Seismic Anisotropy In The Upper Crust In The Intersection Of The Honghe Faults And Xiaojiang Faults

The Xiaojiang faults(XJF),striking north-to-south(NS),and the Honghe faults(HHF),striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.Seismic anisotropy is an important basis for investing the characteristics of deformation in the crust and mantle,migration of deep material,and the dynamic evolution of the lithosphere.When the S wave passes through the crustal anisotropic medium,the obtained splitting parameters can indicate the direction of regional stress and the pattern of complex structures.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array(HX Array)across first-order block boundaries in the southern SYB.By using the waveform data of local earthquakes recorded by temporary seismic stations in the HX Array from December 2017 to October 2019,and by permanent seismic stations of the China National Earthquake Networks from January 2012 to October 2019,this study adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain the seismic anisotropy in the upper crust in the intersection of the HHF and XJF.Combined with kinds of observation data,we investigate the relationship between the anisotropy and faults or stress,and further explore such scientific issues as pattern of faults,tectonic deformation,and dynamic evolution.The results show that the spatial distribution of the directions of polarization of the fast shear-wave(PFS)are obviously deflected at the station HX09 in the Yimen region,along the observation line of the HX Array,which are closely related to the local tectonic.The upper crustal anisotropy can be clearly divided into the east and west along the observation line.The east segment of the HX Array,across the south segment of XJF,the directions of the PFS beneath each station are highly consistency with its dominant directions mainly in the NE,which are inconsistent with the strikes of the faults and the direction of regional principal compressive stress,seem to be uninfluenced by the XJF.This implies that the influence of the SC on the SYB might have passed through the XJF and reached to the Yimen region.The west segment of the HX Array,across the HHF and Lancangjiang fault(LCJF),the dominant direction of the PFS in the NW,consistent with the direction of regional principal compressive stress and the strike of faults.On both sides of the HHF,the dominant direction of the PFS is nearly NW,we speculate the anisotropy in the upper crust is mainly controlled by the aligned minerals along the HHF,also affected by the stress.In the west of the LCJF,the dominant NW direction of the PFS may suggest the direction of the in-situ principal compressive stress.To the south of the HX Array,mainly around the intersection of the south segment of XJF and HHF,the SWS results reported by permanent stations are more complicated,with dominant in the NS direction,parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise,the data from Pms anisotropy analyzed by receiver function and XKS(SKS,SKKS and PKS)splitting,we infer that there might be multiple mechanisms in the crust and the upper mantle as well as in the upper and the lower crust.The west of LCJF,at least beneath the observation area,may have complex pattern of deformation,and the decoupling between the upper and the lower crusts as well as that between the crust and the mantle.Along the HHF and its vicinity,the crust may have formed uniform deformation,which has the characteristics of integration and consolidation,indicating that the HHF might have strong control on the surrounding medium.To the south segment and both sides of the XJF,the anisotropy is similar between the upper crust and the whole crust,and are different from the strike of faults and the direction of stress,but are consistent with the fast-wave azimuthal anisotropy of the lower crust.It is inferred that there may be layered anisotropy due to the severely influenced by the SC,forming the local stress field in the NE direction.Interestingly,the anisotropy of upper and whole crusts has good consistency in PFS orientations in a belt of zone at both sides of the HHF or the XJF.One reason for this phenomenon is that these two large faults could tectonically control wide range and influenced significantly on the anisotropic pattern in the crust.According to the pattern and structure of faults,this paper infers the anisotropy in the crust beneath the HHF and the XJF are controlled by NW and NE aligned cracks,respectively.These two large faults may be closed down or locked up,forming the uniform deformation in the study area.Due to the confliction between the Indian Plate and the TP,as well as the obstruction of the SC,the influence of the HHF and the XJF intersects in the Yimen region,resulting in the delimitation of anisotroic pattern and forming complex stress field.In other words,complex change of anisotropy is attributed to complicated pattern of tectonic and stress.This study demonstrates an effective way to study upper crustal anisotropy with dense temporary seismic array,and reveals the complex spatial distribution of stress and deformation in the intersection of the HHF and XJF.However,owing to the limited amount of data in publicly available earthquake catalogs and the temporary seismic array,the results reported here are preliminary,and we will continue to extract more data in future research.