Studies On Structure,Geomorphology,and Seismology Characteristics Along The Xianshuihe Fault Zone In Kangding

The Xianshuihe Fault zone on the eastern edge of the Tibetan Plateau stands as a large left-lateral strike-slip orogenic belt which features frequent occurrences of intense and large-amplitude earthquake activities.Especially in the Kangding region where the Xianshuihe Fault branch out into the Yalahe,Selaha and Zheduotang faults,the exposure of large-scale Miocene granites indicates frequent geothermal and tectonic activities（e.g.,the 1955 Zheduotang Earthquake（Ms 7.5）and the 2014 Kangding Earthquake（Ms 6.3））.Additionally,the Sichuan-Tibet railway currently under construction has been planned to go through this region,which further motivated the urgent need to understand the regional tectonics,geomorphological characteristics and the earthquake activities in the Kangding region.On the bases of interpretation of high-resolution remote sensing images and DEM data analysis,we selected the Zheduotang Fault zone in the Kangding region as a key research target and,（1）conducted field geological investigation and refined geomorphological mapping,（2）reconstructed the evolutionary history of the terrace structures of the Zheduotang rivers and recognized phases in response to the Zheduotang faulting system,（3）selected a site for exploratory trench and,by the employment of paleoseismological techniques and newly developed chronological methods,reconstructed the sequential events of paleo-earthquakes and their recurrence periods,（4）conducted seismic waveform inversion modelling in attempts to explain the rupture mechanisms in the Zheduotang area which will enhance our understanding in the coupling relationships between the tectonics,geomorphological characteristics and paleo-earthquakes in the Kangding region as well as provide additional bases for future seismic risk assessment and studies on the evolution of tectonic landforms.The results and conclusions of this thesis are detailed as follows:（1）The analysis on the landform distribution suggests a controlling role of the Xianshuihe Fault system on the regional geomorphological characteristics and river development:in the Yalahe fault zone where the slopes are distinctly high and steep,the major controlling tectonic forces are compression and uplift;due to the decreasing vertical component of faulting,the slopes decreases in the Selaha Fault zone and the northwest part of the Zheduotang Fault zone;on both the southwestern side（Gongga shan area）and the northeastern side（Siguniangshan area）of the Moxi Fault zone in the south of Kangding,the slopes change rapidly in space,indicating intense differential uplifting;the distributions and courses of the rivers in the studied region are controlled by both the landform distributions and tectonic movements.（2）At least 5 river terraces have been recognized during the mapping of the tectonic landforms in the Zheduotang area and by detailed analyses on their distribution,structure and genesis,we show that the formation of the terraces as well as the evolution of the Zheduohe river is strongly controlled by the left-lateral right step en-echelon strike slip faults;based on the OSL and ¹⁴C dating results,it is established that the terraces in the Zheduotang area were mainly formed during 3 time intervals（i.e.,～21,000 to～16,000 BCE,～10,000 to～5,500 BCE、～2,400 BCE）and the evolutionary history of the Zheduohe river is therefore reconstructed accordingly;the southern branches of the Zheduohe river have been diverted multiple times,which further reveals the characteristics of the tectonic movements during the time intervals involved.（3）Based on detailed analyses on tectonic geomorphological characteristics,a trench was excavated in a small-scale graben within which the sedimentary units and the fault structures were recognized and served as prominent sedimentary response to the intense tectonic activities in the study area.What’s worth mentioning is the recognition of eight A soil horizons（five in trench CUG-2013-1 and 3 in the old trench）,which allows a general determination of the recurrence period of the paleoseismic events.We finally recognized at least 8 paleoseismic events that happened backwards in time in 1955 CE,～1000 CE,～15 BCE,～1300 BCE,～1850 BCE,～2100 BCE,～2370 BCE and～5500 BCE,respectively.And the recurrent period is estimated to be not exceeding 1000 years.（4）The slip model of the Kangding Ms 6.3 earthquake in 2014 was obtained through the seismic waveform inversion method.The result shows that the NW-SE trending Selaha fault is likely the seismogenic fault where the fracture mainly propagated in the northwest direction and reached its peak slip of～0.31 m at approximately 2.5 km from the epicenter.Most of the slips were released within 20 km to the epicenter.A local maximum of slip was also observed at the epicenter of the Ms5.8 aftershock that occurred on the Zheduotang fault in the southeast,which suggested a triggering relationship between these two earthquakes.（5）According to the results of this study and previous paleoseismic research in the Kangding area,the strong earthquake recurrence period of the Yalahe fault in the past7000 years is about 3000yr while the recurrence periods of the Selaha fault and Zheduotang fault in the past 4000 years are separately about 480yr and 420yr.The latter two faults are more seismically active.Based on the results of previous segmental slip rates of the three faults,the slip rates of the branch faults are scattered and inherit slip components from the main fault.Thus,the city of Kangding,locating near the south intersection of the three faults,still has a strong seismic risk in the future.