Effect of lateral heterogeneity on 2-D Rayleigh wave ZH ratio sensitivity kernels based on the adjoint method: Synthetic and inversion examples

The ratio between vertical and radial amplitudes of Rayleigh waves (hereafter, the Rayleigh wave ZH ratio) is an important parameter used to constrain structures beneath seismic stations. Some previous studies have explored crust and upper mantle structures by joint inversion of the Rayleigh wave ZH ratio and surface wave dispersion. However, all these studies have used a 1-D depth sensitivity kernel, and this kernel may lack precision when the structure varies a great deal laterally. Here, we present a systematic investigation of the two-dimensional (2-D) Rayleigh wave ZH ratio kernel based on the adjoint-wavefield method and perform two synthetic tests using the new kernel. The 2-D ZH ratio kernel is consistent with the traditional 1-D sensitivity kernel but has an asymmetric pattern with a preferred orientation toward the source. The predominant effect caused by heterogeneity can clearly be seen from kernels calculated from models with 2-D heterogeneities, which confirms the necessity of using the new 2-D kernel in some complex regions. Inversion tests using synthetic data show that the 2-D ZH ratio kernel has the potential to resolve small anomalies as well as complex lateral structures.