Anisotropic Velocity Structure Beneath Shikoku, Japan: Insights From Receiver Function and Shear Wave Splitting Analyses.

A combination of receiver function (RF) analysis and shear wave splitting can reveal the anisotropic properties of the Earth's velocity structure. The RF profile exhibits harmonic patterns partially generated from the discontinuities of anisotropic media near the receiver. The anisotropic properties are determined using splitting parameters, which include the fast polarization direction (FPD) and split time. We utilized the seismograms recorded by stations on Shikoku Island, Japan. Also, we applied the Bayesian information criterion to control the spike count of each RF, which consists of a series of spikes generated by time‐domain iterative deconvolution. Anisotropic differences arose between northern and southern regions along the 30‐km iso‐depth line of the top surface of the Philippine Sea Plate. In the southern part of Shikoku Island, the FPDs are sub‐perpendicular to the plunge of the subducting slab, while in the northern part, they are sub‐parallel to the plunge of the slab. Our results indicate that anisotropic strengths are weaker in the tectonic‐tremor band in northwestern Shikoku and stronger in the northern part of central and eastern Shikoku around the no tectonic‐tremor area. These anisotropic variations may characterize Shikoku Island's geological structure. Plain Language Summary: Receiver function (RF) analysis is sensitive to subsurface discontinuities in seismic velocity using shear waves on seismograms. This study estimated a seismic velocity anisotropy by combining RF and shear wave splitting analysis to determine the polarization direction and delay time, characterizing the anisotropic medium. The observed fast direction of anisotropy aligns with the strike of the subducting Philippine Sea Plate in southern Shikoku, while in the northern region, it becomes sub‐parallel to the subducting plate's plunge (N‒S). Additionally, the delay time between the split fast and slow shear waves indicates the strength of anisotropy. In the tectonic‐tremor‐clustered area in northwestern Shikoku, the strengths of anisotropy are regionally weaker than in other areas. Moreover, stronger anisotropy was measured in the deeper northern part of central and eastern Shikoku, where there is a lack of tectonic‐tremor activity. Although several factors contribute to the anisotropy, we concluded that the anisotropy mainly reflects the structure, including the subducting slab, continental crust, and even the mantle wedge. Key Points: Fine‐resolution anisotropic properties have been calculated above slab Moho in Shikoku Island, southwest Japan, using receiver function analysisWeaker anisotropic strength was observed in tectonic‐tremor‐clustered areas with serpentinized mantle wedge, especially in northwestern ShikokuSerpentinization of the mantle and metasomatic reactions lead to hydrofracturing and relatively low anisotropic strength [ABSTRACT FROM AUTHOR]