Browsing by Author "Chen, Min"

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    Low wave speed zones in the crust beneath SE Tibet revealed by ambient noise adjoint tomography
    (American Geophysical Union, 2014) Chen, Min; Huang, Hui; Yao, Huajian; van der Hilst, Rob; Niu, Fenglin
    We present a refined 3D crustal model beneath SE Tibet from ambient noise adjoint tomography. Different from ray-theory-based tomography, adjoint tomography in this study incorporates a spectral-element method (SEM) and takes empirical Green's functions (EGFs) of Rayleigh waves from ambient noise interferometry as the direct observation. The frequency-dependent traveltime misfits between SEM synthetic Green's functions and EGFs are minimized with a preconditioned conjugate gradient method, meanwhile the 3D model gets improved iteratively utilizing 3D finite-frequency kernels. The new model shows 3 - 6% shear wave speed increasing beneath the western Sichuan Basin (SCB) (depth>15 km) and the central Chuan-Dian Block (CDB), and 6 - 12% shear wave speed reduction in the mid-lower crust beneath the northern and the southern CDB. The inferred spatial pattern of low wave speed zones, consistent with possible partial melt, suggests more complex and disconnected geometry than the pervasive narrow zone from the channel flow models.
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    Mantle-driven uplift of Hangai Dome: New seismic constraints from adjoint tomography
    (Wiley, 2015) Chen, Min; Niu, Fenglin; Liu, Qinya; Tromp, Jeroen
    The origin of Hangai Dome, an unusual large-scale, high-elevation low-relief landform in central Mongolia, remains enigmatic partly due to lack of constraints on its underlying seismic structure. Using adjoint tomography—a full waveform tomographic technique—and a large seismic waveform data set in East Asia, we discover beneath the dome a deep low shear wave speed (low-V) conduit indicating a slightly warmer (54 K to 127 K) upwelling from the transition zone. This upwelling is spatially linked to a broader uppermost mantle low-V region underlying the dome. Further observations of high compressional to shear wave speed ratios and positive radial anisotropy in the low-V region suggest partial melting and horizontal melt transport. We propose that the mantle upwelling induced decompression melting in the uppermost mantle and that excess heat associated with melt transport modified the lithosphere that isostatically compensates the surface uplift at upper mantle depths (>80 km).
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    Multiparameter adjoint tomography of the crust and upper mantle beneath East Asia: 1. Model construction and comparisons
    (Wiley, 2015) Chen, Min; Niu, Fenglin; Liu, Qinya; Tromp, Jeroen; Zheng, Xiufen
    We present a 3-D radially anisotropic model of the crust and mantle beneath East Asia down to 900 km depth. Adjoint tomography based on a spectral element method is applied to a phenomenal data set comprising 1.7 million frequency-dependent traveltime measurements from waveforms of 227 earthquakes recorded by 1869 stations. Compressional wave speeds are independently constrained and simultaneously inverted along with shear wave speeds (VSH and VSV) using the same waveform data set with comparable resolution. After 20 iterations, the new model (named EARA2014) exhibits sharp and detailed wave speed anomalies with improved correlations with surface tectonic units compared to previous models. In the upper 100 km, high wave speed (high-V) anomalies correlate very well with the Junggar and Tarim Basins, the Ordos Block, and the Yangtze Platform, while strong low wave speed (low-V) anomalies coincide with the Qiangtang Block, the Songpan Ganzi Fold Belt, the Chuandian Block, the Altay-Sayan Mountain Range, and the back-arc basins along the Pacific and Philippine Sea Plate margins. At greater depths, narrow high-V anomalies correspond to major subduction zones and broad high-V anomalies to cratonic roots in the upper mantle and fragmented slabs in the mantle transition zone. In particular, EARA2014 reveals a strong high-V structure beneath Tibet, appearing below 100 km depth and extending to the bottom of the mantle transition zone, and laterally spanning across the Lhasa and Qiangtang Blocks. In this paper we emphasize technical aspects of the model construction and provide a general discussion through comparisons.