{\rm ZG}\) at (**b**) JYG, (**d**) CNH stations about 0.04 Hz from 2011 to 2012 and (**e**) Dst index. In this case, significant \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) enhancements occurred nearly 2 months prior to the Haixi earthquake and lasted for approximately 1 month (started on 17 April 2013 and ended on 14 May 2013). The maximum polarization values of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) during that period were approximately 1.15 and 0.85, which exceeded the threshold value (\(m+2\sigma\)) by 72% and 57%, respectively. The Dst index and CNH station were also selected for comparisons (Figure 3c-e). The results yield correlation coefficients between \(Y_{\rm ZH}\), \(Y_{\rm ZG}\), and the Dst index were 0.36 and 0.42, respectively, which also implies that the \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) variations are generally not in accordance with the Dst index. Figure 3e shows that the absolute values of the Dst index is small and almost always less than 20 nT when the significant \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) enhancements occur in the GLM station. The primary results indicate that the \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) enhancements in the GLM station show no correlation with Dst or geomagnetic activities. To further illustrate this conclusion, we also obtained \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) in the CNH observatory as a reference station, which is nearly 2530 km from the epicenter. Figure 3c,d show that some minor enhancements of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) occurred at the CNH station in phase with the significant enhancements of the GLM station. However, the amplitude of enhancements at CNH station is very small and does not even reach the annual average. [PERSON] et al. found that the variation amplitude of the \(H\) component within 0.02-0.1 Hz increases with increasing latitude, but the variation amplitude of the \(Z\) component does not [25]. The CNH observatory is located at a higher latitude (44.08 \({}^{\circ}\)N)  
Figure 3: Polarization ratios of \(Y_{\rm ZH}\) at **(a)** GLM, **(c)** CNH stations and \(Y_{\rm ZG}\) at **(b)** GLM, **(d)** CNH observatories about 0.1 Hz during 2013 and **(e)** Dst index. than the GLM station (36.43 \({}^{\circ}\)N). According to [PERSON]'s study, the amplitude of \(S_{\rm H}\) in the CNH station may be greater than that in the GLM station. As the result, the amplitudes of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) in the CNH station are greater than those in the GLM observatory. This means that the synchronized enhancements of GLM and CNH stations may be related to normal geomagnetic field variations. In order to further explore the source of significant enhancements before Haixi \(M_{\rm s}\) 5.4 earthquake at the GLM station, the polarization values of \(Y_{\rm ZH}\) and \(Y_{\rm ZG}\) at CDP station were obtained. The latitude of CDP station is 30.95 \({}^{\circ}\)N, which is lower than that of GLM station. According to the variation characteristics of different components with latitude, if these significant enhancements are caused by normal geomagnetic field variations, a similar increase in polarization value will also occur at the CDP station.