11:30 〜 11:45
[SSS08-04] Surface wave tomography in and around the Sea of Japan using ambient noise data
The Sea of Japan comprises three major oceanic basins (Japan, Yamato, and Tsushima basins) and the North Yamato and Yamato Rises. Detailed seismic structures of back-arc basins like the Sea of Japan provide insights into the subduction of plates along continental margins, the associated back-arc spreading, and the tectonics in East Asia around Japan. The geological feature of the Sea of Japan has been investigated by some earlier studies (e.g., Tamaki et al., 1992) with the Ocean Drilling Program. However, as seen in the thick crust in the Yamato and Tsushima basin, the evolutional history of the Sea of Japan is still controversial.
The seismic stations in the Sea of Japan are sparsely distributed, and shallow seismic events are limited. A large-scale upper mantle structure in the Sea of Japan was investigated with surface waves propagating between stations in Japan and the Eurasian continent (e.g., Yoshizawa et al., 2010). The shallower crustal structure was also investigated using Rayleigh waves from the ambient noise analysis (e.g., Zheng et al., 2011).
In this study, we performed phase velocity measurements and mapping utilizing ambient cross-correlation functions (CCFs) of ambient noises between F-net (NIED) stations in Japan and the temporary stations of the seismic array in NE China (NECESSArray: NorthEast China Extended Seismic Array, 2009-2011). Our dispersion analysis includes many station pairs, and the ray paths cover the entire area of the Sea of Japan. The original continuous records were obtained from over 20,000 station pairs derived from more than 200 stations of NECESSArray and F-net. The CCFs in the period range of 20–60 s (Nakata & Nishida, 2019) were used for the phase speed measurements. For phase speed measurements, the phases extracted from the Fourier transform of the CCFs were corrected considering the asymptotic far-field approximation of the Bessel function (e.g., Lin et al., 2008). The ray paths are approximated as great circles, and the frequency-dependent phase speed maps are expanded in the spherical B-spline functions defined at the spherical triangular grids (e.g., Wang & Dahlen 1995) from 20 to 40 s period.
The checkerboard resolution tests suggest that our models have sufficient lateral resolution beneath the Sea of Japan at scales of approximately 200 km or greater. However, due to the station distribution, the checkerboard tests indicate smearing effects elongated in the northwest-southeast direction due to the biased ray path distributions. The estimated models at 20 s show a high-velocity anomaly across the entire Japan Basin, while low-velocity anomalies are observed along the continental margin and the Japanese islands. These features reflect the velocity contrast between the oceanic lithosphere beneath the Japan Basin and the thicker crustal structures in the surrounding continental and island arcs. Combining the long-period phase speeds from seismic surface waves will help estimate the three-dimensional radially anisotropic S-wave structure of the crust and upper mantle beneath the Sea of Japan.
The seismic stations in the Sea of Japan are sparsely distributed, and shallow seismic events are limited. A large-scale upper mantle structure in the Sea of Japan was investigated with surface waves propagating between stations in Japan and the Eurasian continent (e.g., Yoshizawa et al., 2010). The shallower crustal structure was also investigated using Rayleigh waves from the ambient noise analysis (e.g., Zheng et al., 2011).
In this study, we performed phase velocity measurements and mapping utilizing ambient cross-correlation functions (CCFs) of ambient noises between F-net (NIED) stations in Japan and the temporary stations of the seismic array in NE China (NECESSArray: NorthEast China Extended Seismic Array, 2009-2011). Our dispersion analysis includes many station pairs, and the ray paths cover the entire area of the Sea of Japan. The original continuous records were obtained from over 20,000 station pairs derived from more than 200 stations of NECESSArray and F-net. The CCFs in the period range of 20–60 s (Nakata & Nishida, 2019) were used for the phase speed measurements. For phase speed measurements, the phases extracted from the Fourier transform of the CCFs were corrected considering the asymptotic far-field approximation of the Bessel function (e.g., Lin et al., 2008). The ray paths are approximated as great circles, and the frequency-dependent phase speed maps are expanded in the spherical B-spline functions defined at the spherical triangular grids (e.g., Wang & Dahlen 1995) from 20 to 40 s period.
The checkerboard resolution tests suggest that our models have sufficient lateral resolution beneath the Sea of Japan at scales of approximately 200 km or greater. However, due to the station distribution, the checkerboard tests indicate smearing effects elongated in the northwest-southeast direction due to the biased ray path distributions. The estimated models at 20 s show a high-velocity anomaly across the entire Japan Basin, while low-velocity anomalies are observed along the continental margin and the Japanese islands. These features reflect the velocity contrast between the oceanic lithosphere beneath the Japan Basin and the thicker crustal structures in the surrounding continental and island arcs. Combining the long-period phase speeds from seismic surface waves will help estimate the three-dimensional radially anisotropic S-wave structure of the crust and upper mantle beneath the Sea of Japan.