EOF方法检测GRACE卫星重力结果中的同震重力变化
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摘要
根据GRACE重力场月均值结果,结合300 km平滑半径的扇形滤波器,计算了地面0.5°网格点上的重力扰动的时间变化,并利用经验正交函数方法对重力扰动值进行分析,获得与地震相关的同震重力扰动变化。结果显示,对于苏门答腊、智利和日本三次大地震,采用经验正交函数方法能够很好地获得地震同震重力扰动的变化;而对于汶川地震,由于震级相对较小,同震信号完全湮没在其他信号及误差中,因而无法获得其同震重力变化。为了检验多大的地震引起的同震重力信号能够从GRACE重力场结果中提取出来,以汶川地震为例,将理论计算的地震重力扰动加入到由GRACE重力场观测结果计算的重力扰动中,然后用经验正交函数方法对其进行分析。模拟结果表明,当震级大于Mw8.8时,地震的同震信号能够被分析出来;而当震级小于Mw8.6时,则不能分离出同震信号;震级介于Mw8.6～8.8时,有可能分离出同震信号。
The EOF method was used to extract coseismic gravity signals from GRACE monthly gravity field data.Because the GRACE satellites can't sensor the gravity changes due to vertical displacement of the Earth's surface,the gravity disturbance is used as the target signal.First,the temporal gravity disturbances at the grid points,with 0.5° resolution,on the Earth's surface were calculated according to the GRACE monthly gravity field results,in which the 300 km fan-filter was used.And then the EOF method was applied to these temporal gravity disturbances to obtain the coseismic signals.The results show that the coseismic signals due to the recent 3 huge earthquakes,i.e.occurred in Sumatra,Chile and Japan,were successfully extracted,whereas the coseismic signals due to the Wenchuan earthquake,with relatively smaller magnitude,was not extracted because the signals are inundated by other signals and the error of GRACE data.In order to obtain the knowledge of how large an earthquake whose coseismic signal can be extracted from GRACE data should be,a simulation study was carried out by adding a known signal which was calculated in terms of the focal parameters of the Wenchuan earthquake according to dislocation theory,but with different magnitude.The simulation shows that the coseismic signal can be extracted when the magnitude reaches to Mw8.8 or larger;it is difficult to be extracted when the magnitude is smaller than Mw8.6;and it is possible to be extracted when the magnitude is between Mw8.6 and Mw8.8.
The EOF method was used to extract coseismic gravity signals from GRACE monthly gravity field data.Because the GRACE satellites can't sensor the gravity changes due to vertical displacement of the Earth's surface,the gravity disturbance is used as the target signal.First,the temporal gravity disturbances at the grid points,with 0.5° resolution,on the Earth's surface were calculated according to the GRACE monthly gravity field results,in which the 300 km fan-filter was used.And then the EOF method was applied to these temporal gravity disturbances to obtain the coseismic signals.The results show that the coseismic signals due to the recent 3 huge earthquakes,i.e.occurred in Sumatra,Chile and Japan,were successfully extracted,whereas the coseismic signals due to the Wenchuan earthquake,with relatively smaller magnitude,was not extracted because the signals are inundated by other signals and the error of GRACE data.In order to obtain the knowledge of how large an earthquake whose coseismic signal can be extracted from GRACE data should be,a simulation study was carried out by adding a known signal which was calculated in terms of the focal parameters of the Wenchuan earthquake according to dislocation theory,but with different magnitude.The simulation shows that the coseismic signal can be extracted when the magnitude reaches to Mw8.8 or larger;it is difficult to be extracted when the magnitude is smaller than Mw8.6;and it is possible to be extracted when the magnitude is between Mw8.6 and Mw8.8.
引文
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14 Wahr J,Molenaar M and Bryan F.Time variability of theEarth’s gravity field:Hydrological and oceanic effects andtheir possible detec-tion using GRACE[J].J GeophysRes.,1998,103(B12):30 205-30 229.
15 Zhang Z Z,et al.An effective filtering for GRACE time-var-iable gravity:Fan filter[J].Geophysical Research Letter,2009,36,L17311,doi:10.1029/2009GL039459.
16 Cheng M K and Tapley B D.Variations in the Earth’s ob-lateness during the past 28 years[J].J Geophys Res.,2004,109:doi:10.1029/2004JB003028.
17 Rodell M,et al.The global land data assimilation system[J].Bull Am Met Soc.,2004,85:381-394.
18闫昊明,钟敏,朱耀仲.科学计算程序库WHIGG F90LIB及其地学应用[J].大地测量与地球动力学,2006,(1):109-114.(Yan Haoming,Zhong Min and Zhu Yaozhong.Scientific computation library WHIGG F90 LIB and it’s ap-plication in geosciences[J].Journal of Geodesy and Geody-namics,2006,(1):109-114)
2 Chen J L,et al.GRACE detects coseismic and postseismicdeformation from the Sumatr-Andaman earthquake[J].Geo-physical research letter,2007,34,L13302,doi:10.1029/2007GL030356.
3 Matsuo K and Kosuke H.Coseismic gravity changes of the2011 Tohoku-Oki earthquake[J].Geophysical research let-ter,2011,38,L00G12,Doi:10.1029/2011GL049018.
4 Sun W and Okubo S.Surface potential and gravity changesdue to internal dislocations in a spherical earth-I.theory fora point dislocation[J].Geophysical Journal International,1993,114:569-592.
5周新,孙文科,付广裕.重力卫星GRACE检测出2010年智利Mw8.8地震的同震重力变化[J].地球物理学报,2011,54(7):1 745-1 749(Zhou Xin,Sun Wenkeand Fu Guangyu.Gravity satellite GRACE detects coseismicgravity changes caused by 2010 Chile Mw8.8 earthquake[J].Chinese Journal of Geophysics,2011,54(7):1 745-1 749)
6 Kosuke H and Matsuo K.Coseismic gravity changes of the2010 earthquake in central Chile from satellite gravimetry[J].Geophysical research letter,2010,37,L24306,Doi:10.1029/2010GL045335.
7 Jolliffe I T.Principal component analysis(2nd edn)[M].Springer:New York,2002.
8 Wilks D S.Statistical methods in the atmospheric sciences(2nd edn)[M].Amsterdam:Academic Press,2006.
9 Preisendorfer R W.Principal component analysis in meteor-ology and oceanography[M].Amsterdam:Elsevier,1988.
10 de Viron O,et al.Retrieving earthquake signature in gracegravity solutions[J].Geophysical Journal International,2008,174:14-20.
11 Sun W and Okubo S.Coseismic deformations detectable bysatellite gravity missions:A case study of Alaska(1964,2002)and Hokkaido(2003)earthquakes in the spectraldomain[J].Journal of Geophysical Research,2004,109,B04405,doi:10.1029/2003JB002554.
12 Zhou J C,et al.Reformulation of co-seismic polar motionexcitation and low degree gravity changes:Applied to the2011 Tohoku-Oki Earthquake(Mw9.0)[J].Journal ofGeodynamics,2013,63:20-26.
13周江存,孙和平,徐建桥.用地表和空间重力测量验证全球水储量变化模型[J].科学通报,2009,54(9):1 282-1 289(Zhou Jiangcun,Sun Heping and Xu Jian-qiao.Validating global hydrological models by ground andspace gravimetry[J].Chinese Science Bulletin,2009,54(9):1282-1289.
14 Wahr J,Molenaar M and Bryan F.Time variability of theEarth’s gravity field:Hydrological and oceanic effects andtheir possible detec-tion using GRACE[J].J GeophysRes.,1998,103(B12):30 205-30 229.
15 Zhang Z Z,et al.An effective filtering for GRACE time-var-iable gravity:Fan filter[J].Geophysical Research Letter,2009,36,L17311,doi:10.1029/2009GL039459.
16 Cheng M K and Tapley B D.Variations in the Earth’s ob-lateness during the past 28 years[J].J Geophys Res.,2004,109:doi:10.1029/2004JB003028.
17 Rodell M,et al.The global land data assimilation system[J].Bull Am Met Soc.,2004,85:381-394.
18闫昊明,钟敏,朱耀仲.科学计算程序库WHIGG F90LIB及其地学应用[J].大地测量与地球动力学,2006,(1):109-114.(Yan Haoming,Zhong Min and Zhu Yaozhong.Scientific computation library WHIGG F90 LIB and it’s ap-plication in geosciences[J].Journal of Geodesy and Geody-namics,2006,(1):109-114)
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