# Detection of Geomagnetic Signals as Precursors to Some Earthquakes in China
## 4 Discussion  
[PERSON] et al. asserted that 0.01 Hz is the most useful frequency to detect seismogenic ultralow-frequency emissions [12; 21]. Our results indicate that the frequency inferred from the depth according to the skin effect was also a good candidate to detect ULF anomalies prior to earthquakes. Many researchers have considered the investigation of ULF geomagnetic polarization ratio to be a key parameter to detect possible seismogenic signals and provided many references in this study [3; 4; 12; 14; 16; 18; 26; 27]. Some researchers have used the polarization method to study the characteristics of geomagnetic anomalies before the Guam earthquake (8 August 1993; \(M_{\rm w}\) 7.7), the Biak earthquake (17 February 1996; \(M_{\rm w}\) 8.2), the Izu earthquake swarm (June-August 2000; \(M_{\rm s}\) > 6.0), the Kagoshimaken-Hokuseibu earthquake (26 March 1997; \(M_{\rm s}\) 6.5), and the Nias earthquake (6 September 2018; \(M_{\rm s}\) 5.3). Their study results showed that some significant enhancements in polarization ratios occurred 1-3 months prior to earthquakes like the case of earthquakes near KSH station in this paper. [PERSON], [PERSON] and some researchers also have applied the polarization method to obtained seismo-electromagnetic emissions before the Iwateken Nairiku Hokubu earthquake (3 September 1998; \(M_{\rm s}\) 6.3), the Izu earthquake swarm (April-May 1998; \(M_{\rm s}\) 4.7, \(M_{\rm s}\) 5.7), the Chi-Chi earthquake (21 September 1999; \(M_{\rm w}\) 7.7), and the Kamchatka earthquakes (June 2000-September 2001; \(M_{\rm s}\) = 4.0-6.2). In their studies, the abnormal enhancements of polarization ratios started at a shorter time before the earthquake, about a few days to half a month. Those findings were similar to the result of the Jinta earthquake in this paper. It is worth noting that in the results of previous studies, most of the seismogenic anomalies ended before the earthquake occurred [5; 28; 29; 30], whereas in this paper, the seismomagnetic anomalies continued until two months after the Jinta \(M_{\rm s}\) 5.4 earthquake occurrence. In the earthquake case studies of KSH station, three earthquakes had magnitudes of \(M_{\rm s}\) 7.4, \(M_{\rm s}\) 6.7, and \(M_{\rm s}\) 6.7 with corresponding epicentral distances of 291 km, 207 km, and 155 km, respectively. In terms of anomalous amplitudes of polarization ratios, the maximum amplitudes of \(Y_{\rm ZH}\) of these three earthquakes were 62%, 69%, and 119%, respectively, indicating that the anomalous amplitude increases with decreasing epicenter distance, which may suggest that seismo-electromagnetic emissions are more strongly influenced by distance than magnitude. However, this preliminary conclusion requires further verification due to the limited number of earthquakes studied here. [PERSON] et al. counted all the published earthquake cases in the world and found that when the earthquake magnitude (\(M\)) and epicentral distance (\(R\)) satisfy the relationship of \(0.025R\leq M\)-4, 90% of the earthquakes had seismomagnetic anomalies before them [4]. According to the statistical results of [PERSON] et al., the geomagnetic radiation energy generated by a \(M\)5.0 earthquake can only affect stations within 40 km from the epicenter, and the influence range of a \(M\)6.0 earthquake is only 80 km. This is inconsistent with our results in this paper. Moreover, only 5 out of 37 earthquakes were found to have probable signals of seismo-magnetic anomalies in our studies. This indicates that not all earthquakes are preceded by seismo-electromagnetic anomaly signals. In order to study the characteristics of abnormal earthquakes, we analyzed 37 earthquakes according to their magnitude and epicentral distance. Figure 6a shows all 37 earthquakes in this paper, and red points indicate the four earthquakes with seismogenic anomalies before them.