壳-幔过渡带首波传播特征
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摘要
上地幔速度结构是影响Pn波几何衰减特征的主要因素,本文利用合成理论地震图方法,研究了壳-幔过渡带速度梯度对Pn波几何衰减特征的影响.结果表明,Pn波的衰减特征对上地幔的速度梯度十分敏感,如:1000 km处Pn波的振幅,在相当于将均匀上地幔经展平变换后的等效速度梯度(0.0013 s-1)模型下,约为在半无限空间上叠加一层的简单模型下的10倍;当震中距大于某一距离后,具有速度梯度模型下的Pn波振幅随震中距衰减的速度明显小于无速度梯度的半无限空间模型下Pn波振幅的衰减速度,速度梯度越大,衰减速度越慢,在某一距离范围内,振幅甚至有增大的趋势;当存在速度梯度时,Pn波的几何衰减随频率而变化.上述结果对深入理解Pn波在区域震中距范围内的衰减规律提供了理论依据.
Pn geometric spreading is a dominate reason for Pn attenuation.This report gives a theoretical study of the dependence of Pn geometric spreading on the velocity gradient in the upper mantle.The results show that Pn attenuation is very sensitive to the velocity gradient in the upper mantle.For example,the Pn amplitude at 1000 km is a factor of 10 larger for a model with a weak velocity gradient(equivalent to the earth-flattening transformation of a homogeneous upper mantle) than a model consisting of layer over an infinite halfspace,and the Pn amplitude increases after a critical distance for linear gradient models.These results are similar to those of Sereno(1989).Also,Pn geometric spreading is frequency-dependent for an upper mantle model with a simple linear velocity gradient. The results are important because most methods for estimating Pn attenuation require assumptions regarding geometric spreading.
Pn geometric spreading is a dominate reason for Pn attenuation.This report gives a theoretical study of the dependence of Pn geometric spreading on the velocity gradient in the upper mantle.The results show that Pn attenuation is very sensitive to the velocity gradient in the upper mantle.For example,the Pn amplitude at 1000 km is a factor of 10 larger for a model with a weak velocity gradient(equivalent to the earth-flattening transformation of a homogeneous upper mantle) than a model consisting of layer over an infinite halfspace,and the Pn amplitude increases after a critical distance for linear gradient models.These results are similar to those of Sereno(1989).Also,Pn geometric spreading is frequency-dependent for an upper mantle model with a simple linear velocity gradient. The results are important because most methods for estimating Pn attenuation require assumptions regarding geometric spreading.
引文
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[14]何永锋,陈晓非,张海明.层裂对区域震相Lg波的影响[J].地震学报,2005,48(3):309~316.
[15]何永锋,陈晓非,张海明.地下核爆炸Lg波的激发原理[J].地球物理学报,2005,48(2):367~373.
[16]何永锋,陈晓非,何耀锋,靳平.地下核爆炸Rg波低谷点激发机理[J].地球物理学报,2005,48(3):643~648.
[2]Sereno J.Numerical modeling of Pn geometric spreading andempirically determined attenuation of Pn and Lg phase recor-ded in eastern Kazakhstan[J].Science Applications Interna-tional Corporation,San Diego,California,1989.
[3]Chen X F.A systematic and efficient method of computingnormal modes for multilayered half-space[J].Geophys J Int,1993,115:391~409.
[4]Chen X F.Seismograms synthesis in multi-layered half-spacemedia[Part I.Theoretical formulations].Earthquake Re-search in China,1999,13(2):149~174.
[5]Zhang H,Chen X.Self-adaptive Filon's Integration methodand its application to computing synthetic seismograms[J].Chin Phys Lett,2001,18:313~315.
[6]Zhang H,Chen X,Chang S.An efficient method for compu-ting synthetic seismograms for a layered half-space withsources and receivers at close or same depth[J].Pure ApplGeophysics,2003,160:467~486.
[7]Kennett B L N.Seismic Wave Propagation in Stratified Media[J].Cambridge University Press,New York,1983.
[8]何耀锋,陈蔚天,陈晓非.利用广义反射-透射系数方法求解含低速层水平层状介质模型中面波频散曲线问题[J].地球物理学报,2006,(4):1074~1081.
[9]Yao Z X,Harkrider D G.A generalized reflection-transmis-sion coefficient matrix and discrete wavenumber method forsynthetic seismograms[J].Bull Seism Soc Am,1983,73:1685~1699.
[10]田,陈晓非.利用拟牛顿法和信赖域法联合反演震中分布与一维速度结构[J].地球物理学报,2006,49(3):845~854..
[11]周红,陈晓非.不同尺度地形的SH波频率域响应特征研究[J].地球物理学报,2006,49(1):205~211..
[12]冯德益.地震波理论与应用[M].北京:地震出版社,1988.
[13]张坚,张海明,陈晓非.垂向非均匀介质中首波特征分析[J].地震学报,2002,45():~.
[14]何永锋,陈晓非,张海明.层裂对区域震相Lg波的影响[J].地震学报,2005,48(3):309~316.
[15]何永锋,陈晓非,张海明.地下核爆炸Lg波的激发原理[J].地球物理学报,2005,48(2):367~373.
[16]何永锋,陈晓非,何耀锋,靳平.地下核爆炸Rg波低谷点激发机理[J].地球物理学报,2005,48(3):643~648.
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