sPL,一个近距离确定震源深度的震相
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摘要
实际地震波形观测表明,对于大陆结构相对简单的地壳中的地震而言,有一震相出现在P波和S波之间.一般在30～50km附近发育得较好,其能量主要集中在径向分量,而垂向分量的振幅相对径向要小,切向分量上的振幅很弱,且波形以低频为主,通常没有P波尖锐.在利用FK方法计算合成地震图的基础上,发现该震相是由S波入射到自由地表形成水平传播的P波(文献称为surface P wave,自由地表P波)或者包括S波入射到地表后形成的多次P波或其散射震相.由于该震相是由S波和P波之间耦合而形成,本文将其定义为sPL(s coupled into P)震相.理论波形研究表明,sPL相对直达P波的到时差对震中距离不敏感,而随着震源深度的增加几乎呈线性增加,因此可以很好的约束震源深度.本文以2005年江西九江地震为例,证实了sPL确定震源深度的可行性和可靠性.在观测到sPL震相的情况下,离震源50km以内的一个三分量地震台站的波形就可以帮助获得可靠的震源深度,而不需要精确的震中距离.由于sPL震相出现距离较近,对于较小(三级以上)的地震也可以应用,因此在稀疏台网布局情形下sPL对于确定中小地震深度应该具有很好的应用意义.
A seimic phase is often observed between P and S of continental earthquakes for epicentral distances less than 50km in regions with simple crustal structure. It is stronger on radial component than on vertical component, and its tangential amplitude is weak. The seismic phase is also less impulsive than P waves. From synthetic waveforms, we find that it is generated when S wave goes upward and is post-critically converted to P wave traveling along the free surface. It also may contain multiple conversion or reverberations of P wave near free surface. Because it results from coupling between S and P waves, we define this phase as sPL. For a half-space velocity model, sPL degenerates to surface P wave as defined by Aki and Richards, and it contains less high frequency energy due to its similar nature as head waves. But it may contain multiple reverberations of P wave near critical distance when there is substantial sub-surface velocity gradient. Waveform study shows that differential time between sPL and direct P is insensitive to epicentral distance, but increases almost linearly with the focal depth. It is used to determine the focal depths for a 2005 earthquake sequence in Jiujiang, China. Error analysis with synthetic waveforms indicates that 10% uncertainty in velocity structure will lead to depth error of about 2km or less. Focal depth from sPL has the advantage that it can be obtained from even only one seismic record in distance less than 50km and accurate epicentral distance is not required. It may help a lot to determine focal depth of small earthquakes with magnitude less than M3, thus will be helpful when only sparse seismic stations are available.
A seimic phase is often observed between P and S of continental earthquakes for epicentral distances less than 50km in regions with simple crustal structure. It is stronger on radial component than on vertical component, and its tangential amplitude is weak. The seismic phase is also less impulsive than P waves. From synthetic waveforms, we find that it is generated when S wave goes upward and is post-critically converted to P wave traveling along the free surface. It also may contain multiple conversion or reverberations of P wave near free surface. Because it results from coupling between S and P waves, we define this phase as sPL. For a half-space velocity model, sPL degenerates to surface P wave as defined by Aki and Richards, and it contains less high frequency energy due to its similar nature as head waves. But it may contain multiple reverberations of P wave near critical distance when there is substantial sub-surface velocity gradient. Waveform study shows that differential time between sPL and direct P is insensitive to epicentral distance, but increases almost linearly with the focal depth. It is used to determine the focal depths for a 2005 earthquake sequence in Jiujiang, China. Error analysis with synthetic waveforms indicates that 10% uncertainty in velocity structure will lead to depth error of about 2km or less. Focal depth from sPL has the advantage that it can be obtained from even only one seismic record in distance less than 50km and accurate epicentral distance is not required. It may help a lot to determine focal depth of small earthquakes with magnitude less than M3, thus will be helpful when only sparse seismic stations are available.
引文
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[3] 石耀霖,朱守彪.中国大陆震源机制深度变化反映的地壳-地幔流变特征.地球物理学报,2003,46(3) :359～365 Shi Y L,Zhu S B.Contrast of Rheology in the crust and mantle near Moho revealed by depth variation of earthquake mechanism in continental China.Chinese J.Geophys.(in Chinese),2003,46:359～365
[4] King R.Observations of sPn from Swabian Alb earthquakes at theGRFArray.J.Geophys.,1979,45:337～340
[5] Helmberger D,Engen G.Modeling the long-period body waves from shallow earthquakes at regional ranges.Bull.Seism.Soc.Am.,1980,70:1699～1714
[6] Zonno G,Kind R.Depth determination of North Italian earthquakes using Grafenberg data.Bull.Seism.Soc.Am.,1984,74:1645～1659
[7] Langston C.Depth of faulting during the 1968 Meckering,Australia,earthquake sequence determined from waveform analysis of local seismograms.J.Geophys.Res.,1987,92:1561～I574
[8] Langston C.The SsPmp phase in regional wave propagation.Bull.Seism.Soc.Am.,1996,86:133～143
[9] Zhao L,Helmberger D.Broadband modelling along a regional shield path,Harvard recording of the Saguenay earthquake.Geophys.J.Int.,1991,105:301～312
[10] Zhao L,Helmberger D.Source retrieval from broadband regional seismograms:Hindu Kush region.Phys.Earth Planet.Interiors,1993,78:69～95
[11] Bock G.Depth phases from local earthquakes.BMR J.Aust.Geol.Geophys.,1993,13:275～279
[12] Bock G,Grunthal G,Wylegalla K.The 1985/86 Western Bohemia earthquakes:modeling source parameters with synthetic seismograms.Tectonophysics,1996,261:139～146
[13] Zhu L,Helmberger D.Regional waveform Calibration in the Pamir-Hindu Kush region.J.Geophys.Res.,1997,102:22799～22813
[14] Saikia C.A method for path calibration using regional and teleseismic broadband seismograms:application to the 21 May 1997 Jabalpur,India earthquake(M_W5． 8) .Curt.Sci.,2000,79:1301～1311
[15] Saikia C,Woods B,Thio H.Calibration of the regional crustal waveguide and the retrieval of source parameters using waveform modeling.Pure Appl.Geophys.,2001,158:1301～1338
[16] Bent A L,Perry C.Depths of eastern Canadian earthquakes from regional data.Seism.Res.Lett.,2002,73:273～284
[17] Uski M,Hyvonen T,Korja A,et al.Focal mechanisms of three earthquakes in Finland and their relation to surface faults.Tectonophysics,2003,363:141～157
[18] Ma,Atkinson.Focal depths for small to moderate earthquakes (m_n≥2． 8) in western Quebec,southern Ontario,and Northern New York.Bull.Seism.Soc.Am.,2006,96:609～623
[19] Kim Dineva,Ma,Eaton.The 4 August 2004,Lake Ontario,Earthquake.Seism.Res.Lett.,2006,77:65～73
[20] Kastrup U,Deichmann N,Frohlich A.Evidence for an active fault below the northwestern Alpine foreland of Switzerland.Geophys.J.Int.,2007,169:1273～1288
[21] 房明山,杜安陆,董孝平等.用sPn震相测定近震震源深度.地震地磁观测与研究,1995,16(5) :13～18 Fang M S,Du A L,Dong X P,et al.Determination of focal depth with the sPn phase(in Chinese).Seisrnol Geomag ObserRes.(in Chinese),1995,16(5) :13～18
[22] 任克新,邹立晔,刘瑞丰等.用sPn计算内蒙地震的震源深度.地震地磁观测与研究,2004,25(2) :24～31 RenK X,Zou L H,Liu R F,et al.Application of sPn to focal depth determination in inner Mongolia.Seismologycal and Geomagnetic Observation and Research(in Chinese),2004,25(3) :24～31
[23] 洪星,叶雯燕,邵平荣等.台湾海峡南部一次5． 0级地震的sPn震相分析.地震地磁观测与研究,2006,27(1) :26～31 Hong X,Ye W Y,Shao R P,e al.Simultaneous determination of the phase sPn with multiple station records.Seismological and Geomagnetic Observation and Research(in Chinese),2006,27(1) :26～31
[24] 高立新,刘芳,赵蒙生等.用sPn震相计算震源深度的初步分析与应用.西北地震学报.2007,29(3) :213～244 Gao L X,Liu F,Zhao M S,et al.Primary analysis on the seismic source depth determination using sPn phase and its application.Northwestern Seismological Journal(in Chinese),2007,29(3) .213～244
[25] Langston.An integrated study of crustal structure and regional wave propagation for southeastern missouri.Bull.Seism.Soc.Am.,1994,84:105～118
[26] Aki Richards.Quantitative Seismology(Second Edition).Sausalito,CA:University Science Books,2002
[27] Haskell N A.Radiation pattern of surface waves from point sources in a multi-layered medium.Bull.Seism.Soc.Am., 1964,70:1015～1036
[28] Wang C Y,Herrmann R B.A numerical study of P,SV,and SH wave generation in a plane layered medium.Bull.Seism.Soc.Am.,1980,70:1015～1036
[29] Zhu L,Rivera L A.A note on the dynamic and static displacements from a point source in multi-layered media.Geophys.J.Int.,2002,148:619～627
[30] Magistrale H,McLaughlin K,Day S.A geology-based 3-D velocity model of the Los Angeles basin sediments.Bull.Seismol.Soc.Am.,1996,86:1161～1166
[31] Chong Ni.Near surface velocity and QS structure of the Quaternary sediment in Bohai basin,China.Earthq.Sci.,2009,22:451～458
[32] Brocher T M.Compressional and shear-wave velocity versus depth relations for common rock types in Northern California.Bull.Seismol.Soc.Am.,2008,98(2) :950～968
[33] 吕坚,郑勇,倪四道等.2005年12月26日九江-瑞昌M_S5． 7,M_S4． 8地震的震源机制解与发震构造研究.地球物理学报,2008,51(1) :158～164 Lu J,Zheng Y,Ni S D,et al.Focal mechanisms and seismogenie structure of the M_S5． 7 and M_S4． 8 JiujiangRuichang earthquakes of the Nov.26,2005． Chinese J.Geophys.(in Chinese),2008,51(1) :158～164
[34] 杨中书,曾文敬.利用双差法对2005年江西九江-瑞昌5． 7级地震序列重新定位.地震地磁观测与研究,2007,28(2) :25～31 Yang Z S,Zeng W J.Relocation of the 26 Nov 2005 JiujiangRuichang M5． 7 earthquake sequence using the double diference earthquake location algorithm.Seismological and Geomagnetic Observation and Research(in Chinese),2007,28(2) :25～31
[35] Bouchon.The importance of the surface or interface P wave in near-earthquake studies.Bulletin of the Seismological Society of America,1978,68(5) :1293～1311
[36] Klose C D,Seeber L.Shallow Seismicity in Stable Continental Regions.Seismological Research Letters,2007,78:554～562
[2] 张国民,汪素云,李丽等.中国大陆地震震源深度及其构造含义.科学通报,2002,47(9) :663～670 Zhang G M,Wang S Y,Li L,et al.Focal depth of continental earthquakes in China and its seismotectonic implications.Chinese Science Bulletion(in Chinese),2002,47(9) :663～670
[3] 石耀霖,朱守彪.中国大陆震源机制深度变化反映的地壳-地幔流变特征.地球物理学报,2003,46(3) :359～365 Shi Y L,Zhu S B.Contrast of Rheology in the crust and mantle near Moho revealed by depth variation of earthquake mechanism in continental China.Chinese J.Geophys.(in Chinese),2003,46:359～365
[4] King R.Observations of sPn from Swabian Alb earthquakes at theGRFArray.J.Geophys.,1979,45:337～340
[5] Helmberger D,Engen G.Modeling the long-period body waves from shallow earthquakes at regional ranges.Bull.Seism.Soc.Am.,1980,70:1699～1714
[6] Zonno G,Kind R.Depth determination of North Italian earthquakes using Grafenberg data.Bull.Seism.Soc.Am.,1984,74:1645～1659
[7] Langston C.Depth of faulting during the 1968 Meckering,Australia,earthquake sequence determined from waveform analysis of local seismograms.J.Geophys.Res.,1987,92:1561～I574
[8] Langston C.The SsPmp phase in regional wave propagation.Bull.Seism.Soc.Am.,1996,86:133～143
[9] Zhao L,Helmberger D.Broadband modelling along a regional shield path,Harvard recording of the Saguenay earthquake.Geophys.J.Int.,1991,105:301～312
[10] Zhao L,Helmberger D.Source retrieval from broadband regional seismograms:Hindu Kush region.Phys.Earth Planet.Interiors,1993,78:69～95
[11] Bock G.Depth phases from local earthquakes.BMR J.Aust.Geol.Geophys.,1993,13:275～279
[12] Bock G,Grunthal G,Wylegalla K.The 1985/86 Western Bohemia earthquakes:modeling source parameters with synthetic seismograms.Tectonophysics,1996,261:139～146
[13] Zhu L,Helmberger D.Regional waveform Calibration in the Pamir-Hindu Kush region.J.Geophys.Res.,1997,102:22799～22813
[14] Saikia C.A method for path calibration using regional and teleseismic broadband seismograms:application to the 21 May 1997 Jabalpur,India earthquake(M_W5． 8) .Curt.Sci.,2000,79:1301～1311
[15] Saikia C,Woods B,Thio H.Calibration of the regional crustal waveguide and the retrieval of source parameters using waveform modeling.Pure Appl.Geophys.,2001,158:1301～1338
[16] Bent A L,Perry C.Depths of eastern Canadian earthquakes from regional data.Seism.Res.Lett.,2002,73:273～284
[17] Uski M,Hyvonen T,Korja A,et al.Focal mechanisms of three earthquakes in Finland and their relation to surface faults.Tectonophysics,2003,363:141～157
[18] Ma,Atkinson.Focal depths for small to moderate earthquakes (m_n≥2． 8) in western Quebec,southern Ontario,and Northern New York.Bull.Seism.Soc.Am.,2006,96:609～623
[19] Kim Dineva,Ma,Eaton.The 4 August 2004,Lake Ontario,Earthquake.Seism.Res.Lett.,2006,77:65～73
[20] Kastrup U,Deichmann N,Frohlich A.Evidence for an active fault below the northwestern Alpine foreland of Switzerland.Geophys.J.Int.,2007,169:1273～1288
[21] 房明山,杜安陆,董孝平等.用sPn震相测定近震震源深度.地震地磁观测与研究,1995,16(5) :13～18 Fang M S,Du A L,Dong X P,et al.Determination of focal depth with the sPn phase(in Chinese).Seisrnol Geomag ObserRes.(in Chinese),1995,16(5) :13～18
[22] 任克新,邹立晔,刘瑞丰等.用sPn计算内蒙地震的震源深度.地震地磁观测与研究,2004,25(2) :24～31 RenK X,Zou L H,Liu R F,et al.Application of sPn to focal depth determination in inner Mongolia.Seismologycal and Geomagnetic Observation and Research(in Chinese),2004,25(3) :24～31
[23] 洪星,叶雯燕,邵平荣等.台湾海峡南部一次5． 0级地震的sPn震相分析.地震地磁观测与研究,2006,27(1) :26～31 Hong X,Ye W Y,Shao R P,e al.Simultaneous determination of the phase sPn with multiple station records.Seismological and Geomagnetic Observation and Research(in Chinese),2006,27(1) :26～31
[24] 高立新,刘芳,赵蒙生等.用sPn震相计算震源深度的初步分析与应用.西北地震学报.2007,29(3) :213～244 Gao L X,Liu F,Zhao M S,et al.Primary analysis on the seismic source depth determination using sPn phase and its application.Northwestern Seismological Journal(in Chinese),2007,29(3) .213～244
[25] Langston.An integrated study of crustal structure and regional wave propagation for southeastern missouri.Bull.Seism.Soc.Am.,1994,84:105～118
[26] Aki Richards.Quantitative Seismology(Second Edition).Sausalito,CA:University Science Books,2002
[27] Haskell N A.Radiation pattern of surface waves from point sources in a multi-layered medium.Bull.Seism.Soc.Am., 1964,70:1015～1036
[28] Wang C Y,Herrmann R B.A numerical study of P,SV,and SH wave generation in a plane layered medium.Bull.Seism.Soc.Am.,1980,70:1015～1036
[29] Zhu L,Rivera L A.A note on the dynamic and static displacements from a point source in multi-layered media.Geophys.J.Int.,2002,148:619～627
[30] Magistrale H,McLaughlin K,Day S.A geology-based 3-D velocity model of the Los Angeles basin sediments.Bull.Seismol.Soc.Am.,1996,86:1161～1166
[31] Chong Ni.Near surface velocity and QS structure of the Quaternary sediment in Bohai basin,China.Earthq.Sci.,2009,22:451～458
[32] Brocher T M.Compressional and shear-wave velocity versus depth relations for common rock types in Northern California.Bull.Seismol.Soc.Am.,2008,98(2) :950～968
[33] 吕坚,郑勇,倪四道等.2005年12月26日九江-瑞昌M_S5． 7,M_S4． 8地震的震源机制解与发震构造研究.地球物理学报,2008,51(1) :158～164 Lu J,Zheng Y,Ni S D,et al.Focal mechanisms and seismogenie structure of the M_S5． 7 and M_S4． 8 JiujiangRuichang earthquakes of the Nov.26,2005． Chinese J.Geophys.(in Chinese),2008,51(1) :158～164
[34] 杨中书,曾文敬.利用双差法对2005年江西九江-瑞昌5． 7级地震序列重新定位.地震地磁观测与研究,2007,28(2) :25～31 Yang Z S,Zeng W J.Relocation of the 26 Nov 2005 JiujiangRuichang M5． 7 earthquake sequence using the double diference earthquake location algorithm.Seismological and Geomagnetic Observation and Research(in Chinese),2007,28(2) :25～31
[35] Bouchon.The importance of the surface or interface P wave in near-earthquake studies.Bulletin of the Seismological Society of America,1978,68(5) :1293～1311
[36] Klose C D,Seeber L.Shallow Seismicity in Stable Continental Regions.Seismological Research Letters,2007,78:554～562
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