| The Sichuan-Yunnan block(SYB)is a typical tectonic unit on the SE margin of the Tibetan Plateau(TP),through which material from the TP migrates as a result of the collision of the Indian and Eurasian plates and the blocking of the South China block.The SYB hosts complex geological tectonics,the surface is undulating violently,and strong earthquakes often occur in and around the area.Fault zones in the central part of the SYB cut and intersect each other to form a network of faults,and the 2021 Yangbi Ms6.4 earthquake occurred on the western side of the SYB.Previous studies have shown that there are obvious north-south differences in the deep structure of the SYB,and the Lijiang-Xiaojinhe fault(LXF)is in the transition area of the north-south variation,which has a certain conversion and absorption effect on tectonic morphology and stress-strain.Seismic anisotropy can be used to study the deformation mechanism of the lithosphere and analyse scientific problems such as deep complex structures and dynamical mechanisms.We take the central part of the SYB as the research object and conduct a regional seismic anisotropy study,expecting to obtain more accurate and finer images of crustal seismic anisotropy,thus providing better seismological constraints for the study of regional deep material movement and dynamics patterns.In this paper,31 permanent seismic stations of the China Seismological Network(CSN)located in the study area were used to select three-component data of teleseismic earthquakes with magnitude greater than 5.5 and epicentral distance of 30°-90° to study the P-wave receiver function anisotropy,and we successfully obtain the crustal anisotropy images in the study area.The results show that the crustal thickness in the region varies dramatically,decreasing from north to south,with a significant shift at about 26°N.The results are in good agreement with the topographic relief of the region and the Bouguer gravity anomaly.We infer that the abrupt thickness shift in the northsouth direction in the region is related to deep physical changes near the LXF zone,and that partial melting of material in the southern middle and lower crust may have caused the region to show lateral east-west variability.In general,the results of the crustal mean wave Vp/Vs ratio(Poisson’s ratio)show a clear north-south lateral split in the study area in general.While in the region north of 26°N,the crustal mean Vp/Vs ratio has a clear east-west lateral split.The high wave velocity ratios in the southern part of the LXF zone and the western part of the Anninghe-Zemuhe fault zone correspond to the results of the low velocity zone and high conduction zone obtained from previous studies,indicating that melt or partial melt possibly exists in this region,which may be a channel for the lower crustal flow of material on the TP.Analysis of the comparative results reveals no significant correlation between the Vp/Vs ratio and crustal thickness beneath this region,thus inferring that the variation in Vp/Vs ratio(Poisson’s ratio)in this region may be caused by the mineralogical composition of the rocks and partial melting.The Pms splitting results show that the direction of fast wave polarization in the study area is heterogeneous and may be influenced by multiple factors such as geological structure and crustal material flow.The anisotropy of the crust in the study area is mainly due to the eastward migration of material from the TP,and the differential movement of the brittle upper crust and the ductile middle and lower crust,causing the orientation of the mineral material,resulting in a complex distribution of regional anisotropy.The results of fast-wave polarization directions show that the northern region Pms is consistent with the near-field S-wave polarization directions of the upper crust at 26°N,while the southern region Pms deviates from the near-field S-wave polarization directions of the upper crust.The time difference of the teleseismic XKS(a collective term for SKS,PKS and SKKS)wave splitting is much larger than the Pms wave splitting time,indicating that the anisotropy or medium deformation of the upper mantle is very strong.By comparing the results of previous studies,it can be seen that the heat source causing the violent deformation of the upper mantle may come from the upper mantle.In addition,with the boundary of 26°N.The Pms and XKS fast-wave polarization directions in the northern region are consistent,while there are differences in the Pms and XKS fast-wave polarization directions in the southern region.Based on the different polarization directions in the southern region,we speculate that the crust is decoupled from the mantle deformation.In order to understand the regional upper crustal anisotropy characteristics,Swave splitting analysis of the region near the LXF zone was carried out to study the upper crustal anisotropy characteristics using near-field small earthquake waveform data from 20 transportable stations of the linear mobile seismic array(MCD Array)in the central SYB.As shear wave splitting analysis has strict data screening requirements,this paper uses the REAL method of microseismic identification to identify more earthquakes to update the small earthquake catalogue.The results show that the number of earthquakes obtained by the REAL method is about 2.3 times the number of earthquakes published by the National Earthquake Data Center.The results show that the number of earthquakes with magnitudes of 1-2 and depths of 10-20 km is about 4.2and 4.9 times higher than that of the data centre and that microseismic recognition is good in the area covered by the table array.Shear-wave splitting analysis using the updated seismic catalogue reveals that the fast-wave polarization direction gradually deflects from near-NNW to near-ENE in the study area from west to east,revealing changes in the main compressive stress field.We suggest that the fast-wave polarization direction is controlled by the surrounding fractures and is roughly parallel to the fracture orientation.The anisotropic features of the upper crust are zoned,with good intraregional consistency,with two fast-wave dominant directions within the junction of the Zhongdian fault and Chenghai fault,which may be caused by local tensioning in the region.In the study area,the slow wave time delay generally increases from west to east,roughly bounded by the Zhongdian fault and the Chenghai fault,and is also zoned.The anisotropy of the upper crust in the study area is consistent with the GPS observations of surface deformation and principal compressive stresses,indicating that the anisotropy of the upper crust is strongly influenced by regional tectonics.This paper shows that there is a clear local zoning(or segmentation)of the whole crust anisotropy and upper crust anisotropy in the study area,which is correlated with tectonic morphology,material flow and deep dynamics.As the amount of data used is still limited,the results and understanding obtained in this paper are preliminary,and further experimental studies and comparative analyses are still needed to gain a deeper understanding of the study area. |
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