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), JYG (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 YZH and YZG (YZH, the spectral power ratio of Z to H and YZG, the spectral power ratio of Z to G) occurred five days prior to the Jinta earthquake (Ms 5.4) and continued for about two months. The polarization ratio of YZH reached an abnormally high value about one month before some earthquakes (Ms 7.4, Ms 6.7, Ms 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 YZH and YZG occurred two months before the Haixi earthquake (Ms 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 YZH and YZG in GLM station had nothing to do with the Haixi earthquake.
Highlights
The prelude and occurrence of earthquakes have been shown to possibly coincide with changes in the geomagnetic field, which are called seismomagnetic signals [1,2,3,4,5]
When a micro-fissure expands, void fluids tend to disperse into the new and expanded fissures according to Darcy’s Law. This in turn alters the electrical conductivity of the rocks in the focal zone, which changes the induced geomagnetic field coupled with a variation of the external geomagnetic field [7,8,9]
Earthquakes with the shortest epicentral distance from the observatories were selected as the seismic events and some potential earthquake precursor signatures were obtained from the ultralow-frequency (ULF) records
Summary
The prelude and occurrence of earthquakes have been shown to possibly coincide with changes in the geomagnetic field, which are called seismomagnetic signals [1,2,3,4,5]. 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, Ida and Li 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. 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
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