Crustal Thickness And Vp/Vs Ratio In Sichuan-Yunnan Region And The Middle-southern Segment Of Tan-Lu Fault Zone

The Sichuan-Yunnan region in southwest China is located on the southeastern margin of the Tibetan Plateau.There is High seismicity and serious geological disasters in this region,due to the influence of the collision between the Indian and Eurasian plates.SWChinaCVM-1.0 is the first version of a regional community velocity model for the Sichuan-Yunnan region and aims to provide a reference for seismological research in the region.Based on this model,a high-precision community Moho interface model can be obtained to further expand the applicability of SWChinaCVM-1.0.The middle-southern segment of the Tan-Lu fault zone(TLFZ)is related to the subduction of the west Pacific plate and the destruction of the North China Craton.The study of deep structure in the middle-southern segment of the TLFZ is of great significance for understanding the tectonic evolution of the TLFZ and the seismogenic mechanism of strong earthquakes.Therefore,the crustal thickness and velocity ratio can provide importance constraints for further understanding the tectonic evolution process in the two research regions.In this paper,we collected data from more than 1200 seismic stations in the two research regions and used receiver function method.The results and understanding are as follows:1)Based on SWChinaCVM-1.0,a high-precision community Moho interface model can be obtained using common conversion point(CCP)stacking.Before CCP stacking,it is necessary to check the fit of the practical and theoretical travel times of the Ps phase and its multiples under the constraints of the SWChinaCVM-1.0.Using this method,referred to as H-κ0 stacking,we determined the Moho interface.However,the multiples were poorly fitted with the theoretical travel times at many stations,and the crustal thickness obtained was inconsistent with previous findings.Therefore,we speculate that the SWChinaCVM-1.0 model has some limitations that render it incompatible with the CCP stacking method.2)To further verify this hypothesis,we performed H-κ stacking using the Vp-Vs model,Vp model,Vs model,and the regional average velocity Vp0 and compared the results with those of H-κ0 stacking.The results of the comparison indicate that the SWChinaCVM-1.0 model requires improvements.In addition,by analyzing the SWChinaCVM-1.0 inversion process,we found that the Vs model was more reliable than the Vp model.3)We adopted the crustal thickness obtained using H-κ stacking based on the Vs model as the initial version of a community Moho interface model for the SichuanYunnan region(SWChinaCVM-MOHO-1.0).Results show that the crustal thicknesses varied widely throughout the study area,ranged from 25 to 70 km,and exhibited a general trend of thicker crust in the northwest and thinner crust in the southeast.The results of crustal thickness and Vp/Vs ratio based on Vs model are consistent with those of previous studies.4)We collected data from a total of 233 seismic stations in the middle-southern segment of the Tan-Lu fault zone(TLFZ)and removed the reverberation caused by the sedimentary layer.The crustal thickness and Poisson’s ratio in this research region are obtained using H-κ stacking method.The results show that the distribution of crustal thickness is divided into three segments with Lujiang and Xinyi as the subsection points.The segmentation characteristics in the north-south direction are consistent with that of the geological structure.In the southern segment,the crustal thickness on the west side of the TLFZ is about 5 km thicker than that on the east side.In the middle segment,the thickness difference between the two sides of the TLFZ decreases to about 3 km.In the northern segment,the crustal thickness is characterized by a "thick-thin-thick" and the Moho interface is uplifted beneath the TLFZ.5)According to the Airy isostasy theory,we concluded that:In the southern segment,the crustal thickness is related to the elevation,which largely conforms to the Airy isostasy model.In the middle segment,the elevation is the same,but the crustal thickness is different on both sides of the TLFZ,which indicates the deep structure does not conform to the Airy isostasy model.This result is related to the higher density lithosphere of the North China Craton on the west side of the TLFZ than that of the Yangtze Craton on the east side.Because high-density lithosphere can produce a downward "drag force"which leads to a deeper Moho interface than theoretical.In the northern segment,the Moho interface is deeper than theoretical Moho interface.The deep structure of this area is similar to that of the west side of TLFZ in the middle segment,which is belonging to the North China Craton.6)Within the rupture zone of the Tancheng M 8.5 earthquake,there are three types of structural features:uneven transition of Moho interface,the boundary of the active block,bifurcation and intersection of faults.These structural features make it easier to concentrate stress,thus producing the M 8.5 earthquake in Tancheng in 1668.