The South China Sea (SCS) located at the intersection of the three intercontinental plates of Eurasia, IndiaAustralia, and the Pacific Oceanis, is a typical marginal sea basin formed by the seafloor spreading under the tectonic background of plate convergence. Many crustal-scale studies indicate that the SCS basin has undergone asymmetric spreading, multi-phase ridge jumps, and intense post-spreading volcanic activity. Due to the lack of seismic data in the oceanic basin of the SCS, it remainsunclear about the scale and basin control of the Zhongnan fault, the magma source depth of the SCS basin, and the transport channel after the cessation of seafloor spreading. Phase velocity derived from ambient noise surface wave tomography may provide useful information to shed light on the mechanisms of the aforementioned problems. From October 2019 to July 2020, a 3D Ocean Bottom Seismometers (OBS) passive seismic observation experiment was carried out by the Second Institute of Oceanography, Ministry of Natural Resources (SIOMNR) in a broad area of the SCS. Based on the seismic ambient noise data recorded by 16 OBSs in the SCS basin, we inverted the phase velocity images over a period range of 10–20 s using ambient noise surface wave tomography. Our results indicate that the Zhongnan fault zone is a lithospheric-scalefault, which played a regulating role in the last oceanic ridge transition of the SCS basin from the East Subbasin to the Southwest Subbasin. In addition, the low-velocity body in the north flank of the Southwest Subbasin extends from the post-spreading seamounts on the ocean crust to the uppermost mantle (i.e., about 10–30 km), which indicates an oblique magma migration during the postspreading volcanism.
Abstract Seismic azimuthal anisotropy characterized by shear wave splitting analyses using teleseismic X K S phases (including S K S , S K K S , and P K S ) is widely employed to constrain the deformation field in the Earth's crust and mantle. Due to the near‐vertical incidence of the X K S arrivals, the resulting splitting parameters (fast polarization orientations and splitting times) have an excellent horizontal but poor vertical resolution, resulting in considerable ambiguities in the geodynamic interpretation of the measurements. Here we use P ‐to‐ S converted phases from the Moho and the 410‐ ( d 410) and 660‐km ( d 660) discontinuities to investigate anisotropy layering beneath Southern California. Similarities between the resulting splitting parameters from the X K S and P ‐to‐ S converted phases from the d 660 suggest that the lower mantle beneath the study area is azimuthally isotropic. Similarly, significant azimuthal anisotropy is not present in the mantle transition zone on the basis of the consistency between the splitting parameters obtained using P ‐to‐ S converted phases from the d 410 and d 660. Crustal anisotropy measurements exhibit a mean splitting time of 0.2 ± 0.1 s and mostly NW‐SE fast orientations, which are significantly different from the dominantly E‐W fast orientations revealed using X K S and P ‐to‐ S conversions from the d 410 and d 660. Anisotropy measurements using shear waves with different depths of origin suggest that the Earth's upper mantle is the major anisotropic layer beneath Southern California. Additionally, this study demonstrates the effectiveness of applying a set of azimuthal anisotropy analysis techniques to reduce ambiguities in the depth of the source of the observed anisotropy.
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