Xinfu Li

The distribution of the mid‐to‐lower crustal low‐velocity zone beneath the northeastern Tibetan Plateau revealed from ambient noise tomography

We collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the Chinese Provincial Digital Seismic Networks, A Seismic Collaborative Experiment of Northern Tibet, and the Northeastern Tibet Seismic experiment. Cross correlations of vertical component records are computed to extract the Rayleigh wave empirical Greens functions. Group and phase velocities are then constructed from the empirical Greens functions in 8 to 50 s period. At periods ≤25 s, more than 10% lower velocities are imaged beneath the Qaidam Basin, and high velocities are observed beneath the nonbasin regions. At periods ≥30 s, up to 10% lower velocities are imaged in the Qiangtang and Songpan-Ganze Terranes. From these group and phase velocity maps, a three-dimensional (3-D) Vsv model of the crust is derived. The model shows that the Qiangtang and Songpan-Ganze Terranes have a very thick crust with a prominent low-velocity zone (LVZ) in the middle crust. The LVZ thins out in the vicinity of the eastern Kunlun Mountains, providing a new constraint on the mode of deformation across the Tibetan Plateau. The northwestern Qilian Orogen, where receiver functions reveal a Moho deeper than the surrounding areas, also features a relatively weak midcrustal LVZ, which we interpret as an intracrustal response associated with the shortening between the North China Craton and the Tibetan Plateau.

Radial anisotropy in the crust beneath the northeastern Tibetan Plateau from ambient noise tomography

Through analysis of Rayleigh wave and Love wave Green’s functions estimated from ambient noise tomography, we obtain radial anisotropy and shear wave velocity structure beneath the northeastern Tibetan Plateau. With two hundred and twenty three broadband seismic stations deployed by China Earthquake Administration, a collaborative seismic experiment of northern Tibet (ACSENT) experiment and northeastern Tibet seismic (NETS) experiment provide the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the northeastern Tibetan Plateau. Discrepancies between Love (sh) and Rayleigh (sv) wave velocities show complex anisotropic patterns associated with the dynamic processes of the collision between the Indian and Eurasian plates: (1) In the upper crust, Vsv>Vsh anisotropy is dominant throughout the study area which probably reflects fossil microcracks induced by the uplift, folding and erosion geodynamic processes; (2) in the middle crust, Vsh>Vsv observed beneath the Songpan-Ganzi terrane and the northwestern Qilian orogen correlates well with a mid-crustal low velocity zone (LVZ); (3) at depths deeper than 40 km, Vsh>Vsv is still found in the Songpan-Ganzi terrane. This anisotropy could be caused by the sub-horizontal alignment of anisotropic minerals that has followed the collision between India and Eurasia. However, the northwestern Qilian orogen is associated with Vsv>Vsh anisotropy which may be related to the vertically aligned seismic anisotropic minerals caused by the crustal thickening.

Effects of seasonal changes in ambient noise sources on monitoring temporal variations in crustal properties

Continuous data recorded at 39 broadband stations near the Longmen Shan Fault operated by the China Earthquake Administration from 1 January 2008 to 30 September 2010 are used to study temporal variability in direct surface wave arrivals extracted from ambient noise. We use a cross-correlation technique to compute Empirical green functions (EGFs) for all available station pairs at the frequency range of 0.1 to 0.5Hz. Delay times are measured by cross-correlating reference empirical green functions and moving 60-day stacks of EGFs. By comparing the temporal changes with and without the correction for seasonal variations, our results show that for some station pairs temporal variations were strongly affected by the seasonal variation. After correction for seasonal variations, we measure a 0.5-% maximum velocity drop after the 2008 Ms8.0 earthquake in Sichuan, China. We find that the Sichuan Basin exhibited a larger relative velocity drop than the Tibetan plateau area. Our results suggest that correction for seasonal variation is an important procedure for monitoring temporal variations in crustal properties using the direct arrival surface waves extracted from ambient noise.