Crustal Structure And Azimuthal Anisotropy Across Haiyuan Fault Zone And Its Adjacent Area Derived From Ambient Noise Tomography

Haiyuan fault zone is a large NW-trending boundary fault zone located on the northeastern margin of the Tibetan Plateau.The fault zone is a thrust and left-lateral strikeslip activity combined by a number of secondary faults of different inclinations and the pullapart basins in between.fracture zone.The Haiyuan fault zone and its adjacent areas have complex geological structures.The main geological tectonic units include the Alxa block,the Ordos block,the northeastern Qinghai-Tibet block,the Yinchuan graben,the Qilian orogenic belt,and the West Qinling orogenic belt.In order to explore the deep tectonic deformation of the northeastern margin of the Qinghai-Tibet Plateau and the northeastern migration pattern of Qinghai-Tibet materials,to understand the role of the Haiyuan fault zone in the tectonic movement of the northeastern margin of the Tibetan Plateau,and to provide more seismological insights into the deep tectonic mechanism of large earthquakes.In this study,the continuous records of the regional mobile seismic array and the regional fixed seismic network are used,and the background noise inversion technology is used to carry out research on the crustal velocity structure and anisotropy of the Haiyuan fault zone and its adjacent areas.The data used in this paper come from the following two parts:(1)the continuous waveform data from December 2012 to June 2020 recorded by 21 fixed stations of the Seismic Network in Gansu Province and Ningxia Hui Autonomous Region;(2)China Earthquake The continuous waveform data from December 2012 to October 2014 recorded by 40 stations of the Trans-Haiyuan Fault Zone Mobile Seismic Array(SACHY-Array)set up by the Institute of Earthquake Prediction of the Bureau.Using Ambient noise cross-correlation method,the surface wave phase velocity dispersion curve was extracted,and the Rayleigh wave phase velocity azimuthal anisotropy and the three-dimensional S-wave velocity azimuthal Anisotropic image with a period range of 5-30 s and a resolution of 1°×1° were obtained by inversion.Through the ambient noise imaging of the Haiyuan fault zone and its adjacent areas,combined with the comprehensive analysis of regional geological and geophysical data,this study has obtained the following main results and research understandings:(1)The phase velocity and S-wave velocity of the Haiyuan fault zone are both high and low velocity transition zones.The Hexi Corridor transition zone on the north side of the fault zone shows low velocity anomalies,while the Qilian fold orogenic belt on the south side shows high velocity anomalies.The crustal structure of the Haiyuan fault zone and adjacent areas is complex,and there may be crustal damage in local areas;there are sedimentary layers from the Hexi Corridor to the southern part of the Yinchuan graben and the northwestern part of the Ordos block,showing obvious low-velocity anomalies in the upper crust.Below the Yantongshan fault,the S-wave low-velocity body structure is distributed over 15 km.The fault may be a deep fault in the upper crust.It is speculated that this area may be the front of the NE-trending compression of the Tibetan Plateau.(2)The azimuthal anisotropy of the crust is an important indicator for constraining the process of crustal deformation.The azimuthal anisotropy of the upper crust in the Haiyuan fault zone and its adjacent areas is mainly affected by the orientation of fractures and the distribution of fault zones caused by stress.The two sides of the Haiyuan fault zone near the Ganyanchi Pond are obviously different below 10 km;the south of the Yinchuan graben presents an anisotropic fast wave direction parallel to the graben strike,which is consistent with the extensional structural strike of the graben,which may indicate its the crust is mainly subjected to compression in the north-east direction.The direction of fast-wave anisotropy in the northwest of the Ordos block is ENE,which may be due to the combined action of the NW-SE expansion of the graben and the NE compression of the plateau.(3)The regional S-wave velocity distribution results tend to support the deformation model of upper crustal thickening in the Haiyuan arc structure region in the northeastern margin of the Tibetan Plateau.The difference in azimuthal anisotropy shape distribution indicates that the regional deformation mechanism may be mainly caused by the northeastern margin of the Tibetan Plateau.It is caused by the shear deformation of the crust;research shows that the north-east pushing of the Tibetan Plateau is still the main dynamic source in this area.