极低频台站同震电磁信号特征分析

doi:10.3969/j.issn.0253-4967.2022.03.012

摘要/Abstract

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

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

中图分类号:

P315.722

韩冰, 汤吉, 赵国泽, 王立凤, 董泽义, 范晔, 孙贵成. 极低频台站同震电磁信号特征分析[J]. 地震地质, 2022, 44(3): 753-770.

HAN Bing, TANG Ji, ZHAO Guo-ze, WANG Li-feng, DONG Ze-yi, FAN Ye, SUN Gui-cheng. ANALYSIS OF ELECTROMAGNETIC CO-SEISMIC PHENOMENA OBSERVED IN CSELF STATIONS[J]. SEISMOLOGY AND GEOLOGY, 2022, 44(3): 753-770.

图/表 14

图 1 川滇地区极低频观测台站的位置分布图

Fig. 1 Distribution of CSELF stations in Sichuan and Yunnan.

表1 地震的基本信息

Table 1 General information of earthquakes

发震时刻(UTC)	震级 /M	北纬 /(°)	东经 /(°)	深度 /km	参考位置	震中距 /km
2014-10-07 13:49:39	6.6	23.39	100.46	5	云南景谷	31
2014-10-11 06:05:12	4.7	23.50	100.40	8	云南景谷	34.2
2014-12-05 18:43:44	5.8	23.31	100.50	9	云南景谷	32.7
2014-12-06 10:20:00	5.9	23.32	100.50	10	云南景谷	32.7
2014-12-07 09:23:04	4.7	23.30	100.50	15.4	云南景谷	33.2
2015-11-13 16:55:06	4.6	23.33	100.52	5.3	云南景谷	29.4
2018-09-08 02:31:29	5.9	23.28	101.53	11	云南墨江	85

图 2 台站及地震震中分布图三角形表示极低频台站位置, 圆圈表示地震震中

Fig. 2 Distribution of CSELF stations and earthquakes.

图 3 2015年11月13日景谷M4.6地震发生前后的地震波及电磁波变化时间序列由上到下分别为地震波分量BHE、 BHN、 BHZ和电磁场分量Ex、 Ey、 Hx、 Hy、 Hz, 黑色虚线表示地震发生的时刻

Fig. 3 Seismic and electromagnetic signals observed at Jinggu Station before and after Jinggu M4.6 earthquake on November 13, 2015.

图 4 各地震发生前后的地震波及电磁波变化时间序列空白表示数据缺失, 黑色虚线表示地震发生时刻。其中图a和f进行了GPS校正

Fig. 4 Seismic and electromagnetic signals observed at Jinggu Station before and after each earthquake.

图 5 2018年9月8日墨江M4.1地震发生后的地震波及电磁波变化时间序列

Fig. 5 Seismic and electromagnetic signals observed at Jinggu Station after the Mojiang M4.1 earthquake on September 8, 2018.

图 6 景谷M6.6地震后的时间序列细节图

Fig. 6 The detail of time series after Jinggu M6.6 earthquake.

图 7 振幅比与震级的关系

Fig. 7 Relation of EM amplitude ratio and the magnitude of the earthquakes.

表2 2次地震振幅增强对比值

Table 2 Comparison of enhanced amplitude between the M6.6 and M4.6 earthquakes

震级/M	E_x	E_y	H_x	H_y	H_z
6.6	210.490 5	0	49.553 44	110.139 6	2 091.949
4.6	6.349 204	2.023 256	1.173 469	2.151 22	15.351 94
振幅比值	33.152 27	0	42.228 15	51.198 69	136.266 1
能量比值	1 099.073	0	1 783.217	2 621.306	18 568.45

表3 地震及台站信息

Table 3 Information of earthquake and stations

地震名称	台站及震中距
景谷5.9级地震	景谷台(32.7km)、牟定台(254km)、大理台(272km)
景谷6.6级地震	景谷台(32km)、牟定台(242km)、大理台(262km)、新平台(175km)

图 8 景谷M6.6地震发生前后的电磁场时间序列图空白表示数据缺失; 虚线表示地震发生时刻

Fig. 8 Time series of electromagnetic field during the Jinggu M6.6 earthquake.

图 9 景谷M5.9地震发生前后的电磁场时间序列图虚线表示地震发生时刻

Fig. 9 Time series of electromagnetic field during the Jinggu M5.9 earthquake.

图 10 大理台的视电阻率曲线

Fig. 10 Apparent resistivity curve of Dali Station.

图 11 小波变换图各台站从上到下依次为Ex、 Ey、 Hx、 Hy、 Hz分量的结果

Fig. 11 Wavelet transform diagram.

参考文献 25

[1]	高永新. 2010. 地震电磁场: 基于动电效应的波场模拟[D]. 哈尔滨: 哈尔滨工业大学.
	GAO Yong-xin. 2010. Simulation of earthquake-induced electromagnetic wave field due to the electrokinetic effect[D]. Harbin Institute of Technology, Harbin (in Chinese).
[2]	江鹏. 2021. 磁棒旋转诱导的同震磁场研究[D]. 合肥: 合肥工业大学.
	JIANG Peng. 2021. Study on coseismic magnetic field induced by rotation of coil-type magnetometer[D]. Hefei University of Technology, Hefei (in Chinese).
[3]	汤吉, 詹艳, 王立凤, 等. 2008. 5月12日汶川8.0级地震强余震观测的电磁同震效应[J]. 地震地质, 30(3): 739—745.
	TANG Ji, ZHAN Yan, WANG Li-feng, et al. 2008. Coseismic signal associated with aftershock of the M_S8.0 Wenchuan earthquake[J]. Seismology and Geology, 30(3): 739—748 (in Chinese).
[4]	汤吉, 詹艳, 王立凤, 等. 2010. 汶川地震强余震的电磁同震效应[J]. 地球物理学报, 53(3): 526—534.
	TANG Ji, ZHAN Yan, WANG Li-feng, et al. 2010. Electromagnetic coseismic effect associated with aftershock of Wenchuan M_S8.0 earthquake[J]. Chinese Journal of Geophysics, 53(3): 526—534 (in Chinese).
[5]	王立凤, 朱学会, 赵国泽, 等. 2016. GMS -07电磁观测系统测量注意事项及故障检测[J]. 物探与化探, 40(2): 385—389.
	WANG Li-feng, ZHU Xue-hui, ZHAO Guo-ze, et al. 2016. The operation cautions and troubleshooting of the GMS -07 system in MT survey[J]. Geophysical and Geochemical Exploration, 40(2): 385—389 (in Chinese).
[6]	谢小碧. 1999. 地震波在盆地中的传播特点及其对盆地中震害的影响[J]. 山西地震, 96(1): 1—5.
	XIE Xiao-bi. 1999. Propagation characters of seismic wave in basin and its impact on earthquake disaster[J]Earthquake Research in Shanxi, 96(1): 1—5 (in Chinese).
[7]	徐光晶, 汤吉, 陈小斌, 等. 2009. 云南宁洱 M_S6.4 地震震后电磁效应[J]. 地震地质, 31(2): 305—312. doi: 10.3969/j.issn.0253-4967.2009.02.011. DOI
	XU Guang-jing, TANG Ji, CHEN Xiao-bin, et al. 2009. Electromagnetic effects associated with aftershocks of the M_S6.4 Ning’er earthquake[J]. Seismology and Geology, 31(2): 305—312 (in Chinese).
[8]	赵国泽, 陆建勋. 2003. 利用人工源超低频电磁波监测地震的试验与分析[J]. 中国工程科学, 5(10): 27—33.
	ZHAO Guo-ze, LU Jian-xun. 2003. Monitoring and analysis of earthquake phenomena by artificial SLF waves[J]. Engineering Science, 5(10): 27—33 (in Chinese). DOI URL
[9]	赵国泽, 王立凤, 汤吉, 等. 2010. 地震监测人工源极低频电磁技术(CSELF)新试验[J]. 地球物理学报, 53(3): 479—486.
	ZHAO Guo-ze, WANG Li-feng, TANG Ji, et al. 2010. New experiments of CSELF electromagnetic method for earthquake monitoring[J]. Chinese Journal of Geophysics, 53(3): 479—486 (in Chinese).
[10]	赵国泽, 王立凤, 詹艳, 等. 2012. 地震预测人工源极低频电磁新技术(CSELF)和第一个观测台网[J]. 地震地质, 34(4): 576—585. doi: 10.3969/j.issn.0253-4967.2012.04.004. DOI
	ZHAO Guo-ze, WANG Li-feng, ZHAN Yan, et al. 2012. A new electromagnetic technique for earthquake monitoring-CSELF and the first observational network[J]. Seismology and Geology, 34(4): 576—585 (in Chinese).
[11]	Gao Y X, Zhao G Z, Chong J J, et al. 2020. Coseismic electric and magnetic signals observed during 2017 Jiuzhaigou M_W6.5 earthquake and explained by electrokinetics and magnetometer rotation[J]. Geophysical Journal International, 223(2): 1130—1143. DOI URL
[12]	Grossman A, Morlet J. 1984. Decomposition of Hardy functions into square integrable wavelets of constant shape[J]. SIAM Journal on Mathematical Analysis, 15(4): 723—736. DOI URL
[13]	Hayakawa M, Hobara Y. 2010. Current status of seismo-electromagnetics for short-term earthquake prediction[J]. Geomatics, Natural Hazards and Risk, 1(2): 115—155. DOI URL
[14]	Honkura Y, Isikara A M, Oshiman N. 2000. Preliminary results of multidisciplinary observations before, during and after the Kocaeli(Izmit)earthquake in the western part of the north Anatolian fault zone[J]. Earth, Planets and Space, 52(4): 293—298. DOI URL
[15]	Honkura Y, Satoh H, Ujihara N. 2004. Seismic dynamo effects associated with the M7.1 earthquake of 26 May 2003 off Miyagi Prefecture and the M6.4 earthquake of 26 July 2003 in northern Miyagi Prefecture, NE Japan[J]. Earth, Planets and Space, 56: 109—114. DOI URL
[16]	Huang Q, Ren H, Zhang D, et al. 2015. Medium effect on the characteristics of the coupled seismic and electromagnetic signals[J]. Proceedings of the Japan Academy, 91(1): 17—24.
[17]	Johnston M, Mueller R. 1987. Seismomagnetic observation with the July 8, 1986, M_L5.9 North Palm Springs earthquake[J]. Science, 237(4819): 1201—1203. PMID
[18]	Johnston M, Mueller R, Sasai Y. 1994. Magnetic field observations in the near-field of the 28 June 1992 M_W7.3 Landers, California, earthquake[J]. Bulletin of the Seismological Society of America, 84(3): 792—798. DOI URL
[19]	Johnston M, Sasai Y, Egbert G, et al. 2006. Seismomagnetic effects from the long-awaited 28 September 2004 M6.0 Parkfield earthquake[J]. Bulletin of the Seismological Society of America, 96(4B): 206—220.
[20]	Kumar P, Rawat V S, Patro P K, et al. 2020. Assessment and recognition of pre- and co-seismic electromagnetic signatures from magnetotelluric data: A case study from Koyna-Warna seismoactive region, India[J]. Acta Geophysica, 69:1—15. DOI URL
[21]	Matsushima M, Honkura Y, Oshiman N. et al. 2002. Seismo-electromagnetic effect associated with the İzmit earthquake and its aftershocks [J]. Bulletin of the Seismological Society of America, 92(1): 350—360.
[22]	Morlet J, Arens G, Fourgeau E, et al. 1982. Wave propagation and sampling theory(Part Ⅱ): Sampling theory and complex waves[J]. Geophysics, 47(2): 801—813.
[23]	Okubo K, Takeuchi N, Utsugi M, et al. 2011. Direct magnetic signals from earthquake rupturing: Iwate-Miyagi earthquake of M7.2, Japan[J]. Earth and Planetary Science Letters, 35: 65—72. DOI URL
[24]	Ren H, Wen J, Huang Q, et al. 2015. Electrokinetic effect combined with surface-charge assumption: A possible generation mechanism of coseismic EM signals[J]. Geophysical Journal International, 200(2): 835—848.
[25]	Sun Y C, Uyeshima M, Ren H X, et al. 2019. Numerical simulations to explain the coseismic electromagnetic signals: A case study for a M5.4 aftershock of the 2016 Kumamoto earthquake[J]. Earth, Planets and Space, 71(1): 1—24. DOI URL