The crustal and upper mantle structure beneath the South-North seismic zone from the inversion of Love wave phase velocity

The South-North Seismic Zone (SNSZ) located the eastern margin of Tibetan plateau is an active tectonic zone between the west of the rising plateau and the east of the stable blocks, where the regional tectonic is quite complex and the seismicity is very high. The crustal and mantle structure beneath the SNSZ has the scientific significance for indicating the uplift mechanism of the Tibetan plateau and the interaction between the Tibetan plateau and its adjacent blocks. The tomographic method was used to estimate the crustal and mantle structure beneath the SNSZ. A two-step inversion scheme was adopted to obtain the S-wave velocity structure. Firstly, we constructed phase velocity distributions by inverting pure path dispersions of Love wave at different periods. Secondly, we determined the S-wave velocity structure at different depths from the acquired phase velocities. We collected the seismic waveform data recorded between January 2008 and March 2011 by 292 stations from the Chinese Provincial Digital Seismic Networks, measured the Love wave phase velocity dispersion curves with Frequency-Time Analysis and constructed the distributions of the Love wave phase velocity beneath the SNSZ. From the pure path dispersions of Love wave phase velocity, the one-dimension S-wave velocity was inverted by the linearized inversion program at each node on a 0. 25 degrees x 0. 25 degrees grid. Then these one-dimension S-wave velocity were assembled together to obtain the three-dimension S-wave velocity structure. Our results suggested that the significantly low velocity layer of S-wave existed in Songpan-Garze terrane and Chuandian diamond block. The characteristic of the low velocity anomaly supported the lower crust flow model for explaining the uplift of the Tibetan plateau and its crustal materials movement. The high S-wave velocity anomaly was presented in northwestern Chuandian diamond block at the depth 100 to 120 km, which maybe attribute to the underthrust plate of the India lithosphere along the Himalayas east structural knot. The obvious comparison between the weak lower crust with the low S-wave velocity and the hard upper mantle with high S-wave velocity maybe imply the different tectonic movement and deformation mode between the crust and upper mantle, which provided the condition for the dynamic decoupling of the crust and mantle beneath the northwestern Chuandian diamond block.