Swave Splitting Of The Upper Crust In The Intersection Of Sanjiang Lateral Collision Zone And Lijiang-Xiaojinhe Fault

The Sanjiang lateral collision zone,located on the southeast margin of the Tibetan Plateau,is a key area for the study of the eastward flow of material from the Tibetan Plateau and the eastward insertion of the Indian plate.It has a complex geological structure,intense crustal deformation,frequent seismicity,and rich metal deposits.The intersection zone of the Sanjiang lateral collision zone and the Lijiang-Xiaojin River fault is a key window for studying the Tibetan Plateau’s tectonic structure and the Tethys Himalayan’s tectonic evolution.This region has a distribution of different focal mechanisms of strike-slip and normal,the crustal structure shows an inhomogeneous high-low velocity structure,and there may be lower crustal flows passing beneath the region,revealing a complex deep tectonic and medium deformation feature.Seismic anisotropy is widespread in the crust and upper mantle.The main source of anisotropy in the upper crust,apart from anisotropy caused by tectonics or lithology,is more commonly the structure of fracture-directed alignments under stress.S-wave splitting(SWS)occurs when propagating in an anisotropic medium,resulting in two mutually orthogonal S-wave components.This split S-wave carries a large amount of information about the medium through which the wave propagates.For stressinduced fracture-directed alignment anisotropic structures,information on the regional stress field and fracture structure can be obtained by SWS analysis.To better understand the complex lithospheric structure and strong deformation characteristics within the intersection of the Sanjiang lateral collision zone and the Lijiang-Xiaojinhe fault,this study used near-field seismic waveform data recorded by a temporary areal seismic array(SJ-Array)and permanent seismic stations from November 2018 to December 2020 and applied the SWS analysis technique to investigate the basic characteristics of crustal anisotropy in the intersection of Sanjiang lateral collision zone and Lijiang-Xiaojinhe fault,using earthquake events provided by the China Earthquake Network Center(hereinafter referred to as network catalog)Based on the dense temporary station array(SJ-Array)and the network catalog,this study obtained that in the intersection of the Sanjiang lateral collision zone and the Lijiang-Xiaojinhe fault,the dominant polarization of the fast S-wave is NNW,with a mean polarization orientation of 161.3 °.The zoning characteristic of fast S-wave polarization is apparent.It can be divided into three subzones from the west to the east: A,B,and C,with the mean polarization varying from NNW to NS and subsequently to NNE,in which subzone B is separated into two subzones: Bn and Bs.The normalized time delay of the slow S-wave in the study area is 3.4 ms/km,ranging from 1.9 to 5.2 ms/km at each station.The maximum time delay is on the east side of the large fault.Furthermore,there is a localized zone of strong anisotropy in the studied area.It suggests the local deformation differences are controlled by tectonics.Due to the SWS data in the upper crust must meet specific criteria.The amount of effective SWS data is always in shortage.Therefore,this paper used the Phase Net and REAL methods to construct a microseismic catalog and used VELEST and Hypo DD to do further seismic location to construct a high-resolution seismic catalog.The picked microseismic records are added to the SWS study to ensure the study of small-scale regional anisotropy.After microseismic pick-up and location,a high-resolution seismic catalog of the Sanjiang lateral collisional zone was constructed,and 2923 events were obtained,with a repositioning resolution better than 100 m.The distribution of seismic profiles after relative positioning shows that the hypocenters in the Sanjiang lateral collisional zone are generally shallow,mainly concentrated above 20 km,but there is a cluster of earthquakes at the intersection of profiles AA’ and DD’,with a depth of up to 40km;along the western boundary of the Chuandian block,the hypocenter range from north to south.Along the western boundary of the Chuandian block,the hypocenters become progressively shallower from north to south;on both sides of the Chuandian block boundary,the distribution of hypocenters shows a characteristic of deeper in the west and shallower in the east.The newly obtained high-precision relative localization of small seismic events based on deep learning pickup was used for SWS comparison analysis to obtain fast S-wave polarization directions and slow S-wave time delays for 17 stations,increasing the number of valid records by 228.The results of the SWS study show that the waveforms of seismic events missed by the network catalog are clear and valid.In comparison with the SWS results obtained from the network catalog,the SWS results obtained using the newly obtained(complementary)microseismic catalog show good agreement and complement the SWS parameters obtained based on the network catalog,especially by increasing the consistency of the dominant polarisation direction of the fast S-wave.