Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent

We here present the results of the inverse modeling of crustal S-phases recorded from a 400-km-long seismic profile, with azimuth nearly N30W, from Lianxian, near Hunan Province, to Gangkou Island, near Guangzhou City, Guangdong Province, in the southern margin of South China continent. The finding in this case is that many shot gathers provided by this wide-angle seismic experiment show relatively strong reflected and refracted S-phases, in particular some crustal refractions (Sg waves) and Moho reflections (SmS waves or simply Sm waves). The P-wave velocity structure of the crust and uppermost mantle was already obtained through the interpretation of vertical-component shot gathers. Now, with constraints introduced by the P-wave velocity architecture and after picking up S-wave traveltime data on the seismograms, we have obtained the S-velocity model of the crust by adjusting these traveltimes but keeping the geometry of the crustal reflectors. Our results demonstrate: (1) the average crustal S-velocity is about 3.64 km/s to the northwest of the Wuchuan-Sihui fault, and 3.62 km/s to the southeast of this fault; (2) relatively constant S-velocity of about 3.42 km/s for the upper crust, 3.55 km/s for the middle crust and laterally varying shear velocity around 3.82 km/s for the lower crust; (3) correspondingly, Vp/Vs ratio is 1.73 for the upper crust, 1.71 for the middle crust and 1.74 for the lower crust. Both shear velocities and Vp/Vs ratio correlate well with the major active faults that break the study area, and show significant changes especially in the upper crust. High Poisson’s ratio (1.8) is observed at shallow depth beneath the Minzhong depression to the southeast of the Wuchuan-Sihui fault and the Huiyuan depression in the southern margin of South China continent. In contrast, a very low Vp/Vs ratio (1.68) is observed between 8 and 14 km depth beneath Huiyuan. At deeper depth, a high Vp/Vs ratio (1.76) is observed in the lower crust beneath the Minzhong depression.

S-wave phases are analyzed and interpreted under the constraints imposed by the P-wave velocity model along the Lianxian–Gangkou (L–G) seismic profile by adjusting these traveltimes but keeping the geometry of the crustal reflectors. After S-wave velocity model of Fig. 4 was constructed by carrying out a fitting between field observations and theoretical traveltimes calculated by ray-tracing, the Vp/Vs ratio using directly the P-wave velocity given by Zhang and Wang (2007). Both shear velocities and Vp/Vs ratio correlate well with the major active faults that break the study area. In the illustration we labeled by capital letters (A, B, C, D, E and F) six lithospheric columns. In Fig. 7 we show the velocity–depth functions for P- and S-waves, as well as the Vp/Vs ratio, for each of the well-illuminated crustal portions.Figure optionsDownload as PowerPoint slideHighlights
► Construct S-wave and Vp/Vs model in South China based on P-wave structure.
► Both shear velocities and Vp/Vs ratio correlate well with the major active faults.
► Wuchuan-Sihui fault as the boundary between the Yangtze and Cathaysia blocks.