Abstract:

The geoelectric field is an important geophysical field, and the long-term geoelectric field is also an important component of the Earth's electromagnetic field, which mainly refers to the geoelectric field with a variation period greater than 100,000 seconds. The long-term geoelectric field can dynamically reflect the trend changes of spontaneous electric field, periodic changes of telluric electricity field, and other seasonal disturbances. Furthermore, it can further reveal the basic characteristics and physical mechanisms are related to the sources of the electromagnetic field, through such analysis and research on the changes in the long-term geoelectric field.

In this paper, a systematic study has been conducted on the basic characteristics and variation mechanism of the long-term geoelectric field, which is mainly based on long-term geoelectric field observation data Dulan seismic station in Qinghai Province from 2015 to 2022. Firstly, based on interference preprocessing and reliability analysis, various methods and approaches such as wavelet analysis, median filtering, convolutional filtering, linear fitting are used to filter out high-frequency disturbances in the geoelectric field and to extract and analyze the components of long-term geoelectric field variation. At the same time, based on the analysis results of the abnormal changes in the electric field at the Dawu seismic station before and after the Madoi M_S7.4 earthquake, the possible correlation and main field source mechanism between the abnormal variation of the electric field at the Dulan seismic station and the earthquake has been analyzed and discussed.

The research results show that, from the observation data of geoelectric field at Dulan seismic station: 1)It has relatively strong long-term stability and variation reliability, with the arithmetic mean value of the daily correlation coefficient not less than 0.98, and that of the daily variation difference not more than 0.2mV/km, in the Same measurement direction; 2)The FFT spectrum analysis results include not only short and medium cycle changes such as 24h, 12h, 8h and 6h, but also long cycle changes such as annual cycle, semi-annual cycle and so on; 3)The daily variation range shows seasonal variation with high in summer and low in winter, and the significant variation includes annual cycle, half-year cycle, 27-day cycle, half-month cycle; 4)The trend variation shows a typical periodic annual changing feature as the sine wave change, and the annual extreme value and change amplitude are basically the same; 5)Before and after the Madoi M_S7.4 earthquake, there were obvious abnormal distortions from the annual variation.

Theoretical and mechanism analysis shows that the periodic changes in the daily variation amplitude of the geoelectric field at the Dulan seismic station mainly come from the periodic changes in spatial field sources, and the trend periodic changes of the geoelectric field mainly come from the long-term tidal effect generated by the gravitational action of the sun. During the process of Earth's orbit and rotation around the sun, the periodic changes in direct solar radiation, as well as in the ionospheric plasma concentration and the underground fluid seepage field caused by the gravitational force of the sun, are the important excitation mechanisms for this long-term periodic change of the geoelectric field.

According to the comprehensive analysis, it is believed that the variation of the geoelectric field data observed at the Dulan seismic station has excellent objectivity, reliability, and long-term stability, which can reflect the characteristics of long-term geoelectric field changes and abnormal distortion characteristics of the geoelectric field before and after strong earthquakes. And the change mechanism is in good agreement with the theoretical analysis results and has strong research and application value.

In summary, the research results which carried out on the long-term variation of the geoelectric field, as well as revealed the important mechanisms of the generation and variation of Earth's electromagnetic fields, will have more important enlightening significance and reference value in the multifaceted research work about the geoelectric field observation, such as the site survey, observation system construction, observation data analysis and application et al.

Key words: Dulan seismic station, Long period geoelectric field, Variation characteristics, Madoi M_S7.4 earthquake

摘要：

地电场是重要的地球物理场, 长周期地电场是地球电磁场的重要组成部分。文中针对2015—2022年青海省都兰地震台较长时间尺度的地电场观测数据资料, 采用小波分析、 中值滤波、 卷积滤波及线性拟合等多种方法和途径, 对长周期地电场及其变化特征进行提取和分析。研究结果显示: 1)都兰地震台地电场观测数据具有比较强的长期平稳性和变化可靠性, 同测向日相关系数的算术平均值≥0.98, 日变化差值的算术平均值≤0.2mV/km; 2)FFT频谱分析结果表明, 数据除包含24h、 12h、 8h和6h等中短周期变化外, 同时也包含年周期、 半年周期及3月周期等长周期变化成分; 3)地电场观测值的日变化幅度呈现夏高冬低的季节性变化, 显著性变化包括年周期、 半年周期、 4月周期、 27日周期及半月周期等; 4)观测值的趋势性变化呈现比较典型的周期性年变化特征, 变化形态近似正弦波, 且每年的极值点和变化幅度基本相当; 5)玛多 M_S7.4 地震前后地电场观测值出现了较为明显的破年变化异常畸变现象。机理分析显示, 在地球围绕太阳公转及自转过程中, 太阳直射点的往复改变是长周期地电场变化的重要激发机制。综合分析认为, 都兰地震台较长时间尺度的地电场观测数据变化具有比较优良的客观性、 可靠性及长期变化平稳性, 能够充分反映长周期地电场的基本变化特性, 具有较高的研究和应用价值。

关键词: 都兰地震台, 长周期地电场, 变化特征, 玛多M_S7.4地震