Lithosphere structures of northeast Tibetan Plateau and their geodynamic implications

摘要

In the past ten years, three major earthquakes with devastating impacts have taken place along the margins of the Bayan Har Terrane in NE Tibetan Plateau, China. To understand the geodynamic background controlling the NE Tibetan Plateau, we processed a large amount of broad-band seismic data in and around the Tibetan Plateau, and constructed a three dimensional lithospheric S-wave velocity model using a regional surface-wave tomographic tool. The velocity model produces images of important structures. First, a weak discontinuity at ～110km depth seems to exist, which separates the abnormally thick Tibetan upper-mantle lithosphere into two layers. Second, the upper-mantle lithospheric velocity structures have certain correlations with seismicity and fault activity. Regions with a low-velocity upper-mantle lithosphere have a stronger seismicity than regions with a high-velocity upper-mantle lithosphere. The earthquake focal mechanisms or faults in the interior of the low-velocity regions typically display normal faulting with a strike-slip component, while reverse faults, sometimes with a small strike-slip component, typically occur in the low-to-high velocity transition zone. The correlation of seismicity and fault activity with upper-mantle lithospheric velocity anomalies suggests that the strength of the upper-mantle lithosphere can control the deformation mode of the overlying crust where most earthquakes occur.