Rayleigh phase velocity and azimuthal anisotropy from high dense seismic arrays reveal strong control of block boundary faults on crustal material migration in SW China

Abstract

Located on the southeastern margin of the Tibetan plateau (TP), SW China, the southern Sichuan-Yunnan block (SYB) is intersected by boundary faults as the NW-trending Red River fault (RRF) and the nearly NS-trending Xiaojiang fault (XJF). It is a crucial zone for exploring severe crustal deformation and complicated tectonic movement. Using ambient noise data from multiple temporary seismic arrays and regional permanent seismic stations within the southern SYB, we can obtain the high-resolution S-wave velocity and azimuthal anisotropy by the direct inversion method. However, the Rayleigh phase velocity can reveal the structural characteristics of specific range of thickness at various periods. To evaluate the reliability of the direct inversion method in forward calculation, this study further calculates the Rayleigh phase velocity and azimuthal anisotropy at periods of 5–40 s. Compared with the S-wave velocity and azimuthal anisotropy, the forward Rayleigh phase velocity and azimuthal anisotropy illustrate similar patterns. Compared with the traditionally obtained Rayleigh phase velocity and azimuthal anisotropy, the forward results show good consistency, providing more details of the low-velocity anomalies and the azimuthal anisotropy owing to the dense seismic stations. A notable difference from previous results is that the low-velocity anomaly beneath the XJF does not cross over the southern segment of the RRF, resulting in the mechanically weak crustal materials distributed along the faults showing the fast axis of local complex pattern. This may indicate that the RRF strongly controls the deep structures.