| The Dangjiang fault is an important part of the southern boundary zone of the Bayan Har block and a seismogenic structure for strong earthquakes,which is also the transformation connecting structure between the western end of the Ganzi-Yushu fault and the Fenghuoshan fault.Its strongly late Quaternary tectonic deformation has played an important role in regulating and absorbing the internal deformation of the Tibet Plateau.However,due to the limitation of working conditions,the research foundation of the late Quaternary tectonic deformation of the Dangjiang fault is relatively poor,which makes us hard to reasonably evaluate its future strong earthquake risk and the strain on the southern boundary zone of the Bayan Har block and clearly to understanding of the geodynamic processes of allocation and tectonic transformation.This paper takes the investigation of the"surface rupture zone of the Dangjiang fault in CE 1738"as the starting point,with the help of historical books and textual research and interpretation of high-resolution remote sensing images.micro-geomorphic analysis of faults,large-scale geological and landform mapping,high-definition aerial photography of unmanned aerial vehicles and data modeling analysis,trenching and excavation of surface faults,Paleo-earthquake identification,sampling and processing of 14C and luminescence samples,etc,were be used to field geological survey in Dangjiang fault.focus on the tectonic deformation characteristics and geomorphic response of the Dangjiang fault since the late Quaternary.The main understandings are as follows:(1)The coseismic surface rupture characteristics of the difficult historical earthquake of CE 1738 on the Dangjiang fault were be investigated for the first time by means of unmanned aerial vehicles photography,trench excavation and large-scale mapping.historical records show that a strong historical earthquake occurred in the Dangjiang area,northwest of Yushu,Qinghai in CE 1738,which caused the death of about 260 people and had a great impact.However,limited by conditions such as high-altitude,oxygen lack and inconvenient transportation,the research on its coseismic deformation characteristics and main parameters has controversial.The earliest historical data set the macro-epicenter of this historical earthquake near Yushu,Qinghai,with an estimated magnitude of M6?.The results of this study show that the macro-epicenter of the earthquake is located in the area from Wanqing Village to Dangjiang Rong village in Zhiduo county,and a coseismic surface rupture zone with a length of about 120 km was formed in the area between Dangjiang County and Yushu County.The maximum left-handed coseismic displacement is about 3.6±0.1 m,the average left-handed coseismic displacement is about 2.7±0.1 m,and the maximum vertical coseismic displacement is 0.9±0.1 m.A series of left-lateral strike-slip fault fault triangles,river terraces,shutter ridges,fault steep ridges,fault rock ridges and gullies developed along the rupture zone.based on the analogy of earthquake cases,comprehensive analysis of the empirical formula of coseismic surface rupture length and maximum coseismic displacement,it is estimated that the magnitude of this earthquake can reach M 7?.(2)The geometric and kinematic characteristics of the tectonic deformation of the Dangjiang fault since the late Quaternary were studied based on field geological and geomorphological surveys.The Dangjiang fault has many geomorphological features such as ditch,gate ridge,gullies,terraces and alluvial-proluvial fans,and geological bodies and geomorphological units of different scales have recorded evidence of strong earthquake surface ruptures with multiple dislocations,which indicate that the Dangjiang fault has experienced multiple left-handed repeated rupture events of strong earthquakes during the Quaternary period.In this study,the left-lateral displacement and the corresponding chronological data of three typical different geomorphological points of the Dangjiang fault were selected at the same time.It is believed that the left-lateral slip rate of the Dangjiang fault since the Holocene is about 7.72±1.15 mm·(6-1;According to the related research results and linear fitting,the left-lateral slip rate of the fault since the late Pleistocene is about6.72±0.78 mm·(6-1(R~2=0.962).(3)Based on geomorphic units and geological bodies of different scales,the geomorphological response to the late Quaternary tectonic deformation of the Dangjiang fault was discussed from three different time scales:short,medium and long term.On a short time scale,the Dangjiang fault was affected by a series of strong earthquakes in the Holocene,and micro-topography such as left-handed gullies,terraced ridges,and fault water systems were formed on the surface,with an average single left-handed displacement of about 2.7±0.1 m.On the medium time scale,through the interpretation and mapping of DEM data with a resolution of 12.5 m in the Dangjiang area and high-definition aerial photography modeling data of unmanned aerial vehicles,it is revealed that the maximum accumulated dislocation of the left-lateral gullies of the Dangjiang fault can reach 280 m.,indicating that the left-lateral strike-slip of the Dangjiang fault since the middle of the Late Pleistocene has formed a cumulative displacement of hundreds of meters.From a longer time scale,the three first-order tributaries of Tongtianhe passing through the Dangjiang fault were affected by the left-lateral activity of the Dangjiang fault,and the displacements from west to east were?5 km,?16 km,and?6 km,respectively.It indicates that the Dangjiang fault has experienced long-term repetitive left-lateral strike-slip activity at least since the early Quaternary(?1.0 Ma).(4)Based on the strong earthquake recurrence interval of the Dangjiang fault,its seismic activity potential and the strong earthquake risk of the southern boundary of the Bayan Har block were be discussed.In the past three decades,at least 10destructive earthquakes of magnitude M 7 or above have occurred in the boundary zone of the Bayan Har block.It is considered to be the most seismically active and most dangerous area in mainland china,and its future strong earthquake risk should be quite highly attention.According to the maximum horizontal coseismic displacement of 3.6±0.1 m and the Holocene slip rate of 7.72±1.15 mm·(6-1 obtained from the investigation of the surface rupture zone of the Dangjiang fault,the estimated recurrence interval is about 466±84 a.At the same time,combined with the previous estimates on the recurrence interval of strong earthquakes on the fault of356±41 a and 290±24 a.It's considered that the elapsed time(284 years)after the rupture of the M 7?strong earthquake on the Dangjiang fault in 1738 is basically close to its recurrence interval.Based on the analysis of the existing earthquake migration model and strong earthquake recurrence interval analysis,there may be a potential strong earthquake rupture gap in the Dangjiang fault,and the risk of strong earthquake rupture in the next ten years or a little longer is very high.The danger of strong earthquakes at the southern border of the body cannot be underestimated.The results of this paper are expected to more comprehensively and quantitatively analyze the tectonic deformation characteristics of the Dangjiang fault during the late Quaternary period and evaluate its strong earthquake risk,and at the same time help to understand the strain distribution and tectonic transformation of the southern boundary zone of the Bayan Har block.and discuss its role and mechanism in absorbing and regulating the internal deformation of the Tibet Plateau. |
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