## 1 Introduction  
[PERSON] et al. [1] found some correlations between electromagnetic variations and earthquakes as early as the 1960s. They covered a wide frequency range from direct current (DC) to very high frequency [2]. Ground-based observations of electromagnetic disturbances in the ultralow frequency (ULF, f \(<\) 10 Hz) range are considered one of the most promising means to monitor precursory signatures with earthquakes, since the larger skin depth is comparable to the depth of earthquakes [3]. [PERSON] et al. [4] found ultralow-frequency (ULF \(\leq\) 10 Hz) electromagnetic radiation emission during rock fracture via rock fracture experiments. [PERSON] et al. [5] studied the 1993 Ms 7.1 Guam earthquake (depth: \(\sim\)60 km) using the polarization (as the ratio of vertical and horizontal components) method with geomagnetic ULF data at the Guam Observatory, which is approximately 65 km from the epicenter. The results showed that the ratio exhibited a pronounced anomaly only approximately one month prior to the earthquake, which suggested that radiation during this period most likely represented a geomagnetic precursor of the earthquake within the \(\sim\)0.02 to \(\sim\)0.05 Hz frequency range (with a maximum value of \(\sim\)0.1 nT). [PERSON] et al. [6] analyzed data from 4.5 years before and after an Ms 6.1 earthquake on 3 September 1998, in Japan using a ULF magnetometer network based on the Japanese ground-based ULF monitoring system. They found that the spectral density ratio of the horizontal component to the vertical component (the polarization ratio) exhibited an anomalous change two weeks prior to the earthquake and concluded that this anomalous change may be related to the signal of the coming earthquake. [PERSON] et al. [3] used ground-based magnetic field data to investigate the anomalies associated with large earthquakes. They found a notable increase in the ratio of the horizontal component power spectra of Pc3 pulsation near the epicenter to that at remote reference stations in the weeks prior to an Ms 6.4 earthquake on 12 May 1999, in Japan. Moreover, the anomalies were concentrated in the horizontal component rather than the vertical component. [PERSON] et al. [7] showed that the polarization data of the geomagnetic vertical intensity exhibited annual variation. They observed a favorable correspondence between high polarization values at the Kashi station after annual variation correction with earthquakes around the station within 2 months, and most earthquakes occurred within \(\sim\)1 month. [PERSON] et al. [8] used the polarization method to extract ULF geomagnetic anomalies prior to several medium or strong earthquakes near the Chengdu station with second-sampled observation data from the FHDZ-M15 geomagnetic observation system. They found that the phenomenon of increasing polarization values within two months corresponded well to Ms \(>\) 5.0 earthquakes around the station. The polarization anomaly amplitude before thrust earthquakes was larger than that before strike-slip earthquakes. [PERSON] et al. [9] analyzed the spatiotemporal variation characteristics of the geomagnetic polarization anomaly before the 2022 Menyuan Ms 6.9 earthquake in Qinghai based on second-sampled data from 14 geomagnetic stations in the Gan-Qingchuan area. Their results showed that a pronounced geomagnetic polarization anomaly emerged at the end of October 2021 and that the Menyuan earthquake occurred 73 days after the anomaly. The epicenter occurred near the threshold line of the pronounced polarization anomaly, and the geomagnetic polarization anomaly exhibited certain spatiotemporal variation characteristics. [PERSON] et al. [10] focused on the geomagnetic energy variation in the Z component observed at the Kakioka (KAK) station, Japan, during 2001-2010. They investigated the relationship between the energy of ULF geomagnetic signals of the frequency around 0.01 Hz enhancements and earthquake activities.