Fault Behavior And Seismic Hazard Of The Xianshuihe Fault Zone In Eastern Tibetan Plateau

The Xianshuihe Fault Zone is a significant fault zone in the eastern margin of the Tibetan Plateau that exhibits considerable differential movement and intense tectonic deformation.Serving as the boundary between the Bayan Har Block and the Sichuan-Yunnan Block,it regulates their movements and deformations and may participate in the seismic activities within the Bayan Har seismic cluster.This fault structure is prominent,characterized by high slip rates,frequent and intense seismic activities,and a relatively dense population in neighboring areas.Moreover,it is an active fault zone traversed by the famous Sichuan-Tibet Highway(Route 317 and 318)and the Sichuan-Tibet Railway,making it one of the most seismically active fault zones of significant concern for studying the seismic potential and seismic hazard of large fault zones within mainland China.The northwestern segment of the Xianshuihe Fault Zone is a rare example of an intercontinental fault that exhibits multiple slip behaviors,including high-magnitude earthquakes with co-seismic ruptures and aseismic slip,along with unique features such as termination of rupture on relatively straight fault traces.Understanding the fault behavior of this segment is crucial for analyzing the seismic potential and future seismic hazards of large earthquakes.Additionally,the Sichuan-Yunnan region plays a pivotal role in studying the theory of eastward material escape from the Tibetan Plateau.And the Xianshuihe Fault Zone serves as an ideal natural laboratory for investigating deep-seated dynamics of the plateau,mechanisms of strong continental earthquakes,deformation patterns of the Tibetan Plateau,rheological characteristics of the lower crust,and their relationship with earthquakes.However,fault movements and deformations exhibit distinct characteristics in different time and spatial scales.A single set of observational data and methods is insufficient to describe fault behavior comprehensively and can only depict features of a specific region and time period.To fully characterize the behavior of the Xianshuihe Fault Zone from multiple dimensions,it is necessary to use data and methods from various temporal and spatial scales,focus on multiple earthquake stages including co-seismic,post-seismic and inter-seismic periods,consider the behavior of the fault on the surface,within fault planes,and in the deep.Therefore,this paper takes the Xianshuihe Fault Zone as the research object,based on studies of paleoseismic and historical earthquakes,constructs the spatiotemporal distribution of large earthquake ruptures,analyzes the fault movements and stress states on the fault plane,and studies the relationship between deep rheological structures and earthquakes.Starting from the northwestern segment of the Xianshuihe Fault Zone,which exhibits different fault behaviors such as large earthquake ruptures and aseismic slip,the paper explores the impacts of fault behavior on rupture,propagation,and termination of large earthquakes,evaluates the seismogenic capacity of the Xianshuihe Fault Zone,and further analyzes fault behavior and seismic hazard from different temporal and spatial scales.The paper’s specific research contents and main findings are as follows:(1)The spatiotemporal distribution of large earthquake ruptures T.Optical image data and airborne Li DAR data were used for research on the 1973 Luhuo M7.6earthquake that occurred in a geometrically simple section and the 1955 Zhaduotang M7.5 earthquake that occurred in a geometrically complex parallel branch section of the Xianshuihe Fault Zone,both of which have well-preserved surface ruptures.High-resolution image interpretation and detailed field investigations were combined to finely depict the geometric patterns and displacement characteristics of the co-seismic surface ruptures of these two historical earthquakes.The historical earthquake surface rupture database was expanded to provide a basis for understanding seismic rupture processes and analyzing earthquake disasters.Excavation of paleoseismic trenches was carried out in the Luhuo and Bamei sections of the Xianshuihe Fault Zone,including two sets of trenches at Kemu(T1,T2)and Longdeng(T3).Work included interpretation of trenches,sample collection,dating,and identification of paleoseismic events.The Kemu trench group revealed 5-7 events,while the Longdeng trench revealed three events.Based on a thorough collection and collation of existing historical and paleoseismic research results,a spatial distribution model of large earthquakes on the Xianshuihe Fault Zone was constructed.(2)Building upon research into the spatiotemporal distribution of large earthquake ruptures,co-seismic displacement,fault segmentation models,and a layered viscoelastic medium model were established at the eastern boundary of the Sichuan-Yunnan rhombic block.We further calculated Coulomb stress evolution over time caused by co-seismic dislocations,post-seismic viscous relaxation,and interseismic tectonic loading on the fault surfaces of the Xianshuihe,Anninghe,Zemuhe,and Xiaojiang fault zones.The results indicated that the seismic hazard of the Bamei,Selaha,Kangding,and Moxi segments of the Xianshuihe fault should be given attention.A continuous GPS observation profile consisting of five stations was established across the aseismic segment to analyze the fault’s deformation characteristics and seismic hazards using data from multiple sources such as GPS and In SAR.The results based on In SAR data from 2014 to 2019 show that slip rates on different segments of the Xianshuihe Fault Zone varied greatly.And between Daofu and Bamei,Kangding,and Moxi segments,the degree of locking is the highest,and the deepest locking depth could reach up to 22 km.(3)This paper provides the distribution of viscosity coefficients at different depths,varying profiles across the eastern boundary of the Sichuan-Yunnan block,and various observation points located in different structural positions.In addition,a profile of viscous structures along the eastern boundary of the Sichuan-Yunnan rhombic block was extracted.The results indicate that the lower crust of the Xianshuihe Fault Zone is weaker than that of the Anninghe,Zemuhe,and Xiaojiang faults.(4)Based on the In SAR interseismic(2014-2020)and coseismic deformation fields of the Luding earthquake with a magnitude of 6.8 in 2022,this paper estimates the interseismic fault slip rate,locking depth,and coseismic slip distribution.Results show apparent shear deformation features on both sides of the Moxi fault,with strain more concentrated in the southern segment than in the northern segment.The Moxi fault showed obvious locking before the earthquake.And the Luding earthquake was a high-angle strike-slip earthquake with predominantly left-lateral movement,with a maximum slip of 1.71 meters at a depth of 10 kilometers.Fault stress study results indicate that this earthquake did not ultimately reduce the earthquake risk of the Xianshuihe Fault Zone,causing a significant Coulomb stress increase in the northern segments of the Moxi fault,Anninghe fault,and Daliangshan fault,warranting attention.(5)Based on different spatial-temporal scale data and methods,this paper comprehensively analyzes seismic hazards of the Xianshuihe Fault Zone from the spatial,temporal,and intensity perspective of major earthquakes occurring.Considering the results of stress evolution in the earthquake cycle and locking of fault plane obtained by geodetic data inversion,the seismic hazard of the Bamei,Kangding,and Moxi segments of the Xianshuihe Fault Zone needs special attention.Considering stress disturbance caused by large earthquakes in surrounding areas,the quantitatively corrected probability of large earthquake occurrence shows higher probabilities for the Bamei,Yalahe,Selaha,Kangding,and Moxi segments of the Xianshuihe Fault Zone.This paper attempts to simulate the possibility of an aseismic slip section and a large earthquake rupture section of the northwest area of the Xianshuihe Fault Zone being involved in earthquake rupture through numerical simulation.Dynamic rupture process simulations show there is a possibility of two sections cascade rupturing in one earthquake when the initial stresses of the Luhuo fault and Daofu fault are both high.If a cascade rupture occurs in the northwest section of the Xianshuihe Fault Zone,the maximum magnitude may exceed M_w7.9,exceeding the understanding of its seismogenic capacity from historical earthquake research.