Abstract

We perform high‐resolution 3D shear‐wave structures of the crust and upper mantle using continuous wave data recorded at 170 stations from national and regional networks in the southeastern Tibetan plateau. First, empirical Green’s functions are obtained between all the possible station pairs. Then Rayleigh‐wave group velocity and phase velocity dispersion curves are retrieved using the frequency–time analysis method. The Rayleigh‐wave group velocity images at periods from 8 to 50 s and phase velocity images at periods from 8 to 40 s are inverted on a 1°×1° grid. Finally, the 1D shear‐wave structure beneath each grid and the 3D shear‐wave velocity structure are acquired by linear interpolation. Our results indicate that low‐velocity layers (LVLs) are ubiquitous in the middle–lower crust beneath the southeastern Tibetan plateau but vary in depth (20–35 km), thickness (5–20 km), and strength (5.4%–10.8% velocity reduction) in different blocks. The 3D geometry of the LVLs is complex and cannot be interpreted as a model with a uniform flow channel. The shear velocity in the Chuandian block is unevenly distributed, indicating that the block is inhomogeneous. The 12 May 2008 Wenchuan and 20 April 2013 Lushan earthquake sequences (Ms≥3.0) all occurred in the crustal layers in which LVLs are inhibited by the strong Sichuan basin and in which the topography and crustal thickness vary sharply. This formed the earthquake preparation environment of the two earthquakes. The Ningnan–Qiaojia pull‐apart region is characterized by low shear‐wave velocity in the shallow crust, which agrees well with the sedimentary features of the upper crust. A relatively high velocity is observed in the central part of the low‐velocity Sichuan basin in the shallow layers, indicating the basement beneath the Sichuan basin may be uplifted around the central part of the basin.

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