美国南加州地壳、上地幔三维P波速度成像——莫霍面起伏的影响
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摘要
应用南加州地震台网记录的 6 347次区域地震的 14 6 42 2个P波到时数据 ,并结合其它地震学方法所得到的莫霍面深度资料 ,在考虑莫霍面起伏变化下 ,确定了该区 0～ 35km深度内的精细三维P波速度模型 .检测板分辨率实验结果说明了反演结果的可靠性 .该模型提供了有关区域结构的重要信息 ,浅层P波速度结构与地表的地质特征相关 ,非常准确地反映了山脉盆地、地形地貌的差异 ;各深度层上的速度结构表现为分块特征 ,反映了以大型活动断裂为界勾画出的分区 ,如洛杉矶盆地、文图拉盆地、莫哈韦沙漠、半岛山脉、圣华金山谷、内华达山区及思尔顿凹槽区域整体变化的特征 .圣安德列斯断层成为研究区内明显的边界 ,几个深度层上可以看到两侧的速度存在明显的差异 ,而横跨该断裂的几个剖面图上则显示了位于断层西南侧的太平洋板块区域速度较高、不均匀性较大 ,地震活动较深 ;位于断层东北部的北美板块速度偏低 ,非均匀性稍弱 ,地震活动性则较弱 .比较平缓莫霍界面和起伏莫霍界面下反演得到的结果可以说明 :莫霍界面的起伏对中下地壳层位速度结构的影响是明显的 ,尤其是在莫霍面深度变化较大的部位 ,这种影响更为强烈 .带莫霍面形状的模型能更精确地计算射线路径和理论到时 ,从而使反演后的残差更小、结果更精确
We use 146 422 P wave arrival times from 6347 local earthquakes recorded by the Southern California Seismic Network to determine a detailed three dimensional P wave velocity structure of 0～35 km depth. We have taken into account the Moho depth variations which were obtained by Seismological methods. Checkerboard tests suggest that our inversion results are reliable. Our models provide new information on regional geological structures of Southern California. Shallow layer P wave velocity structure correlates with surface geological features and expresses well variations of surface topography of the mountains and basins. The velocity structure at each layer is characterized by block structures divided by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Peninsular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all round block. San Andreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there are strong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variations than to the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inversions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found that the topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Moho topograph is considered, a more reasonable tomographic result can be obtained and the resulting 3 D velocity model fits the data better.
We use 146 422 P wave arrival times from 6347 local earthquakes recorded by the Southern California Seismic Network to determine a detailed three dimensional P wave velocity structure of 0～35 km depth. We have taken into account the Moho depth variations which were obtained by Seismological methods. Checkerboard tests suggest that our inversion results are reliable. Our models provide new information on regional geological structures of Southern California. Shallow layer P wave velocity structure correlates with surface geological features and expresses well variations of surface topography of the mountains and basins. The velocity structure at each layer is characterized by block structures divided by large faults. Ventura Basin, Los Angeles Basin, Mojave Desert, Peninsular Ranges, San Joaquin Valley, Sierra Nevada, and Salton Trough show respectively all round block. San Andreas Fault becomes an obvious boundary of the region. To its southwest, the velocity is higher, and there are strong heterogeneity and deeper seismicity; but to its northeast, the velocity is lower and shows less variations than to the southwest, the seismicity is shallower. To investigate the effect of the Moho geometry we conducted inversions for two cases: one for flat Moho geometry, another for a Moho with lateral depth variations. We found that the topography of the Moho greatly affects the velocity structure of the middle and lower crust. When the Moho topograph is considered, a more reasonable tomographic result can be obtained and the resulting 3 D velocity model fits the data better.
引文
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SauberJW ,ThatcherW ,SolomonSC .1986.GeodeticmeasurementsofdeformationinthecentralMojaveDesert,California[J].JGeophysRes,91:12683～12693
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ZhuL ,KanamoriH .2000.MohodepthvariationinsouthernCaliforniafromteleseismicreceiverfunctions[J].JGeophysRes,105:2969～2980
ZhaoD ,HasegawaA ,HoriuchiS .1992.TomographicimagingofPandSwavevelocitystructurebeneathnortheasternJapan[J].JGeophysRes,97:19909～19928
ZhaoD ,KanamoriH ,HumphresE .1996.Simultaneousinversionoflocalandteleseismicdataforthecrustandmantlestruc tureofsouthernCalifornia[J].PhysEarthPlanetInteri,93:191～214
ZhaoD ,HasegawaA ,KanamoriH .1994.DeepstructureofJapansubductionzoneasderivedfromlocal,regionalandteleseis micevent[J].JGeophysRes,99:22313～22329
HearnTM ,ClaytonRW .1986.LateralvelocityvariationinsouthernCaliforniaⅠ.ResultfortheuppercrustfromPgwave[J].BullSeisSocAmer,76:495～509
HumphreysE ,ClaytonRW ,HagerBH .1984.AtomographicimageofmantlestructurebeneathsouthernCalifornia[J].GeophysResLett,11:625～627
HaukssonE .2000.CrustalstructureandseismicitydistributionadjacenttothePacificandNorthAmericaplateboundaryinSouthCalifornia[J].JGeophysRes,105:13875～13903
HuangJ ,ZhaoD ,ZhengS .2002.LithosphericstructureanditsrelationshiptoseismicandvolcanicactivityinsouthwestChina[J].JGeophysRes,107(B10):2255
HearnTM .1984.PntraveltimeinsouthernCalifornia[J].JGeophysRes,89:1843～1855
HadleyD ,KanamoriH .1977.Seismicstructureofthetranserverranges,California[J].GeolSocAmBull,88:1469～1478
IrwinWP .1990.Geologyandplate tectonicdevelopment[A].In:WallaceREed.TheSanAndreasFaultSystem,Cali fornia[C].USGeolSurvProfPap,1515:156～189
JonesCH ,KaramoriH ,RoeckerSW .1994.Missingrootsandmantle′drips′:regionalPnandteleseismicarrivaltimesinthesouthernSierraNevadaandvicinity,California[J].JGeophysRes,99:4567～4601
KanamoriH ,HadleyD .1975.CrustalstructureandtemporalvelocitychangeinsouthernCalifornia[J].PureApplGeophys,113:257～280
MagistraleH ,KaramoriH ,JonesC .1992.Forwardandinversethree dimensionalPwavevelocitymodelsofthesouthernCali forniacrust[J].JGeophysRes,97:14115～14135
PaigeCC ,SaundersMA .1982.LSQR :Analgorithmforsparselinearequationsandsparseleastsquares[J].AssocComputMachTransMathSoftware,8:43～71
ParsonsT ,McarthyJ.1996.CrustalanduppermantlevelocitystructureoftheSaltonTrough,SouthernCalifornia[J].Tec tonics,15:456～471
RuppertS ,FliednerMM ,ZandtG .1998.ThincrustandactiveuppermantlebeneaththesouthernSierraNevadainthewest ernUnitedStates[J].Tectonophysics,286:237～252
RichardsDingerKB ,ShearerPM .1997.EstimatingcrustalthicknessinsouthernCaliforniabystackingPmParrivals[J].JGeophysRes,102:15211～15224
SauberJW ,ThatcherW ,SolomonSC .1986.GeodeticmeasurementsofdeformationinthecentralMojaveDesert,California[J].JGeophysRes,91:12683～12693
ThurberCH .1983.EarthquakeIocationsandthree dimensionalcrustalstructureintheCoyoteLakearea,centralCalifornia[J].JGeophysRes,88:8226～8236
ZhuL ,KanamoriH .2000.MohodepthvariationinsouthernCaliforniafromteleseismicreceiverfunctions[J].JGeophysRes,105:2969～2980
ZhaoD ,HasegawaA ,HoriuchiS .1992.TomographicimagingofPandSwavevelocitystructurebeneathnortheasternJapan[J].JGeophysRes,97:19909～19928
ZhaoD ,KanamoriH ,HumphresE .1996.Simultaneousinversionoflocalandteleseismicdataforthecrustandmantlestruc tureofsouthernCalifornia[J].PhysEarthPlanetInteri,93:191～214
ZhaoD ,HasegawaA ,KanamoriH .1994.DeepstructureofJapansubductionzoneasderivedfromlocal,regionalandteleseis micevent[J].JGeophysRes,99:22313～22329
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