Observations Of Shear-wave Splitting On Tengchong Volcanic Area,Yunnan

The unstable state of underground magma bodies or volcanic eruptions have a major impact on continental crust evolution and society.Magma intruding into the crust and even ejecting from the surface will be affected by the stress of the crust to a certain extent,and the magma activity will in turn affect the stress state of the crust?The Tengchong volcanic area is located on the southeastern margin of the Tibet Plateau.It is one of the youngest active volcanic areas in China and an ideal area to study the interaction between magmatic activity and crustal stress.In this study,the seismic anisotropy measured by shear wave splitting analysis is used as a tool to study the stress state of the upper crust in the Tengchong volcanic area.These results are of great significance in understanding the interaction between magmatic activity and stress.In this study,using the local seismic data recorded by 15 fixed and temporary seismic stations in the Tengchong volcanic area from 2008.10 to 2019.7,the shear wave splitting analysis was performed using the MFAST method,and we obtained the anisotropic shear wave splitting parameters of middle and upper crust in this area finally.In addition,we used the local seismic travel time recorded by 10 fixed stations around Tengchong volcano,the Vp/Vs value of the study area was 1.68.This study used the data of 8 volcanic stations and 5 temporary stations in the Tengchong volcanic area for the first time,compared with the existing results of local seismic anisotropy(Zhang et al.,2018;Shi et al.,2015),revealed the lateral variation characteristics of the anisotropy of the upper and lower crust in the Tengchong volcanic area more clearly.Studies have shown that the main contribution of crustal anisotropy comes from the middle and upper crust,and the fast wave polarization directions of different stations in the study area vary greatly,which seems to reflect the combined effect of tectonic and regional stress fields.By comparing the polarization direction,stress direction and fault direction of the fast wave obtained by the splitting analysis,we deduced that the overall fast wave polarization direction in the west side of the Tengchong volcanic fault zone with strong seismic activity is near the N-S direction,while the overall fast-wave polarization direction of the east side region is NE-SW,which is basically the same as the direction of regional compressive stress given by Zhao et al.,(2013),suggesting that the anisotropy of the upper and lower crust in the study area is mainly subject to compressive stress caused by directional cracks.Combined with the low crustal wave velocity ratio in the study area,it can be speculated that the local variations in the crustal stress field in the study area may be caused by the expansion of the gas-rich intermediate acid magma in the upper crust.On the other hand,at some stations outside the Tengchong volcanic area,the phenomenon that the polarization direction of the fast wave is inconsistent with the main compressive stress,known faults and other tectonic trends is observed,suggesting that its anisotropy is the result of the interaction of the structure or the structure and the regional stress field.In addition to this,the teleseismic waveform data recorded by 10 fixed stations in Tengchong volcanic area in Yunnan were also splitting analyzed by Splitlab method.Although this study did not obtain effective splitting parameters due to the influence of waveform data quality and other factors,the research results show that the back azimuths of these invalid seismic events are concentrated in the SEE-NWW and SSW directions,which are consistent/vertical with the fast wave polarization direction of the study area given by the predecessors,proving other scholars' s upper mantle study in this area,that is,the polarization direction of the fast mantle in the upper mantle of the Tengchong volcanic area is NWW-SEE.