[S-SS07] Seismic wave propagation: Theory and Application

[SSS07-P08] Azimuthal dependence of phase velocities of surface waves extracted from ambient seismic noise recorded at S-net stations

*Ryota Takagi¹, Kiwamu Nishida² (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2.Earthquake Research Institute, The University of Tokyo)

Keywords:Japan Trench, Anisotropy, S-net, Ambient noise, Surface wave

Azimuthal anisotropy of surface waves has provided insight into deformation processes in crust and mantle in both continental and oceanic areas (e.g., Fry et al. 2010; Takeo et al. 2014). Toward estimating anisotropy structure in the forearc oceanic region along the Japan Trench, we investigate the azimuthal dependence of phase velocities of Rayleigh waves extracted from ambient seismic noise recorded at S-net stations. As ambient noise surface waves are sensitive to shallow structures and potentially resolve the depth variations in anisotropy, surface wave study may complement shear wave splitting analysis and body-wave anisotropy tomography (Uchida et al. 2020; Wang et al. 2022). We extracted surface waves propagating between pairs of S-net stations by the cross-correlation functions of 3-year ambient noise records and measured two-station phase velocities by fitting theoretical cross spectra to observed ones (Takagi & Nishida, 2022). The azimuthal dependence of phase velocities is investigated with subarrays. The subarrays consist of stations whose water depths are more than half and less than twice the reference stations and within a radius of 150 km from the reference stations. The lateral heterogeneity of phase velocity within the subarrays is corrected using the isotropic phase velocity maps. We determined anisotropy amplitude and direction by fitting a curve consisting of isotropic, 2θ, and 4θ terms, where θ represents the azimuth of station pairs, to the observed phase velocities. The fundamental-mode Rayleigh-wave phase velocities at 10 s show trench parallel fast directions for the 2θ dependency. The peak-to-peak amplitude of 2θ terms is ~4% at the northern and southern parts of the Japan Trench and at the Kuril Trench and ~1–2% at the central part of the Japan Trench. The 2θ term amplitude is smaller than 1% at 13 s. The 4θ terms are comparable in magnitude to the 2θ terms at 10 s and 13 s. At the period of 20 s, the 4θ term amplitude is ~5% and significantly larger than the 2θ terms. To correctly interpret the azimuthal dependence of phase velocities and obtain anisotropic structures, we plan to examine the influence of the non-isotropic source distributions on the phase velocity measurements for this observation.