oa Estimation of S-wave anisotropy in the Nankai Trough using active and passive seismic dataset observed by DONET

In The Nankai Trough seismogenic zone, cabled real-time observation system, DONET and IODP C0002G borehole observatory, have been operating to monitor seismic activity, crustal deformation and tsunami propagation since Aug. 2011 and Jan. 2013, respectively. For elucidating dynamic processes from preseismic state to the generation of mega-thrust earthquake, which occurs repeatedly in subduction zones, it is important to observe and monitor the stress state, i.e., a key parameter governing its fault dynamics in the vicinity of seismogenic fault. In this study, we performed active and passive seismic data processing to obtain seismic anisotropy, as a proxy of stress state, by using dataset acquired by three-component seismometers installed in the DONET and IODP C0002G observatories. In the passive data processing, we applied a seismic interferometry method to ambient noise records acquired by horizontal components of each seismometer. After the application of cross-dipole analysis to the acquired records, several coherent events have become visible. These events were perceived to be reflected S-wave from each layer below seafloor, and S-wave splitting caused by seismic anisotropy was observed. We then estimated anisotropy direction and amplitude beneath each seismometer in shallow sediment layer. For the active seismic dataset that was acquired in azimuthally aligned airgun shot locations for each seismometer performed in Nov. 2013, we steered horizontal records for each pair of shot and seismometer to form radial and transverse components to each shot location at a distance of ca. 3km from the seismometer. In the steered records, P-S converted waves from bottom of shallow sediments were clearly visible. The horizontal axis of symmetry to fast S-wave direction was estimated through the fitting of a simple sinusoidal curve to the aximuthal amplitude distribution in radial and transverse components for S-wave anisotropy in the shallow sediments in the Nankai Trough. We finally compared the obtained S-wave anisotropy from the passive data with the active one. The comparison in the order of anisotropy and the orientation of the horizontal axis of symmetry showed good agreement with each other, especially in landward area. Some differences in the complicated structure zone were probably caused by the signal-to-noise ratio deterioration due to the influence of the dimensionality of the sub-seafloor structure and the seafloor topography around the observatory in the active dataset, and to the contamination of seafloor microseisms in the passive data. We now plan to develop a new scheme including layer stripping method, 3-D rotation method, etc., to improve the quality of our analysis.