ANALYSIS OF COSEISMIC VARIATIONS IN MAGNETIC FIELD OF THE LUSHAN MS7.0 EARTHQUAKE IN 2013

doi:10.3969/j.issn.0253-4967.2020.06.003

Abstract

Abstract: Stress perturbation related to earthquakes could cause the anomalous changes of magnetic field. Numerous observation data from high-precision magnetometers before and after earthquakes have revealed obvious coseismic changes in the geomagnetic field during earthquakes. In order to assess the ability of observing geomagnetic variations related to earthquakes with present geomagnetic stations, the coseismic geomagnetic variations corresponding to the 2013 M_S7.0 Lushan earthquake are presented. Precise measurements of geomagnetic fields have been obtained at a continuous geomagnetic station, Chengdu(CDP), which is approximately 99km from the epicenter of the M_S7.0 earthquake. The 12^th Generation International Geomagnetic Reference Field is used to reduce the secular variation of geomagnetic field. The geomagnetic station, Wuhan(WHN)and Lanzhou(LZH), which are approximately 1 100km and 650km from the epicenter of M_S7.0 earthquake respectively, are regarded as reference stations to reduce the diurnal variation of geomagnetic field. A corrected method based on observed data from CDP, WHN and LZH is also used to correct the effect of short-period geomagnetic variations of external origin. Minute average values of CDP from April 16, 2013 to April 23, 2013 is used to analyze the coseismic geomagnetic changes because the ΣKp indices in this period are less than 16 which indicates that there are no strong magnetic disturbances, solar flares, or solar storms. The result reveals that coseismic geomagnetic variation at CDP is little, which is about +0.09nT and +0.04nT, with WHN and LZH as reference stations. Meanwhile, the coseismic geomagnetic variation is +0.02nT using the geomagnetic values sampled per second observed in 20s before and after earthquake from CDP.
The observed coseismic geomagnetic variations are generally interpreted in terms of the piezomagnetic effect in rocks, which are most probably generated by sudden stress changes resulting from earthquake rupturing. According to slip model of 2013 M_S7.0 Lushan earthquake which is constrained by GPS data, the coseismic stress changes are calculated. Then, piezomagnetic magnetic field in the epicenter and surrounding area of the 2013 M_S7.0 Lushan earthquake is calculated based on linear model of magnetization and stress. In the model, we assume that the initial magnetization is consistent with the present magnetic field, which has an inclination and declination of 47.2° and -1.7°(westward). The in-situ Curie depth is chosen as 30km because the magnetized rocks beneath 30km contribute negligibly to the ground geomagnetic field. The calculated piezomagnetic effect shows that the vertical component is downward and horizontal component is mainly southward resulting from stress release in footwall of faults. The vertical component is upward and the horizontal component is mainly northward in hanging wall of faults. When the stress sensitivity is 1×10^-3MPa^-1 and magnetization is 1A/m,the calculated piezomagnetic field is about 5nT, 1nT and +0.007nT in the epicenter, 15km away from epicenter and at CDP, respectively. According to coseismic geomagnetic variations acquired from observed data and calculated data, it is obvious that stations (sites) located close enough to the epicenter could record coseismic signals above several nano Tesla. This implies that it would be difficult to observe geomagnetic signals associated with moderate earthquakes with present geomagnetic stations in this area and we need more improved observations. The piezomagnetic model tended to underestimate the observed coseismic geomagnetic variations with stress sensitivity of 1×10^-3MPa^-1 and magnetization of 1A/m. In this study,a uniform initial magnetization of 1A/m is expected to be reasonable according to aeromagnetic data and ground-based geomagnetic data, which indicates a relatively small magnetic anomaly. In this case, the stress sensitivity of 2×10^-3~3×10^-3MPa^-1 is suitable for explaining the observed coseismic geomagnetic variations. This result provides new data for coseismic geomagnetic variations. We can evaluate the initial magnetization, the stress sensitivity and slip model of earthquake if more observed data in near-field are available.

Key words: Lushan earthquake, geomagnetic field observation, piezomagnetic effect, coseismic geomagnetic variations

摘要： 同震地壳应力变化能够引起地磁场的变化。文中利用成都地磁台连续观测资料分析了芦山M_S7.0地震前后的地磁场变化特征, 并根据芦山M_S7.0地震的同震破裂模型和岩石磁化强度-应力的线性模型, 计算了芦山M_S7.0地震同震引起的稳定地磁场变化。分均值分析结果显示成都地磁台观测的同震稳定地磁变化非常微弱, 以武汉台和兰州台为参考, 同震稳定地磁变化分别为+0.09nT和+0.04nT; 秒观测值分析结果显示同震稳定地磁场变化为+0.02nT。压磁效应计算结果显示, 断裂带下盘磁场垂直分量以向下变化为主, 断裂带上盘磁场垂直分量以向上变化为主, 水平分量的变化则比较散乱。当应力响应系数为1×10^-3MPa^-1、岩石磁化强度为1A/m时, 计算得到震中稳定地磁场总强度变化达5nT, 而距离震中约15km处为1nT, 在距离震中99km的成都地磁台处为+0.007nT。以上结果为同震地磁变化研究提供了新的数据。

关键词: 芦山M_S7.0地震, 地磁观测, 压磁效应, 同震磁场变化

CLC Number:

P315.72⁺1

SONG Cheng-ke, ZHANG Hai-yang. ANALYSIS OF COSEISMIC VARIATIONS IN MAGNETIC FIELD OF THE LUSHAN M_S7.0 EARTHQUAKE IN 2013[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(6): 1301-1315.

宋成科, 张海洋. 2013年芦山M_S7.0地震同震地磁变化分析[J]. 地震地质, 2020, 42(6): 1301-1315.

References

[1] 陈斌, 袁洁浩, 王璨, 等. 2017. 流动地磁监测数据处理流程[J]. 地震研究, 40(3): 335—339.
CHEN Bin, YUAN Jie-hao, WANG Can, et al. 2017. Data processing flowchart of Chinese mobile geomagnet monitoring array[J]. Journal of Seismological Research, 40(3): 335—339(in Chinese).
[2] 郝锦绮, 黄平章, 张天中, 等. 1989. 岩石剩余磁化强度的应力效应[J]. 地震学报, 11(4): 381—391.
HAO Jin-qi, HUANG Ping-zhang, ZHANG Tian-zhong, et al. 1989. The stress effect on remanent magnetization of rocks[J]. Acta Seismologica Sinica, 11(4): 381—391(in Chinese).
[3] 何畅, 廖晓峰, 祁玉萍, 等. 2017. 2017年8月8日九寨沟M_S7.0地震前成都台地磁谐波振幅比异常分析[J]. 中国地震, 33(4): 575—581.
HE Chang, LIAO Xiao-feng, QI Yu-ping, et al. 2017. Study on anomalous variation of the geomagnetic harmonic wave amplitude ratios of the Chengdu seismic station before the M_S7.0 Jiuzhaigou earthquake on August 8, 2017[J]. Earthquake Research in China, 33(4): 575—581(in Chinese).
[4] 黄禄渊, 张贝, 瞿武林, 等. 2017. 2010智利Maule特大地震的同震效应[J]. 地球物理学报, 60(3): 972—984.
HUANG Lu-yuan, ZHANG Bei, QU Wu-lin, et al. 2017. The co-seismic effects of 2010 Maule earthquake[J]. Chinese Journal of Geophysics, 60(3): 972—984(in Chinese).
[5] 廖绍欢, 李雪浩, 胡俊明, 等. 2016. 成都地震基准台FHDZ-M15仪与GM4仪观测资料对比分析[J]. 四川地震, (4): 15—20.
LIAO Shao-huan, LI Xue-hao, HU Jun-ming, et al. 2016. Comparative analysis of observed data recorded by both geomagnetic instrument FHDZ-M15 and GM4 at Chengdu seismic standard station[J]. Earthquake Research in Sichuan, (4): 15—20(in Chinese).
[6] 廖晓峰, 何康, 张明东, 等. 2018. 基于平滑伪魏格纳-维勒时频分析的地磁数据研究[J]. 地震, 38(1): 107—116.
LIAO Xiao-feng, HE Kang, ZHANG Ming-dong, et al. 2018. Time-frequency analysis of geomagnetic data based on smooth pseudo-Wigner-Ville distribution[J]. Earthquake, 38(1): 107—116(in Chinese).
[7] 庞亚瑾, 程惠红, 张怀, 等. 2019. 青藏高原东缘岩石圈结构对现今地表垂向运动影响的数值分析[J]. 地球物理学报, 62(4): 1256—1267.
PANG Ya-jin, CHENG Hui-hong, ZHANG Huai, et al. 2019. Numerical analysis of the influence of lithospheric structure on surface vertical movements in eastern Tibet[J]. Chinese Journal of Geophysics, 62(4): 1256—1267(in Chinese).
[8] 解滔, 刘杰, 卢军, 等. 2018. 2008年汶川M_S8.0地震前定点观测电磁异常回溯性分析[J]. 地球物理学报, 61(5): 1922—1937.
XIE Tao, LIU Jie, LU Jun, et al. 2018. Retrospective analysis on electromagnetic anomalies observed by ground fixed station before the 2008 Wenchuan M_S8.0 earthquake[J]. Chinese Journal of Geophysics, 61(5): 1922—1937(in Chinese).
[9] 熊盛青, 杨海, 丁燕云, 等. 2016. 中国陆域居里等温面深度特征[J]. 地球物理学报, 59(10): 3604—3617.
XIONG Sheng-qing, YANG Hai, DING Yan-yun, et al. 2016. Characteristics of Chinese continent Curie point isotherm[J]. Chinese Journal of Geophysics, 59(10): 3604—3617(in Chinese).
[10] 张凌, 杜爱民, 郎雪, 等. 2016. 汶川地震前的地磁场日变化特征分析[J]. 地球物理学报, 59(3): 952—958.
ZHANG Ling, DU Ai-min, LANG Xue, et al. 2016. Characteristics of geomagnetic regular diurnal variation before Wenchuan earthquake[J]. Chinese Journal of Geophysics, 59(3): 952—958(in Chinese).
[11] Bhattacharyya B K. 1964. Magnetic anomalies due to prism-shaped bodies with arbitrary polarization[J]. Geophysics, 29(4): 517—531.
[12] Carmichael R S. 1977. Depth calculation of piezomagnetic effect for earthquake prediction[J]. Earth and Planetary Science Letters, 36(2): 309—316.
[13] Currenti G, Del Negro C, Johnston M J S, et al. 2007. Close temporal correspondence between geomagnetic anomalies and earthquakes during the 2002—2003 eruption of Etna volcano[J]. Journal of Geophysical Research: Solid Earth, 112(B9): B09103.
[14] Hamano Y. 1983. Experiments on the stress sensitivity of natural remanent magnetization[J]. Journal of Geomagnetism and Geoelectricity, 35(5): 155—172.
[15] Hao J Q, Hastie L M, Stacey F D. 1982. Theory of the seismomagnetic effect: A reassessment[J]. Physics of the Earth and Planetary Interiors, 28(2): 129—140.
[16] Hattori K, Takahashi I, Yoshino C, et al. 2004. ULF geomagnetic field measurements in Japan and some recent results associated with Iwateken Nairiku Hokubu earthquake in 1998[J]. Physics and Chemistry of the Earth, 29(4-9): 481—494.
[17] Jiang Z, Wang M, Wang Y, et al. 2014. GPS constrained coseismic source and slip distribution of the 2013 M_W6.6 Lushan, China, earthquake and its tectonic implications[J]. Geophysical Research Letters, 41(2): 407—413.
[18] Johnston M J S, Mueller R J. 1987. Seismomagnetic observation during the 8 July 1986 magnitude 5.9 North Palm Springs earthquake[J]. Science, 237(4819): 1201—1203.
[19] Johnston M J S, Mueller R J, Sasai Y. 1994. Magnetic field observations in the near-field the 28 June 1992 M_W7.3 Landers, California, earthquake[J]. Bulletin of the Seismological Society of America, 84(3): 792—798.
[20] Johnston M J S, Sasai Y, Egbert G D, 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): S206—S220.
[21] Kean W F, Day R, Fuller M, et al. 1976. The effect of uniaxial compression on the initial susceptibility of rocks as a function of grain size and composition of their constituent titanomagnetite[J]. Journal of Geophysical Research, 81(5): 861—872.
[22] Martin R J. 1980. Is piezomagnetism influenced by microcracks during cyclic loading?[J]. Journal of Geomagnetism and Geoelectricity, 32(12): 741—755.
[23] Mueller R J, Johnston M J S. 1990. Seismomagnetic effect generated by the October 18, 1989, M_L7.1 Loma Prieta, California, earthquake[J]. Geophysical Research Letters, 17(8): 1231—1234.
[24] Nagata T. 1969. Basic magnetic properties of rocks under the effects of mechanical stresses[J]. Tectonophysics, 9(2-3): 167—195.
[25] Nishida Y, Sugisaki Y, Takahashi K, et al. 2004. Tectonomagnetic study in the eastern part of Hokkaido, NE Japan: Discrepancy between observed and calculated results[J]. Earth Planets and Space, 56(11): 1049—1058.
[26] Okubo A, Oshiman N. 2004. Piezomagnetic field associated with a numerical solution of the Mogi model in a non-uniform elastic medium[J]. Geophysical Journal International, 159(2): 509—520.
[27] 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, 305(1-2): 65—72.
[28] Oshiman N. 1990. Enhancement of tectonomagnetic change due to non-uniform magnetization in the Earth's crust: Two-dimensional case studies[J]. Journal of Geomagnetism and Geoelectricity, 42(5): 607—619.
[29] Revol J, Day R, Fuller M D. 1977. Magnetic behavior of magnetite and rocks stressed to failure: Relation to earthquake prediction[J]. Earth and Planetary Science Letters, 37(2): 296—306.
[30] Sasai Y. 1980. Application of the elasticity theory of dislocations to tectonomagnetic modelling[J]. Bulletin of the Earthquake Research Institute, University of Tokyo, 55(2): 387—447.
[31] Sasai Y. 1991. Tectonomagnetic modeling on the bases of the linear piezomagnetic effect[J]. Bulletin of the Earthquake Research Institute, University of Tokyo, 66(4): 585—722.
[32] Sasai Y. 1994. Piezomagnetic fields produced by dislocation sources[J]. Surveys in Geophysics, 15(4): 363—382.
[33] Sasai Y, Ishikawa Y. 1978. Changes in the geomagnetic total force intensity associated with the anomalous crustal activity in the eastern part of the Izu Peninsula(2): The Izu-Oshima-Kinkai earthquake of 1978[J]. Bulletin of the Earthquake Research Institute, University of Tokyo, 53(2): 893—923(in Japanese).
[34] Sasai Y, Ishikawa Y. 1997. Seismomagnetic models for earthquakes in the eastern part of Izu Peninsula, central Japan[J]. Annals of Geophysics, 40(2): 463—478.
[35] Stacy F D. 1964. The seismomagnetic effect[J]. Pure and Applied Geophysics, 58(1): 5—22.
[36] Takla E M, Yomoto K, Liu J Y, et al. 2011. Anomalous geomagnetic variations possibly linked with the Taiwan earthquake(M_W=6.4)on 19 December 2009[J]. International Journal of Geophysics, 211:1—10.
[37] Thebault E, Finlay C C, Alken P, et al. 2015a. Evaluation of candidate geomagnetic field models for IGRF -12[J]. Earth, Planets and Space, 67(1): 787—804.
[38] Thebault E, Finlay C C, Beggan C D, et al. 2015b. International geomagnetic reference field: The 12th generation[J]. Earth, Planets and Space, 67(1): 79—98.
[39] Utada H, Shimizu H, Ogawa T, et al. 2011. Geomagnetic field changes in response to the 2011 off the Pacific coast of Tohoku earthquake and tsunami[J]. Earth and Planetary Science Letters, 311(1-2): 11—27.
[40] Wang R, Lorenzo-Martín F, Roth F. 2006. PSGRN/PSCMP: A new code for calculating co- and post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory[J]. Computers and Geosciences, 32(4): 527—541.
[41] Yamazaki K. 2013. Improved models of the piezomagnetic field for the 2011 M_W9.0 Tohoku-oki earthquake[J]. Earth and Planetary Science Letters, 363:9—15.