【摘要】 在阿波罗月震记录中普遍存在着强烈持久的尾波信号,这样的波形特征无法用均匀分层月球模型解释.一个普遍被接受的解释是月震尾波由月球浅层结构对月震波的散射引起.我们采用基于交错网格的伪谱和有限差分混合方法模拟研究非均匀上月壳对月震波的散射效应,在此基础上解释月震尾波的形成机制,并估计出上月壳速度扰动的强度.我们发现,在均匀分层模型基础上,进一步考虑上月壳中的非均匀结构对月震波的散射效应,能有效地解释月震信号中强烈持久的尾波.我们认为月震尾波可能是由上月壳中的低波速、低衰减和散射这三个因素的共同作用所引起.采用不同的扰动标准差模拟上月壳的非均匀性,并比较模拟波形与真实月震图的相似程度,我们发现上月壳中速度扰动的标准差应该在3%到5%之间,很可能接近于3%.更多还原

【Abstract】 The Apollo lunar seismic data generally exhibits a long and strong coda,which masks seismic phases other than initial P and S waves and causes difficulties in analyses of secondary waves.Recent numerical simulation of 2D global seismic wave propagation in a whole-Moon model showed that lunar seismic coda couldn’t be produced by a layered Moon model.A widely accepted explanation is that lunar seismic coda is resulted from scattering in a near-surface layer,which could be several to more than twenty kilometers thick.This study presents numerical simulations of scattering effects caused by small-scale heterogeneity in upper Moon crust in order to explain the coda.Wave equations in velocity-stress form are solved in a 2Dcylindrical coordinate system.The spatial derivatives are calculated by a hybrid pseudospectral and finite difference method defined on staggered grid.The background layered model is based on recent published seismic velocity and attenuation models of the Moon.In order to simulate the heterogeneity of upper Moon crust,von-Karman auto correlation function is used to generate velocity fluctuations,superimposed onthe background velocity.We firstly check the effectiveness of scattering in upper Moon crust to reproduce lunar seismic coda.Wavefields are calculated for an upper Moon crust model without scattering and one with scattering.Calculations are performed for a 71 km shallow moonquake and an 867 km deep moonquake.Comparison of the wavefield snapshots generated by the two models shows that wave energy can last a very long time in the upper Moon crust due to scattering in this layer.Comparison of waveforms shows that scattering in upper Moon crust can effectively produce longduration coda.Comparison with Apollo seismograms shows that the duration and strength of coda caused by scattering in upper Moon crust can be well consistent with observations.The mechanism which produces lunar seismic coda is tentatively discussed. We propose that scattering,low attenuation and low velocity in the upper Moon crust may all contribute to the lunar seismic coda.Effects of fluctuant strength on synthetic coda are discussed.Different standard deviations of velocity fluctuations are set for upper Moon crust:1%,3%,5%,7%and 9%.For both shallow and deep moonquakes,wavefields are simulated for each model and the characteristic decay times of synthetic coda are calculated at four epicentral distances:22°,45°,67.5°and 90°.The characteristic decay times firstly increase with standard deviation of velocity fluctuations and then decrease.The characteristic decay times reach the largest value where standard deviation equals to 3%.Finally,similarities between synthetics and Apollo lunar seismograms are measured via correlation coefficient in order to estimate the most realistic strength of velocity fluctuations.We propose that the standard deviation of velocity fluctuations should be between 3% and 5%,and it′s likely close to 3%.更多还原