High-resolution Shear Wave Velocity Structure beneath the Western Sichuan from Ambient Noise Array Tomography

Since 2006, the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, has deployed a dense seismograph array in SW China in order to understand the earthquake dynamics in this region. Using about one year continuous recordings at 156 stations over western Sichuan(100°~105°E, 29°~32°N), we obtained the surface wave empirical Green’s functions from ambient noise cross-correlation, and then measured about 10,000 Rayleigh wave phase velocity dispersion curves. Rayleigh wave phase velocity maps (2-35s) are constructed using the generalized linear inversion method with lateral resolution about 20-30 kilometers. Finally, we invert for the 3-D shear wave velocity structure in the crust using the Neighborhood Algorithm with constraints (e.g., Moho depth) from the receiver function inversions (Liu et al., 2008). The results show significant differences of the crustal structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. The phase velocity maps at short periods also show clear boundaries around the Longmenshan faults and Xianshuihe faults. The Sichuan basin shows significant higher velocity feature than the Chuandian and Songpan-Garze block at the middle and lower crustal depth, and appears much slower in the upper crust, suggesting the existence of a thick sediment layer. And the crust thickness of Sichuan basin becomes thinner from south to north. In the Songpan-Garze block, a low-velocity layer exists in the middle and lower crust. In the Chuandian block exists a high-velocity layer in the middle crust near the Xianshuihe faults, but a low-velocity layer in the lower crust. Our study manifests that Sichuan basin vertically contacts with the Songpan-Garze block without obvious westward subduction, although the crust of the Songpan-Garze block seems mechanically much weaker as inferred from the widespread crustal low-velocity zone (Liu et al, 2008).