Seismic Anisotropy Of The Upper Crust On The Northeastern Margin Of The Tibetan Plateau

The northeastern margin of the Tibetan Plateau is situated at the intersection of the Tibetan,South China,Ordos,and Alxa blocks.This area serves as s forefront for material uplift and outward extension for the plateau.With its intricate geological structure and frequent strong earthquakes,it represents an essential region for studying both crustal uplift-expansion and deep dynamics associated with the Tibetan Plateau.To gain a better understanding of the lithospheric structure and deformation characteristics in the northeastern margin of the Tibetan Plateau and its adjacent areas,this study utilizes near-field microseismic waveform data recorded by permanent networks in Gansu,Ningxia,Shaanxi and Sichuan(form October 2018 to November2021),as well as temporary seismic array deployed by the China Earthquake Administration’s Institute of Earthquake Forecasting along Haiyuan fault zone(from November 2012 to October 2014).By employing shear wave splitting analysis methods on small events form earthquake catalogs provided China Seismic Network Center and microseismic events picked up by temporary seismic arrays,we investigate the basic anisotropic features within upper crust for these regions.In this study,a total of 78 seismic stations(55 permanent and 23 temporary)were obtained,yielding 1490 valid records for fast-wave polarization directions and time delays.The primary fast-wave polarization directions within the study area are exhibiting distinct zoning features.This study area can be divided into five regions with the Haiyuan fault zone and its northern side of Yinchuan rift basin further subdivided into two subregions.Surrounding the Yinchuan rift basin,the fast shear-wave polarizations align with the NE direction and show better consistency with the structural stress field orientation.The fast shear-wave polarizations directions of the station near the Haiyuan fault zone,is roughly parallel to the fault’s strike.On the eastern side of Tibetan block,the fast-wave polarizations are NW-oriented,aligning with principal stress directions and geodetic data revealing structural stress directions.At northern Longmenshan fault zone stations exhibit both NW and ENE alignments polarizations due to combined effects from regional tectonic stress fields and faults.Adjacent to Qinling orogenic belt,stations exhibit near east-west oriented fast shear-wave polarizations that coincide with seismogenic mechanisms revealing structural stress directions.Along the periphery of Weihe basin,stations present fast-wave polarization orientations in NE direction consistent with primary compressional stress directions in North China Block.For the temporary seismic station data,by identifying microseismic events,the effective shear wave analysis data added is 1.6 times greater than that of valid earthquake catalog event data.The analysis reveals that fast-wave polarization directions in temporary arrays exhibit a northwest orientation,which aligns with the Haiyuan fault’s trend.This demonstrates the controlling role of the Haiyuan fault on anisotropic directionality at nearby stations.The permanent station time delays obtained in this study range from 0.9 to 5.6ms/km,with an average value of 2.0 ms/km,while the average normalized time delay for temporary seismic array stations ranges between 1.34 and 4.52 ms/km,averaging at2.35 ms/km.Among the five regions(A-E),region A has the largest average time delay;regions C,D,and E have comparable time delays;region B is similar to region A in terms of time delay but has a relatively larger As zone.Strong anisotropy areas are observed along tectonic boundaries that reveal structural variations.The western and southern edges of Ordos Block exhibit strong anisotropy due to their obstruction by stable Ordos Block against northeastern extension from Tibetan Plateau,resulting in significant crustal deformation within these areas.Eastern Kunlun fault stations display prominent anisotropic characteristics closely related to regional tectonic stresses.In the Qilian-Hexi Corridor area,stronger anisotropy indicates substantial crustal deformation caused by collision and compression between Qilian block and Alashan block.The intersection zone of Xianshuihe fault and Ganzi-Yushu fault also demonstrates heightened levels of anisotropy intensity which highlights its close relationship with active regional structures.The fast-wave polarization directions obtained from permanent stations and temporary seismic arrays in this study reveal noticeable differences on both sides of the Haiyuan fault zone.In conjunction with existing regional geophysical research results,our findings support the notion that the Haiyuan fault zone serves as a boundary for north-eastward crustal extension within the Tibetan Plateau.