The velocity structure of the crust beneath Liaoning province and the Bohai sea in China was imaged using ambient seismic noise recorded by 73 regional broadband stations. All available three-component time series from the 12-month span between January and December 2013 were cross-correlated to yield empirical Green's functions for Rayleigh and Love waves. Phasevelocity dispersion curves for the Rayleigh waves and the Love waves were measured by applying the frequencytime analysis method. Dispersion measurements of the Rayleigh wave and the Love wave were then utilized to construct 2D phase-velocity maps for the Rayleigh wave at 8-35 s periods and the Love wave at 9-32 s periods, respectively. Both Rayleigh and Love phase-velocity maps show significant lateral variations that are correlated well with known geological features and tectonics units in the study region. Next, phase dispersion curves of the Rayleigh wave and the Love wave extracted from each cell of the 2D Rayleigh wave and Love wave phase-velocity maps, respectively, were inverted simultaneously to determine the 3D shear wave velocity structures. The horizontal shear wave velocity images clearly and intuitively exhibit that the earthquake swarms in the Haicheng region and the Tangshan region are mainly clustered in the transition zone between the low-and high-velocity zones in the upper crust, coinciding with fault zones, and their distribution is very closely associated with these faults. The vertical shear wave velocity image reveals that the lower crust downward to the uppermost mantle is featured by distinctly high velocities, with even a high-velocity thinner layer existing at the bottom of the lower crust near Moho in central and northern the Bohai sea along the Tanlu fault, and these phenomena could be caused by the intrusion of mantle material, indicating the Tanlu fault could be just as the uprising channel of deep materials.
The Bohai Sea is a seismically active region in China, and the studying of the velocity structure of Bohai Sea and its relationship with earthquake may be helpful to the analysis and prediction of earthquakes. Now, ambient noise tomography is an effective tool to obtain crust and upmost mantle structure. In this paper, the S-wave velocity model of the crust beneath the Bohai Sea and its surrouding areas in China was constructed applying ambient noise tomography method, with steps of calculating cross correlations of all possible vertical-component data recorded in 2013 year, retrieving Empirical Green's Functions (EGFs) for Rayleigh wave, measuring and assessing phase velocity-dispersion curves, creating the phase-velocity maps for the 8-35 s period of the Rayleigh wave, constructing the phase velocity maps and inverting the S-wave velocity structure. We work on the statistics and analysis of the characteristic relationship between crustal S-wave velocity and the temporal and spatial distribution of small earthquakes. The results from all available vertical profiles along different directions reveal that small earthquakes usually occur at the edge of the low-velocity anomaly within the shallow crust in a certain velocity range and, under special conditions, even in a distinct velocity contour. The locations of occurrence are closely related to undulating changes of the corresponding Moho morphology and the locally high Vs anomalies within the middle crust.
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