Seismic sedimental structure beneath S-net stations : Application of Rayleigh-wave V/H extracted from noise cross-correlations

Abstract:

Recent development of OBS networks such as S-net and DONET now enables us to measure ambient noise cross-correlations of high-resolution in the sea. Spatiotemporal variations of seismic velocity and anisotropy in marine sediment help us to understand dynamic processes associated with plate interaction in a subduction zone such as aligned cracks caused by the bending of a subducting plate, fluid migration and dehydration of constituent minerals. The ellipticity of Rayleigh waves ( i.e., the ratio of vertical and horizontal amplitude, V/H ) is useful to study seismic velocity of sedimentary layers because V/H is more sensitive to shallow structures than conventional phase or group dispersion data (e.g., Lin et al., 2014).

In this study, first we calculated ellipticity at the seafloor and formulated their sensitivity kernels for several models including the sea. Next, we applied ambient noise cross-correlations to the S-net data to extract Rayleigh waves. We measured V/H values and inverted them to 1-D sedimental models in for the region of S-net stations.

We used continuous three-month records from 1 January, 2017 to 31 March at subarray stations S3 of S-net. We divided them into each of one day long, downsampled it from the original 100 Hz to 4 Hz samplings with an antialiasing filter, and took cross-correlations between two selected stations with 2-40 sec bandpass filtering and one-bit processing (e.g., Bensen et al., 2007). We then compute the zero-lag cross-correlation coefficients of the vertical component and radial component phase-delayed by the Hilbert transform. We selected the data with zero-lag cross-correlations coefficient higher than 0.7 to invert them to new 1-D sedimental models (e.g., Tanimoto and Rivera., 2007) for their find inversion to 1-D seismic models of sedimentary layers.

As a result, cross-correlations among S3 stations perpendicular to the trench axis clearly show surface wave signals with relatively good symmetry of causal and anti-causal waveforms in the period range from 4 to 8 sec. Their group velocity is very small, only about 0.2-0.3 km/s. This signal probably correspond to Scholte waves. Scholte waves propagate parallel to the water-sediment interface, when the shear velocity of the solid is lower than the compressional velocity in the water (1.5 km/s), that is, very loose sediment. Their V/H spectral ratios show high zero-lag cross-correlation coefficients, corresponding to PSD peaks in the period range from 4 to 8. The energy of Scholte waves should be concentrated in this period range. We obtained a new 1-D sedimental model for stations S3 of S-net. P and S wave velocities in a shallow sediment are 1.97km/s and 0.65 km/s respectively. The estimated S wave velocity seems to explain the observed low phase velocity of Scholte waves.

Since the S-wave velocity of the shallow ocean-bottom sediment is considered to vary significantly in space and time, the measurement of V/H with ambient noise data of S-net should be useful to retrieve and spatial-temporal variations.