偏离大圆路径传播对四川西部面波相速度成像的影响

Effect of off-great-circle propagation on surface wave phase velocity tomography in western Sichuan

  • 摘要: 面波成像是研究地壳上地幔横波速度结构的一种重要方法.通常,面波相速度或群速度成像假设面波沿大圆路径传播.但是,在地下介质速度结构变化较大时,面波会偏离大圆路径传播,从而导致基于大圆路径假设下的面波成像结果存在一定的误差. 我们采用基于射线追踪的面波成像方法,研究了面波的偏离大圆路径传播对四川西部地区面波相速度成像结果的影响.使用快速行进法(fast marching method)进行面波传播路径的射线追踪,采用子空间反演法(subspace inversion)进行迭代反演,对理论模型合成数据和川西台阵的短周期背景噪声相速度频散数据进行成像分析,并与使用大圆路径传播的成像结果进行对比.对理论模型的测试结果表明,当速度结构变化较大时,基于偏离大圆路径传播的面波成像能够更好地恢复模型异常.对川西台阵的真实数据反演结果显示:在短周期为6 s时,基于偏离大圆路径传播的反演方法较基于大圆路径传播的反演方法所获得的相速度异常的幅度更大些,在四川盆地区域两者的差异接近0.2 km/s;在周期为10 s时,两种反演方法的差异显著减小,基本都在0.1 km/s以内.这主要是因为6 s周期的面波相速度对复杂的上地壳浅层结构更为敏感,从而使得面波的偏离大圆路径传播效应对反演结果的影响更为显著.本文结果表明,当某一周期不同路径的面波相速度测量值变化较大,例如相对于平均相速度的异常超过10%时,则需考虑采用基于偏离大圆路径传播的面波成像方法,否则速度异常较大区域的反演结果可能会造成较大的偏差.

     

    Abstract: Surface wave tomography is a very important tool to study shear wave velocity structures of crust and upper mantle. For surface wave phase or group velocity tomography we usually assume that surface waves propagate along great-circle paths. However, when the velocity structure has large variations, surface waves will propagate along off-great-circle paths, and therefore there may exist considerable errors in tomographic inversion results based on great-circle propagation of surface waves. We use the ray-tracing-based surface wave tomography method to analyze how off-great-circle propagation of surface waves influences the results of phase velocity tomography in western Sichuan. During the inversion, the fast marching method is used to track the ray paths of surface waves and the subspace inversion scheme is used for the inversion step. We use synthetic data from a theoretical model and short period ambient noise phase velocity dispersion data from the western Sichuan array for the analysis, and we compare the inversion results based on off-great-circle propagation and great-circle propagation of surface waves. Results of the synthetic tests show that the off-great-circle tomographic method recovers the anomaly better when the variation of the velocity structure is large.The inversion of real data from the western Sichuan array reveals that for the 6 s period data, the differences between the phase velocity tomographic results from the off-great-circle propagation and great-circle propagation methods are rather obvious, which are close to 0.2 km/s in the Sichuan basin. For the 10 s period data, differences between the two methods are much smaller, which are all less than 0.1 km/s.The main reason is due to that the 6 s period data are more sensitive to the complex shallow structure of upper crust, so the effect of off-great-circle propagation on the inversion results is more remarkable. Our results indicate that the off-great-circle propagation effect should be considered in the tomography if the observed phase velocities along different paths have large variations, for example, more than 10% of the average velocity, otherwise the inversion results of regions with large velocity anomalies might have considerable bias.

     

/

返回文章
返回