Rayleigh Wave Tomography of Southeastern Tibet

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision. A full understanding of this process requires investigation of the crust and mantle structures in the Himalayas and the Tibet Plateau. Seismic data recorded at 51 stations from the Namche Barwa broadband seismic experiment in southeastern Tibet, which operated between July 2003 and October 2004, allows for constructing high resolution seismic models in the area.

The goal of this study is to image 3D shear wave velocity variation beneath southeastern Tibet from surface wave tomography. We use earthquakes in a distance range of 30 to 120 degrees with magnitude greater than 5.5. 40 of these events provide high quality Rayleigh wave data. Fundamental mode Rayleigh waves are obtained at 15 frequencies ranging from 7 to 50 mHz. We adopt a two-step inversion approach. First, a two-plane-wave inversion technique is applied to calculate phase velocities. Our preliminary results show that the average phase velocity in southeastern Tibet varies from 3.80 km/s at the period of 20 s to 4.24 km/s at 143 s. 2D variation of phase velocity will be computed using the same method. Then a 3D shear wave velocity model will be obtained from the inversion of the 2D phase velocity dispersions.

From this Rayleigh wave tomographic study, we anticipate a high-resolution 3D model of crust and upper mantle beneath southeastern Tibet. The model may reveal the collision architecture of the Indian and Eurasian continental plates beneath the Tibetan Plateau and provide independent constraints to geodynamic models for building collisional orogens.