Strong ground motions from large earthquakes are capable of damaging near-surface sediments and promoting notable reductions in their seismic velocity structures. These velocity reductions can be monitored using either body waves or surface waves from repeatable seismic sources, such as repeating earthquakes (REs) or ambient seismic noise. Here, we compile a decade-long catalog of REs since the 2004 M_w 9.2 Sumatra Earthquake, and monitor the temporal velocity changes from Rayleigh waves (δV_LR) and Love waves (δV_LQ). We observe a δV_LR of −0.16% and δV_LR/δV_LQ ratio of ∼6, inconsistent with velocity reductions in isotropic media. To reconcile the observations, we carry out analyses of sensitivity kernels of surface waves in isotropic and vertical transversely isotropic (VTI) media and forward waveform modeling. The modeling reveals that the observed large δV_LR/δV_LQ ratio can be explained by strong dβ_V (−4%) and weak dβ_H (−0.615%) reductions and an increase in radial anisotropy in the near surface. These changes are best explained by a 2% increase in crack density of aligned horizontal cracks in overpressured sediments near the compressive subduction zone forearc. Temporal variations of δV_LR/δV_LQ ratios and radial anisotropy after consecutive great earthquakes are consistent with laboratory experiments under cyclic loading and unloading.

中文翻译：

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Rayleigh-Love 差异突出了 2004 年苏门答腊大地震后近地表径向各向异性的时间变化

大地震产生的强烈地面运动能够破坏近地表沉积物并促进其地震速度结构的显着降低。可以使用来自可重复地震源（例如重复地震 (RE) 或环境地震噪声）的体波或表面波来监测这些速度降低。在这里，我们编制了自 2004 年M _w 9.2 苏门答腊地震以来长达十年的 RE 目录，并监测瑞利波 ( δV _LR ) 和洛夫波( δV _LQ )的时间速度变化。我们观察δV _LR的-0.16％和δV _LR / δV _LQ比率约为 6，与各向同性介质中的速度降低不一致。为了调和观察结果，我们对各向同性和垂直横向各向同性 (VTI) 介质中的表面波的灵敏度内核和正向波形建模进行了分析。建模表明，观察到的大δV _LR / δV _LQ比率可以用强dβ _V (-4%) 和弱dβ _H (-0.615%) 减少以及近地表径向各向异性增加来解释。这些变化的最好解释是压缩俯冲带前弧附近超压沉积物中对齐的水平裂缝的裂缝密度增加了 2%。δV _LR / 的时间变化连续大地震后的δV _LQ比和径向各向异性与循环加载和卸载下的实验室实验一致。