汶川地震区地壳速度相对变化的环境噪声自相关研究
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摘要
2006年中国地震局地质研究所地震动力学国家重点实验室在川西地区(100°E～105°E,26°N～32°N)布设了由297个宽频带地震台组成的密集流动地震观测台阵.本文利用川西流动地震台阵29°N以北地区的137个台站2007年1月至2008年10月的连续三分量地震环境噪声记录,研究了汶川地震震前震后地壳速度变化特征.借助川西台阵的密度优势,我们针对单台三分量噪声自相关函数和单台不同分量间的互相关函数,利用互谱移动窗技术,在0.33～1Hz频带范围内测量了经过50天滑动平均的相关函数与长时间平均参考相关函数的走时变化率,进而求得地壳浅部速度随时间的相对变化,并得到其空间分布特征.本文结果表明,利用环境噪声自相关方法可以得到与互相关方法基本一致的同震速度变化分布图像,同震速度变化分布与同震体应变具有明显相关性.具有更高空间分辨率的噪声自相关研究进一步发现,在鲜水河断裂和龙门山断裂交汇区存在同震速度增加区,这个区域与同震库伦应力变化和地表形变观测预测的周边断层库伦应力增加区一致.我们的研究发现,同震库伦应力增加效应的持续时间大约为2个月左右,在此之后,区域应力场逐渐恢复为普遍的应力下降.
A temporal seismic array consisted of 297 broadband seismographs was deployed in Western Sichuan(100°～105°E,26°～32°N) in 2006,and the observation covered the great 2008 Wenchuan earthquake.We used the continuous three-component ambient noise data from January,2007 to October,2008,recorded at the 137 stations north of 29°N,to study the crust seismic velocity changes before and after the earthquake.For every single station,three autocorrelation functions(ACF) and three cross correlation functions(CCF) are calculated and stacked in 10 days for three components of noise data respectively.Then we estimated the relative velocity changes by measuring travel time shifts between the 50-day-moving-average stacks and the reference empirical correlation functions.We obtained the characteristic of spatial distribution of relative velocity changes caused by Wenchuan earthquake.Our results indicate that ACF analysis can get similar coseismic velocity changes pattern with the CCF method,and the distribution of coseismic velocity changes is closely correlated with the volumetric strain changes during the Wenchuan earthquake.We also found an area of velocity increase in the region where the Longmenshan Fault zone adjoins the Xianshuihe Fault zone.This area is consistent with the faults that the Coulomb stress increased by the occurrence of the Wenchuan earthquake as predicted by the source mechanism and surface deformation.We also found that the crustal velocity increase lasted for about 2 months and then decreased with ubiquitous stress release in the studying region.
A temporal seismic array consisted of 297 broadband seismographs was deployed in Western Sichuan(100°～105°E,26°～32°N) in 2006,and the observation covered the great 2008 Wenchuan earthquake.We used the continuous three-component ambient noise data from January,2007 to October,2008,recorded at the 137 stations north of 29°N,to study the crust seismic velocity changes before and after the earthquake.For every single station,three autocorrelation functions(ACF) and three cross correlation functions(CCF) are calculated and stacked in 10 days for three components of noise data respectively.Then we estimated the relative velocity changes by measuring travel time shifts between the 50-day-moving-average stacks and the reference empirical correlation functions.We obtained the characteristic of spatial distribution of relative velocity changes caused by Wenchuan earthquake.Our results indicate that ACF analysis can get similar coseismic velocity changes pattern with the CCF method,and the distribution of coseismic velocity changes is closely correlated with the volumetric strain changes during the Wenchuan earthquake.We also found an area of velocity increase in the region where the Longmenshan Fault zone adjoins the Xianshuihe Fault zone.This area is consistent with the faults that the Coulomb stress increased by the occurrence of the Wenchuan earthquake as predicted by the source mechanism and surface deformation.We also found that the crustal velocity increase lasted for about 2 months and then decreased with ubiquitous stress release in the studying region.
引文
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[12]Vidale J E,Li Y G.Damage to the shallow Landers faultfrom the nearby Hector Mine earthquake.Nature,2003,421(6299):524-526.
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[15]Snieder R,Grêt A,Douma H,et al.Coda waveinterferometry for estimating nonlinear behavior in seismicvelocity.Science,2002,295(5563):2253-2255.
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[18]Chen J H,Froment B,Liu Q Y,et al.Distribution ofseismic wave speed changes associated with the 12May 2008Mw7.9Wenchuan earthquake.Geophys.Res.Lett.,2010,37:L18302,doi:10.1029/2010GL044582.
[19]刘志坤,黄金莉.利用背景噪声互相关研究汶川地震震源区地震波速度变化.地球物理学报,2010,53(4):853-863.Liu Z K,Huang J L.Temporal changes of seismic velocityaround the Wenchuan earthquake fault zone from ambientseismic noise correlation.Chinese J.Geophys.(in Chinese),2010,53(4):853-863,doi:10.3969/j.issn.0001-5733.2010.04.010.
[20]Xu Z J,Song X D.Temporal changes of surface wavevelocity associated with major Sumatra earthquakes fromambient noise correlation.PNAS,2009,106(34):14207-14212,doi:10.1073/pnas.0901164106.
[21]刘启元,李昱,陈九辉等.汶川Ms8.0地震:地壳上地幔S波速度结构的初步研究.地球物理学报,2009,52(2):309-319.Liu Q Y,Li Y,Chen J H,et al.Wenchuan Ms8.0earthquake:preliminary study of the S-wave velocity structure ofthe crust and upper mantle.Chinese J.Geophys.(in Chinese),2009,52(2):309-319.
[22]郭飚,刘启元,陈九辉等.川西龙门山及邻区地壳上地幔远震P波层析成像.地球物理学报,2009,52(2):346-355.Guo B,Liu Q Y,Chen J H,et al.Teleseismic P-wavetomography of the crust and upper mantle in Longmenshanarea,west Sichuan.Chinese J.Geophys.(in Chinese),2009,52(2):346-355.
[23]李昱,姚华建,刘启元等.川西地区台阵环境噪声瑞利波相速度层析成像.地球物理学报,2010,53(4):842-852.Li Y,Yao H J,Liu Q Y,et al.Phase velocity arraytomography of Rayleigh waves in western Sichuan fromambient seismic noise.Chinese J.Geophys.(in Chinese),2010,53(4):842-852.
[24]齐少华,刘启元,陈九辉等.汶川Ms8.0地震:龙门山断裂两侧地壳各向异性的初步研究.地震地质,2009,31(3):377-388.Qi S H,Liu Q Y,Chen J H,et al.Wenchuan earthquakeMs8.0:Preliminary study of crustal anisotropy on both sidesof the Longmenshan faults.Seismology and Geology(inChinese),2009,31(3):377-388.
[25]刘启元,陈九辉,李顺成等.汶川Ms8.0地震:川西流动地震台阵观测数据的初步分析.地震地质,2008,30(3):584-596.Liu Q Y,Chen J H,Li S C,et al.The Ms8.0 Wenchuanearthquake:Preliminary results from the Western Sichuanmobile seismic array observations.Seismology and Geology(in Chinese),2008,30(3):584-596.
[26]Lobkis O I,Weaver R L.On the emergence of the Green′sfunction in the correlations of a diffuse field.J.acoust.Soc.Am.,2001,110(6):3011-3017.
[27]Weaver R L,Lobkis O I.Diffuse fields in open systems andthe emergence of the Green′s function.J.acoust.Soc.Am.,2004,116(5):2731-2734.
[28]Snieder R.Extracting the Green′s function from the correlation ofcoda waves:a derivation based on stationary phase.Phys.Rev.E,2004,69(4):046610.
[29]Roux P,Sabra K G,Kuperman W A,et al.Ambient noisecross correlation in free space:theoretical approach.J.acoust.Soc.Am.,2005,117(1)79-84.
[30]Sabra K G,Gerstoft P,Roux P,et al.Extracting time-domain Green's function estimates from ambient seismicnoise.Geophys.Res.Lett.,2005,32:L03310,doi:10.1029/2004GL021862.
[31]Campillo M,Paul A.Long-range correlations in the diffuseseismic coda.Science,2003,299(5606):547-549,doi:10.1126/science.1078551.
[32]Roux P,Sabra K G,Gerstoft P,et al.P-waves from cross-correlation of seismic noise.Geophys.Res.Lett.,2005,32:L19303,doi:10.1029/2005GL023803.
[33]Hadziioannou C,Larose E,Coutant O,et al.Stability ofmonitoring weak changes in multiply scattering media withambient noise correlation:Laboratory experiments.J.Acoust.Soc.Am.,2009,125(6):3688-3695.
[34]Fréchet J,Martel L,Nikolla L,et al.Application of thecross-spectral moving-window technique(CSMWT)to theseismic monitoring of forced fluid migration in a rock mass.Int.J.Rock Mech.Min.Sci.Geomech.Abstr.,1989,26(3-4):221-233.
[35]Roux P.Passive seismic imaging with directive ambientnoise:application to surface waves and the San Andreas Faultin Parkfield,CA.Geophys.J.Int.,2009,179(1):367-373,doi:10.1111/j.1365-246X.2009.04282.x.
[36]Bensen G D,Ritzwoller M H,Barmin M P,et al.Processingseismic ambient noise data to obtain reliable broad-bandsurface wave dispersion measurements.Geophys.J.Int.,2007,169(3):1239-1260.
[37]Parsons T,Ji C,Kirby E.Stress changes from the 2008Wenchuan earthquake and increased hazard in the Sichuanbasin.Nature,2008,454(7203):509-510,doi:10.1038/nature07177.
[38]万永革,沈正康,盛书中等.2008年汶川大地震对周围断层的影响.地震学报,2009,31(2):128-139.Wan Y G,Shen Z K,Sheng S Z,et al.The influence of 2008Wenchuan earthquake on surrounding faults.Acta SeismologicaSinica(in Chinese),2009,31(2):128-139.
[2]张培震,徐锡伟,闻学泽等.2008年汶川8.0级地震发震断裂的滑动速率、复发周期和构造成因.地球物理学报,2008,51(4):1066-1073.Zhang P Z,Xu X W,Wen X Z,et al.Slip rates andrecurrence intervals of the Longmenshan active fault zone andtectonic implications for the mechanism of the May 12Wenchuan earthquake,2008,Sichuan,China.Chinese J.Geophys.(in Chinese),2008,51(4):1066-1073.
[3]Xu X W,Wen X Z,Yu G H,et al.Coseismic reverse-andoblique-slip surface faulting generated by the 2008 Mw7.9Wenchuan earthquake,China.Geology,2009,37(6):515-518,doi:10.1130/G25462A.1.
[4]张勇,冯万鹏,许力生等.2008年汶川大地震的时空破裂过程.中国科学(D辑):地球科学,2008,38(10):1186-1194.Zhang Y,Feng W P,Xu L S,et al.Spatiotemporal ruptureprocess of the 2008 Wenchuan great earthquake.Science inChina(Series D):Earth Sciences(in Chinese),2008,38(10):1186-1194.
[5]Ji C,Hayes G.Preliminary result of the May 12,2008Mw7.9eastern Sichuan,China earthquake,US Geol.Surv.2008,http://earthquake.usgs.gov/eqcenter/eqinthenews/2008/us 2008ryan/finite fault.php[2010-12-28].
[6]王卫民,赵连锋,李娟等.四川汶川8.0级地震震源过程.地球物理学报,2008,51(5):1403-1410.Wang W M,Zhao L F,Li J,et al.Rupture process of theMs8.0Wenchuan earthquake of Sichuan,China.Chinese J.Geophys.(in Chinese),2008,51(5):1403-1410.
[7]陈九辉,刘启元,李顺成等.汶川Ms8.0地震余震序列重新定位及其地震构造研究.地球物理学报,2009,52(2):390-397.Chen J H,Liu Q Y,Li S C,et al.Seismotectonic study byrelocation of the Wenchuan Ms8.0 earthquake sequence.Chinese.J.Geophys.(in Chinese),2009,52(2):390-397.
[8]Shen Z K,Sun J B,Zhang P Z,et al.Slip maxima at faultjunctions and rupturing of barriers during the 2008Wenchuanearthquake.Nature Geoscience,2009,2(10):718-724.
[9]Zhang P Z,Wen X Z,Shen Z K,et al.Oblique,high-angle,listric-reverse faulting and associated development of strain:The Wenchuan earthquake of May 12,2008,Sichuan,China.Annu.Rev.Earth Planet.Sci.,2010,38(1):353-382.
[10]Niu F,Silver P G,Daley T M,et al.Preseismic velocitychanges observed from active source monitoring at theParkfield SAFOD drill site.Nature,2008,454(7201):204-208,doi:10.1038/nature07111.
[11]Rubinstein J L,Uchida N,Beroza G C.Seismic velocityreductions caused by the 2003 Tokachi-Oki earthquake.J.Geophys.Res.,2007,112:B05315,doi:10.1029/2006jb004440.
[12]Vidale J E,Li Y G.Damage to the shallow Landers faultfrom the nearby Hector Mine earthquake.Nature,2003,421(6299):524-526.
[13]Shapiro N M,Campillo M.Emergence of broadband Rayleighwaves from correlations of the ambient seismic noise.Geophys.Res.Lett.,2004,31:L07614,doi:10.1029/2004GL019491.
[14]Sens-Schnfelder C,Wegler U.Passive image interferometryand seasonal variations of seismic velocities at Merapi volcano,Indonesia.Geophys.Res.Lett.,2006,33:L21302,doi:10.1029/2006GL027797.
[15]Snieder R,Grêt A,Douma H,et al.Coda waveinterferometry for estimating nonlinear behavior in seismicvelocity.Science,2002,295(5563):2253-2255.
[16]Wegler U,Sens-Schnfelder C.Fault zone monitoring withpassive image interferometry.Geophys.J.Int.,2007,168(3):1029-1033.
[17]Brenguier F,Campillo M,Hadziioannou C,et al.Postseismicrelaxation along the San Andreas Fault at Parkfield fromcontinuous seismological observations.Science,2008,321(5895):1478-1481.
[18]Chen J H,Froment B,Liu Q Y,et al.Distribution ofseismic wave speed changes associated with the 12May 2008Mw7.9Wenchuan earthquake.Geophys.Res.Lett.,2010,37:L18302,doi:10.1029/2010GL044582.
[19]刘志坤,黄金莉.利用背景噪声互相关研究汶川地震震源区地震波速度变化.地球物理学报,2010,53(4):853-863.Liu Z K,Huang J L.Temporal changes of seismic velocityaround the Wenchuan earthquake fault zone from ambientseismic noise correlation.Chinese J.Geophys.(in Chinese),2010,53(4):853-863,doi:10.3969/j.issn.0001-5733.2010.04.010.
[20]Xu Z J,Song X D.Temporal changes of surface wavevelocity associated with major Sumatra earthquakes fromambient noise correlation.PNAS,2009,106(34):14207-14212,doi:10.1073/pnas.0901164106.
[21]刘启元,李昱,陈九辉等.汶川Ms8.0地震:地壳上地幔S波速度结构的初步研究.地球物理学报,2009,52(2):309-319.Liu Q Y,Li Y,Chen J H,et al.Wenchuan Ms8.0earthquake:preliminary study of the S-wave velocity structure ofthe crust and upper mantle.Chinese J.Geophys.(in Chinese),2009,52(2):309-319.
[22]郭飚,刘启元,陈九辉等.川西龙门山及邻区地壳上地幔远震P波层析成像.地球物理学报,2009,52(2):346-355.Guo B,Liu Q Y,Chen J H,et al.Teleseismic P-wavetomography of the crust and upper mantle in Longmenshanarea,west Sichuan.Chinese J.Geophys.(in Chinese),2009,52(2):346-355.
[23]李昱,姚华建,刘启元等.川西地区台阵环境噪声瑞利波相速度层析成像.地球物理学报,2010,53(4):842-852.Li Y,Yao H J,Liu Q Y,et al.Phase velocity arraytomography of Rayleigh waves in western Sichuan fromambient seismic noise.Chinese J.Geophys.(in Chinese),2010,53(4):842-852.
[24]齐少华,刘启元,陈九辉等.汶川Ms8.0地震:龙门山断裂两侧地壳各向异性的初步研究.地震地质,2009,31(3):377-388.Qi S H,Liu Q Y,Chen J H,et al.Wenchuan earthquakeMs8.0:Preliminary study of crustal anisotropy on both sidesof the Longmenshan faults.Seismology and Geology(inChinese),2009,31(3):377-388.
[25]刘启元,陈九辉,李顺成等.汶川Ms8.0地震:川西流动地震台阵观测数据的初步分析.地震地质,2008,30(3):584-596.Liu Q Y,Chen J H,Li S C,et al.The Ms8.0 Wenchuanearthquake:Preliminary results from the Western Sichuanmobile seismic array observations.Seismology and Geology(in Chinese),2008,30(3):584-596.
[26]Lobkis O I,Weaver R L.On the emergence of the Green′sfunction in the correlations of a diffuse field.J.acoust.Soc.Am.,2001,110(6):3011-3017.
[27]Weaver R L,Lobkis O I.Diffuse fields in open systems andthe emergence of the Green′s function.J.acoust.Soc.Am.,2004,116(5):2731-2734.
[28]Snieder R.Extracting the Green′s function from the correlation ofcoda waves:a derivation based on stationary phase.Phys.Rev.E,2004,69(4):046610.
[29]Roux P,Sabra K G,Kuperman W A,et al.Ambient noisecross correlation in free space:theoretical approach.J.acoust.Soc.Am.,2005,117(1)79-84.
[30]Sabra K G,Gerstoft P,Roux P,et al.Extracting time-domain Green's function estimates from ambient seismicnoise.Geophys.Res.Lett.,2005,32:L03310,doi:10.1029/2004GL021862.
[31]Campillo M,Paul A.Long-range correlations in the diffuseseismic coda.Science,2003,299(5606):547-549,doi:10.1126/science.1078551.
[32]Roux P,Sabra K G,Gerstoft P,et al.P-waves from cross-correlation of seismic noise.Geophys.Res.Lett.,2005,32:L19303,doi:10.1029/2005GL023803.
[33]Hadziioannou C,Larose E,Coutant O,et al.Stability ofmonitoring weak changes in multiply scattering media withambient noise correlation:Laboratory experiments.J.Acoust.Soc.Am.,2009,125(6):3688-3695.
[34]Fréchet J,Martel L,Nikolla L,et al.Application of thecross-spectral moving-window technique(CSMWT)to theseismic monitoring of forced fluid migration in a rock mass.Int.J.Rock Mech.Min.Sci.Geomech.Abstr.,1989,26(3-4):221-233.
[35]Roux P.Passive seismic imaging with directive ambientnoise:application to surface waves and the San Andreas Faultin Parkfield,CA.Geophys.J.Int.,2009,179(1):367-373,doi:10.1111/j.1365-246X.2009.04282.x.
[36]Bensen G D,Ritzwoller M H,Barmin M P,et al.Processingseismic ambient noise data to obtain reliable broad-bandsurface wave dispersion measurements.Geophys.J.Int.,2007,169(3):1239-1260.
[37]Parsons T,Ji C,Kirby E.Stress changes from the 2008Wenchuan earthquake and increased hazard in the Sichuanbasin.Nature,2008,454(7203):509-510,doi:10.1038/nature07177.
[38]万永革,沈正康,盛书中等.2008年汶川大地震对周围断层的影响.地震学报,2009,31(2):128-139.Wan Y G,Shen Z K,Sheng S Z,et al.The influence of 2008Wenchuan earthquake on surrounding faults.Acta SeismologicaSinica(in Chinese),2009,31(2):128-139.
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