[S-SS04] Seismological advances in the ocean

[SSS04-P04] Nankai Trough phase velocity tomography using differential-time measurements for noise interferometry from DONET array

*Xin Liu^1,2, Ayako Nakanishi², Gou Fujie², Takashi Nakamura³, Takashi Tonegawa² (1.University of Hong Kong, 2.Japan Agency for Marine-Earth Science and Technology, 3.Central Research Institute of Electric Power Industry)

Keywords:ambient noise, differential adjoint tomography, Nankai Trough

S-wave velocity structure beneath Ocean Bottom Seismometers is difficult to estimate using traditional active-source surveys as the airgun source at the sea surface does not generate S wave directly. The S wave is sensitive to pore fluids in rocks and provides important structural information for oceanic subduction zone. It can also enhance the seafloor ground motion predictions and earthquake hypocenter location, which are important for the Nankai Trough area towards earthquake and tsunami hazard mitigation. The phase velocity tomography derived from surface wave data is the preliminary result before the S-wave velocity tomography and provides similar structure information as S wave.

In this study, we perform Rayleigh wave phase velocity tomography based on differential-time measurements from ambient noise interferometry. Considering two receivers very close to each other and a virtual source farther away, the differential-time kernel is insensitive to noise sources (Liu 2020). Consequently, it shifts more sensitivity towards velocity structure anomaly. We apply this technique to the vertical components of DONET array deployed on the seafloor above the accretionary prism of Nankai Trough. The resulting phase velocity images (0.05-0.2 Hz) show more details of lateral variations with reduced bias related to non-uniform noise source distributions. In addition, we apply the ambient noise differential adjoint tomography method to phase velocity tomography. The updated results will be presented at meeting.