利用面波振幅谱确定四川九寨沟M7.0地震震源深度
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摘要
准确的震源深度对理解板块构造过程,地震断层结构,非天然地震识别等有重要意义.地震面波远场振幅公式显示面波振幅谱与震源机制、观测方位和震源深度有密切关系.当震源机制与观测方位确定后,可以利用面波振幅谱与震源深度唯一依赖关系,确定震源深度.浅源地震面波振幅谱在0.01~0.08 Hz之间的某个很窄的频段会出现陷波现象即振幅能量衰减,陷波位置与震源深度和频率有密切关系.本研究分析了不同震源机制解的理论地震图陷波现象出现的位置与震源深度的关系.通过理论地震图计算,得到不同震源深度的远场面波振幅谱,与实际观测波形对比,并进行深度定位.理论地震图面波振幅谱显示,震源深度的微小变化会引起面波振幅谱的变化,因此可以精确确定浅源地震的震源深度.震源机制解与震源深度具有耦合现象,为了分析震源机制的扰动对震源深度定位的影响,反演了九寨沟地震的震源机制解,并对走向、倾角、滑动角分别增加±10°的扰动,结果显示震源机制解的较小扰动对深度定位影响不大.为了消除低频噪声、剪切波、短周期面波等对面波振幅谱的影响,利用AK135速度模型对地震波形进行去频散处理.本研究利用瑞利(Rayleigh)面波振幅谱能量衰减特征对2017年8月8日四川九寨沟M7.0地震震源深度进行确定,得到本次地震震源深度为8.2 km.
The accurate source depth of earthquake is important for understanding plate-tectonic processes and structure of seismic faults, identifying natural earthquake and non-natural earthquake. The amplitude formula of seismic surface wave shows that the amplitude of surface wave is closely related to the focal mechanism, the azimuth from epicenter to station and the source depth. When the focal mechanism and the observation position are determined, the excitation energy of surface wave is sensitive to source depth. The surface wave amplitude spectrum can be applied to determine the focal depth. The amplitude energy of shallow earthquake shows the "notch" phenomenon in band of frequency between 0.01 and 0.08 Hz in amplitude spectra, i.e. the attenuation of amplitude energy. The position of the frequency-dependent "notch" is related to the source depth. In this study, the relationship between the position of the "notch" and the source depth is analyzed by theoretical seismogram. We obtain the amplitude spectrums of the far-field at different source depths and then locate the source depth by means of comparing with observed seismic waveforms. By the amplitude spectrum of theoretical surface wave, small changes in the source depth can cause the variation of the amplitude spectrum so that the source depth of shallow earthquake can be accurately determined. The focal mechanism is coupled with the source depth. In order to analyze the influence of the disturbance of source mechanism to source depth determination, the focal mechanism of Jiuzhaigou earthquake was inversed, with disturbance of 10 degrees to strike, dip and rake. The results show only rather small changes in locating depth. In order to eliminate the influence of low frequency noise, shear wave and surface wave of short-period on amplitude spectra, this study adopts the AK135 velocity model to remove the dispersion. In this study, the seismic source depth of the Jiuzhaigou M7.0 earthquake of 8 August 2017 in Sichuan province, China, was determined at8.2 km by the energy attenuation characteristics of Rayleigh amplitude spectrum.
The accurate source depth of earthquake is important for understanding plate-tectonic processes and structure of seismic faults, identifying natural earthquake and non-natural earthquake. The amplitude formula of seismic surface wave shows that the amplitude of surface wave is closely related to the focal mechanism, the azimuth from epicenter to station and the source depth. When the focal mechanism and the observation position are determined, the excitation energy of surface wave is sensitive to source depth. The surface wave amplitude spectrum can be applied to determine the focal depth. The amplitude energy of shallow earthquake shows the "notch" phenomenon in band of frequency between 0.01 and 0.08 Hz in amplitude spectra, i.e. the attenuation of amplitude energy. The position of the frequency-dependent "notch" is related to the source depth. In this study, the relationship between the position of the "notch" and the source depth is analyzed by theoretical seismogram. We obtain the amplitude spectrums of the far-field at different source depths and then locate the source depth by means of comparing with observed seismic waveforms. By the amplitude spectrum of theoretical surface wave, small changes in the source depth can cause the variation of the amplitude spectrum so that the source depth of shallow earthquake can be accurately determined. The focal mechanism is coupled with the source depth. In order to analyze the influence of the disturbance of source mechanism to source depth determination, the focal mechanism of Jiuzhaigou earthquake was inversed, with disturbance of 10 degrees to strike, dip and rake. The results show only rather small changes in locating depth. In order to eliminate the influence of low frequency noise, shear wave and surface wave of short-period on amplitude spectra, this study adopts the AK135 velocity model to remove the dispersion. In this study, the seismic source depth of the Jiuzhaigou M7.0 earthquake of 8 August 2017 in Sichuan province, China, was determined at8.2 km by the energy attenuation characteristics of Rayleigh amplitude spectrum.
引文
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2 Stein S,Wiens D A.Depth determination for shallow teleseismic earthquakes:Methods and results.Rev Geophys,1986,24:806–832
3 Heyburn R,Selby N D,Fox B.Estimating earthquake source depths by combining surface wave amplitude spectra and teleseismic depth phase observations.Geophys J Int,2013,194:1000–1010
4 Basham P W,Ellis R M.The composition of P codas using magnetic tape seismograms.Bull Seismol Soc Amer,1969,59:473–486
5 Gao Y,Zhou H L,Zheng S H,et al.1997.Preliminary discussion on implication of determination on source depth of earthquake.Earthq Res China,1997,13:321–329[高原,周蕙兰,郑斯华,等.测定震源深度的意义的初步讨论.中国地震,1997,13:321–329]
6 Geiger L.Probability method for the determination of earthquake epicenters from the arrival time only.Bull St Louis Univ,1912,8:60–71
7 Xu L S,Du H L,Yan C,et al.A method for determination of earthquake hypocentroid:Time-reversal imaging technique I-Principle and numerical tests(in Chinese).Chin J Geophys,2013,56:1190–1206[许力生,杜海林,严川,等.一种确定震源中心的方法:逆时成像技术(一)——原理与数值实验.地球物理学报,2013,56:1190–1206]
8 Xu L S,Yan C,Zhang X,et al.A method for determination of earthquake hypocentroid:Time-reversal imaging technique—An examination based on people-made earthquakes(in Chinese).Chin J Geophys,2013,56:4009–4027[许力生,严川,张旭,等.一种确定震源中心的方法:逆时成像技术(二)——基于人工地震的检验.地球物理学,2013,56:4009–4027]
9 Zhao B,Shi Y,Gao Y.Seismic relocation,focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu Ms7.1 earthquake and its aftershocks.Earthq Sci,2012,25:111–119
10 Zhao B,Gao Y,Huang Z B,et al.Double difference relocation,focal mechanism and stress inversion of Lushan Ms7.0 earthquake sequence(in Chinese).Chin J Geophys,2013,56:3385–3395[赵博,高原,黄志斌,等.四川芦山Ms7.0地震余震序列双差定位、震源机制及应力场反演.地球物理学报,2013,56:3385–3395]
11 Sun L,Zhang M,Wen L.A new method for high-resolution event relocation and application to the aftershocks of Lushan Earthquake,China.J Geophy Res:Solid Earth,2016,121:2539–2559
12 Tsai Y B,Aki K.Precise focal depth determination from amplitude spectra of surface waves.J Geophys Res,1970,75:5729–5744
13 Fox B D,Selby N D,Heyburn R,et al.Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra.Geophys J Int,2012,191:601–615
14 Aki K,Richards P G.Quantitative Seismology.Sausalito,California:University Science Books,2002
15 Herrin E,Goforth t.Phase-matched filters:Application to the study of Rayleigh waves.Bull Seism Sos Am,1977,67:1259–1275
16 Tapponnier P,Peltzer G,Le Dain A Y,et al.Propagating extrusion tectonics in Asia:New insights from simple experiments with plasticine.Geology,1982,10:611–616
17 Gao Y,Wang Q,Zhao B,et al.A rupture blank zone in middle south part of Longmenshan Faults:Effect after Lushan Ms7.0 earthquake of 20 April2013 in Sichuan,China.Sci China Earth Sci,2013,57:2036–2044[高原,王琼,赵博,等.龙门山断裂带中南段的一个破裂空段——芦山地震的震后效应.中国科学:地球科学,2013,43:1038–1046]
18 Gao Y,Wu Z L,Liu Z,et al.Seismic source characteristics of nine strong earthquakes from 1988 to 1990 and earthquake activity since 1970 in the Sichuan-Qinghai-Xizang(Tibet)zone of China.Pure Appl Geophys,2001,57:1423–1443
19 Zhao B,Shi Y,Gao Y.Relocation of aftershocks of the Wenchuan Ms8.0 earthquake and its implication to seismotectonics.Earthq Sci,2011,24:107 –113
20 Yi G X,Long F,Vallage A,et al.Focal mechanism and tectonic deformation in the seismogenic area of the 2013 Lushan earthquake sequence,southwestern China(in Chinese).Chin J Geophys,2016.59:3711–3731[易桂喜,龙锋,Vallage A,等.2013年芦山地震序列震源机制与震源区构造变形特征分析.地球物理学报,2016,59:3711–3731]
2 Stein S,Wiens D A.Depth determination for shallow teleseismic earthquakes:Methods and results.Rev Geophys,1986,24:806–832
3 Heyburn R,Selby N D,Fox B.Estimating earthquake source depths by combining surface wave amplitude spectra and teleseismic depth phase observations.Geophys J Int,2013,194:1000–1010
4 Basham P W,Ellis R M.The composition of P codas using magnetic tape seismograms.Bull Seismol Soc Amer,1969,59:473–486
5 Gao Y,Zhou H L,Zheng S H,et al.1997.Preliminary discussion on implication of determination on source depth of earthquake.Earthq Res China,1997,13:321–329[高原,周蕙兰,郑斯华,等.测定震源深度的意义的初步讨论.中国地震,1997,13:321–329]
6 Geiger L.Probability method for the determination of earthquake epicenters from the arrival time only.Bull St Louis Univ,1912,8:60–71
7 Xu L S,Du H L,Yan C,et al.A method for determination of earthquake hypocentroid:Time-reversal imaging technique I-Principle and numerical tests(in Chinese).Chin J Geophys,2013,56:1190–1206[许力生,杜海林,严川,等.一种确定震源中心的方法:逆时成像技术(一)——原理与数值实验.地球物理学报,2013,56:1190–1206]
8 Xu L S,Yan C,Zhang X,et al.A method for determination of earthquake hypocentroid:Time-reversal imaging technique—An examination based on people-made earthquakes(in Chinese).Chin J Geophys,2013,56:4009–4027[许力生,严川,张旭,等.一种确定震源中心的方法:逆时成像技术(二)——基于人工地震的检验.地球物理学,2013,56:4009–4027]
9 Zhao B,Shi Y,Gao Y.Seismic relocation,focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu Ms7.1 earthquake and its aftershocks.Earthq Sci,2012,25:111–119
10 Zhao B,Gao Y,Huang Z B,et al.Double difference relocation,focal mechanism and stress inversion of Lushan Ms7.0 earthquake sequence(in Chinese).Chin J Geophys,2013,56:3385–3395[赵博,高原,黄志斌,等.四川芦山Ms7.0地震余震序列双差定位、震源机制及应力场反演.地球物理学报,2013,56:3385–3395]
11 Sun L,Zhang M,Wen L.A new method for high-resolution event relocation and application to the aftershocks of Lushan Earthquake,China.J Geophy Res:Solid Earth,2016,121:2539–2559
12 Tsai Y B,Aki K.Precise focal depth determination from amplitude spectra of surface waves.J Geophys Res,1970,75:5729–5744
13 Fox B D,Selby N D,Heyburn R,et al.Shallow seismic source parameter determination using intermediate-period surface wave amplitude spectra.Geophys J Int,2012,191:601–615
14 Aki K,Richards P G.Quantitative Seismology.Sausalito,California:University Science Books,2002
15 Herrin E,Goforth t.Phase-matched filters:Application to the study of Rayleigh waves.Bull Seism Sos Am,1977,67:1259–1275
16 Tapponnier P,Peltzer G,Le Dain A Y,et al.Propagating extrusion tectonics in Asia:New insights from simple experiments with plasticine.Geology,1982,10:611–616
17 Gao Y,Wang Q,Zhao B,et al.A rupture blank zone in middle south part of Longmenshan Faults:Effect after Lushan Ms7.0 earthquake of 20 April2013 in Sichuan,China.Sci China Earth Sci,2013,57:2036–2044[高原,王琼,赵博,等.龙门山断裂带中南段的一个破裂空段——芦山地震的震后效应.中国科学:地球科学,2013,43:1038–1046]
18 Gao Y,Wu Z L,Liu Z,et al.Seismic source characteristics of nine strong earthquakes from 1988 to 1990 and earthquake activity since 1970 in the Sichuan-Qinghai-Xizang(Tibet)zone of China.Pure Appl Geophys,2001,57:1423–1443
19 Zhao B,Shi Y,Gao Y.Relocation of aftershocks of the Wenchuan Ms8.0 earthquake and its implication to seismotectonics.Earthq Sci,2011,24:107 –113
20 Yi G X,Long F,Vallage A,et al.Focal mechanism and tectonic deformation in the seismogenic area of the 2013 Lushan earthquake sequence,southwestern China(in Chinese).Chin J Geophys,2016.59:3711–3731[易桂喜,龙锋,Vallage A,等.2013年芦山地震序列震源机制与震源区构造变形特征分析.地球物理学报,2016,59:3711–3731]