| In this paper,we first study the velocity structure imaging of south China using ambient noise.The Yangtze and the Cathaysia Blocks collided to form the South China Craton.The suture zone in the southwestern region of the blocks remains controversial.Improving the definition of the boundary between these two blocks will help further characterize the regional tectonic framework and geologic history.We firstly obtained the 2D phase velocity of Rayleigh and Love waves at periods of 4 – 25 seconds using records of seismic ambient noise observed at 457 stations in South China from 2014 to 2016.The phase velocity maps at periods of 4 – 12 seconds are consistent with geological structures like basins and folds in the upper crust.In the phase velocity maps,at periods of 16 – 25 seconds,the eastern region mainly displays high-velocity anomalies related to its thinner crustal thickness.In the southwestern region,Rayleigh waves identify high-velocity anomalies,and Love waves identify significant low-velocity anomalies.This velocity anomaly difference implies the existence of radial anisotropy in these regions.Then the three-dimensional model of shear wave velocity and azimuthal anisotropy is generated by performing a direct inversion technique based on the dispersion curves of the Rayleigh wave at 4 – 40 s.Our model reveals a prominent area of strong azimuthal anisotropy in the crust between the Yangtze and Cathaysia Blocks.The fast polarization axes in this belt correspond to the NE-trending arc-shaped orogen between the two blocks.According to the apparent margins of this strongly anisotropic belt,the Yangtze-Cathaysia suture zone is delineated with the lines traversing: Shaoxing-Jiangshan-Pingxiang-Longsheng-Hechi,and Shitai-Jiujiang-Yueyang-Dayong-Jishou.In vertical section,deep crustal low-velocity anomalies distribute from this strongly anisotropic belt to the whole study region.From our analysis,and combined with geological observations of the widespread magmatism in the late Mesozoic in South China driven by the magma underplating and deep crustal melting,we hypothesize that the subducted Pacific plate's dehydration causes mantle upwelling and underplating of hot material to the basal crust beneath the suture zone.As a result,the deep crust is partially melted across a broad region.Ambient noise imaging can only get the static velocity structure,and investigating the dynamic changes of the subsurface medium is essential for comprehending the earthquake preparation and occurrence process.We study the temporal change of the subsurface medium in the 2021 Yangbi earthquake source area and surrounding areas.Using the vertical component continuous waveform data from June 2019 to January 2022 recorded by 36 stations located in the epicenter and surrounding areas,we obtained the time-dependent curve of the relative wave velocity of the regional subsurface medium.The results show that there are relative velocity decreases after the Yangbi earthquake observed in most stations whose paths pass through or close to the epicenter.There is generally no obvious coseismic velocity drop in the station pairs away from the epicenter area,and velocity increases were observed in a few station pairs.Then we calculated the average value of the relative velocity among all the stations and found that the average velocity of the hypocenter and surrounding underground medium decreased by 0.015% after the Yangbi earthquake,and after that,the average velocity rebounded briefly.In the half-year after the earthquake,the average velocity shows a downward trend,which may reflect the process of subsurface stress unloading caused by the Yangbi earthquake sequence. |
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