Abstract
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. Phase-velocity dispersion curves for the Rayleigh waves and the Love waves were measured by applying the frequency-time 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.
Highlights
The Tancheng-Lujing (Tanlu: illustrated in Fig. 1b) fault zone is considered as the deep faults that penetrate the Earth’s crust into the upper mantle (Wang et al 2000), and go straight through the Bohai sea into Liaoning Province along the NNE direction from the south to the north
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
Jia et al (2009) proposed that the entirely different crustal structures between the stable Yanshan uplift with high velocity in the north and the incompact rift basin (NCB in our study) with low velocity in the south make the transitional zone between the tectonic zoning line of the faults and Yanshan uplift provide a favourable structural environment occurrence of earthquakes in the eastern plain region (NCP in our study) of Zhangjiakou-Bohai belt (Tangshan region in our study), which is similar to the result of our analysis
Summary
The Tancheng-Lujing (Tanlu: illustrated in Fig. 1b) fault zone is considered as the deep faults that penetrate the Earth’s crust into the upper mantle (Wang et al 2000), and go straight through the Bohai sea into Liaoning Province along the NNE direction from the south to the north. The earthquake-based surface-wave tomography has a poor ability to obtain reliable constraints on crustal structure at shallow depths; the joint land-sea seismic survey is restricted by the locations of the profiles and fails to reflect the overall change of the Bohai sea’s deep structure. The theoretical work and applications of ANT have been published by Campillo and Paul (2003), Weaver and Lobkis (2004), Weaver (2005), Shapiro and Campillo (2004), Shapiro et al (2005), Roux et al (2005) and Sabra et al (2005) This method has been applied in the areas involved in our study area, the results only obtain local linear crustal velocity structures (Cheng et al 2011) or reflect major overall structures (e.g. Sun et al 2010; Zheng et al 2011). We inverted threedimensional (3-D) shear wave velocity structures from the surface down to 46 km based on the Rayleigh and Love wave velocities simultaneously and discuss their geological implications and relationship to the time-space distribution of earthquake swarms
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