Structural heterogeneities in the Southeast Tibetan Plateau:implications for the 2008 M7.9 Wenchuan earthquake generation

摘要

We collected a large number of P-wave and Swave arrival-time data from 14,474 local earthquakes and 4,782 teleseismic events to simultaneously invert the three-dimensional (3-D) P-wave and S-wave velocity structures under the southeastern margin of Tibet. Our velocity models confirmed the major features delineated by previous local and global Pwave tomographic approaches as well as surface wave studies, and revealed some new features of the crust and mantle structural heterogeneities relative to the generation of the 2008 Wenchuan earthquake (Mw7.9). Slow anomalous Vp and Vs were generally imaged at upper crust under the Sichuan basin, showing well agreement with the surface geological studies that the foreland basin contains primarily Mesozoic and Paleozoic sedimentary rocks with thickness greater than several kilometers. The Yangtze Platform is characterized as a cratonic-like lithospheric body subducting into the southeast margin of the Tibetan Plateau with a southwestward dipping trend to a depth of 400 km in a sharp contrast to low-velocity anomalies beneath regions of the Songpan-Ganzi Fold System, the Northwest Qiangtang and Indochina Blocks at these depths. The devastating Wenchuan earthquake occurred at a junction between the high and low velocity zone where the mechanical strength of materials could be much weaker than the normal sections of the seismogenic layer due to the penetration of fluids from the lower crust and upper mantle. Such fluids perhaps reduced the mechanical strength of the fractured rock matrix in the source area and altered the local shear stress regime, and thus triggered the 2008 Wenchuan earthquake.