Abstract:

With the support of the wireless electro-magnetic method(WEM)project, the control source extremely low frequency(CSELF)continuous observation network, which includes 30 electromagnetic stations in Beijing capital area(BCA)and the southern section of the North-South Seismic Belt in China, was built for recording the artificial and nature source singles. The natural source observation of the network was started in July 2013 and December 2013 in batches and the electromagnetic field was recorded continually with a sampling rate of 16Hz. Until now, the co-seismic electromagnetic signals have been recorded repeatedly in several stations. In this paper seven co-seismic electromagnetic signals recorded at Jinggu station and co-seismic electromagnetic signals associated with two strong earthquakes recorded at different stations surrounding the epicenter are studied.

It is found that the variation of the EM filed is similar to the seismogram, and the amplitude of the co-seismic EM signal is much larger than the background signal generated by earth induction, and the intensity of the vertical magnetic field is about ten times as big as the horizontal electromagnetic field. For co-seismic EM signals recorded at the same station, the relationship between the amplitude of electromagnetic field and the magnitude of the earthquake is basically linear in logarithmic domain. Meanwhile, the amplitude of electromagnetic field is also affected by focal depth of the earthquake and distance between the stations and the epicenter. When the epicenter distance is close, the amplitude of the co-seismic signal caused by the earthquake with shallow focal depth is higher. When the focal depth is similar, the amplitude of electromagnetic co-seismic signal caused by the earthquake closer to the station is larger.

For the co-seismic EM signals associated with a same earthquake recorded by different stations, the larger the epicenter distance is, the later the signal appears and the longer the duration is. However, the signal amplitude is not only affected by the epicenter distance, but also related to the near-surface medium at the observation point. The electromagnetic co-seismic signals observed at Dali station which is the farthest away from the epicenter of Jinggu earthquake show the characteristics of large amplitude, long duration, and low dominant frequency. This may be related to the electrical structure near the surface of Dali Platform. The electromagnetic field signals of the 5 components of Jinggu, Muding and Dali stations before and after the Jinggu earthquake of magnitude 5.9 were transformed by wavelet transform. Finally, the wavelet spectrum with the horizontal axis as time and the vertical axis as frequency was obtained to indicate the time-frequency changes of the abnormal electromagnetic signals of the same seismic wave. According to the wavelet analysis and combining with the time series before and after the Jinggu earthquake of M_S5.9, the energy enhancement mainly occurs in the shear wave and surface wave periods, while the P-wave is not obvious in the wavelet energy spectrum due to its small amplitude, and only some weak enhancement with scattered frequency can be observed. The main frequency of electromagnetic co-seismic signal is between 1Hz and 2Hz. At the beginning of the co-seismic signal, there are high frequency components, and the high frequency gradually decreases with the increase of epicenter distance. Moreover, compared with electric field, magnetic field can record more abundant high-frequency information. This may have to do with different dominant mechanisms for electric and magnetic field generation.

In this paper, several earthquakes recorded at Jinggu station and electromagnetic co-seismic phenomena caused by two strong earthquakes at Jinggu station are summarized and analyzed. The results show that the variation of co-seismic electromagnetic signal is very complicated, and its starting time, duration, amplitude, and frequency range have some rules, but some stations show their particularity under multiple seismic events, so it is difficult to discuss the mechanism of its generation. However, in terms of observation phenomena, the electromagnetic field variation data observed continuously by extremely low frequency stations give us a more comprehensive understanding of the Earth’s electromagnetic field itself and the electromagnetic signals related to earthquakes. The accumulation of more seismic-related electromagnetic phenomena and the support of theoretical simulation can deepen the understanding of electromagnetic field variation before, during and after the earthquake.

Key words: CSELF network, co-seismic electromagnetic signals, wavelet transform

摘要：

文中对景谷台站记录到的7次同震电磁现象及2次较强地震在周围几个极低频台站引起的同震电磁现象进行了研究, 发现了总体形态与地震波相近的电磁波, 其幅度远大于地球感应产生的背景信号, 且垂直磁场强度约为水平磁场的10倍。对于同一台站记录到的多次同震电磁信号, 幅值与震级在对数域基本满足线性关系, 同时也受震源深度与震中距影响。 在同一地震中, 震中距越大, 台站记录到的同震电磁信号出现的时间越晚, 持续时间也相对较长, 但信号的幅值不仅受到震中距的影响, 还与观测点的近地表介质有关。通过小波能量谱可以看出, 同震电磁信号的主要频率为1~2Hz, 在同震信号初始阶段高频成分较多, 并表现出随震中距增加频率降低的特点; 同时相对于电场, 磁场记录的高频信息更加丰富。2014年景谷M6.6地震发生时, 在距离震中很近的台站记录到比同震信号更强的尖峰信号, 其在地震波到达之初出现。推测偶然的电磁强干扰与地面震动互相叠加是引起电磁信号强烈变化的原因。

关键词: 极低频电磁台网, 同震电磁信号, 小波变换