Rayleigh‐Love Discrepancy Highlights Temporal Changes in Near‐Surface Radial Anisotropy After the 2004 Great Sumatra Earthquake

Abstract

AbstractStrong 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 Mw 9.2 Sumatra Earthquake, and monitor the temporal velocity changes from Rayleigh waves (δVLR) and Love waves (δVLQ). We observe a δVLR of −0.16% and δVLR/δVLQ 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 δVLR/δVLQ ratio can be explained by strong dβV (−4%) and weak dβH (−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 δVLR/δVLQ ratios and radial anisotropy after consecutive great earthquakes are consistent with laboratory experiments under cyclic loading and unloading.