Insights Into The Development Characteristics And Genetic Mechanism Of Long-runout And High-speed Coseismic Landslides

The eastern-southern margin of the Qinghai Tibet Plateau is located in the abdomen of the East Tethys tectonic domain.The strong tectonism,orogeny,crustal movement and magmatism in this region result in intensive seismic and tectonic activities,and this make this area the typical landslide prone area,especial for the large landslides whose scales are larger than 1 million cube meters.As the roof of the world,the Qinghai Tibet Plateau is an important strategic region in China,numerous important basic projects are also planned in this region,the Sichuan-Tibet railway could be the most important one.Geological factors such as deep canyon,high ground stress,frequent strong earthquakes and rock fragmentation make the Sichuan-Tibet Railway be a famous large landslide prone and concentration area.The Luanshibao landslide,which located in the Litang County,just crosses the Sichuan-Tibet Railway,and could be a typical example charactering intensive distribution of faults,complex microterrains,large scale,long runout and etc.Analysis of Luanshibao landslide could effectively reveal the geohazard-inducing background,genetic mechanism and potential hazard identification,which is of great significance for the long-term safety of infrastructure such as Sichuan Tibet railway and G318 national highway.Thus,based on multi field investigation,UAV mapping,three-dimensional tilt photography,scientific research drilling,exploratory trench,sample dating,numerical simulation,physical experiment and comprehensive data,the formation mechanism and dynamic process of high-speed long runout landslide are studied.Some main conclusions could be summarized as follows:（1）Genetic conditions of large landslides.According to field investigations and relative literature,55 typical landslides were collected.The typical characteristics are summarized by systematically analyzing the geological lithology,structural characteristics,spatial position,square quantity,relative height difference,moving distance,geomorphic characteristics,moving speed and other factors.Weredefine the high-speed and long-range landslides by analyzing its typical characteristics;this paper systematically summarizes the genetic conditions of high-speed and long-runout landslides,such as broken hard rock mass,located in the extreme earthquake area and close to the seismogenic fault,special topographic and geomorphic types,etc.（2）Analysis ofdeposit features of source area,deposit area and so on.Luanshibao landslide is located at the top of the thin ridge with three faces facing the free.The lithology is gray white medium fine-grained biotitegranodiorite,granular structure and massive structure.The H/L=0.207,which belongs to a typical long runout and high speed avalance.The volume of landslide is 7.485×10⁸m³.Based on the micro geomorphic characteristics and movement differences,the Luanshibao landslide can be divided into four subareasof I-sliding source area,rear edge and side edge bedrock exposed area,II-multi-level scarp area,III-secondary accumulation landslide area and IV-landslide accumulation area.Each region alsocould be divided into several small areas.Through scientific drilling,exploratory trench,field geological survey and other works,this paper analyzes the characteristicsof stratification,grain sequence and the alluvium and diluvium under the deposit body.（3）Revealing the characteristics ofpaleoseismic chronology and entrainment.Seven large paleo-seismic exploration trenches were excavated on the East-West Yidun-Litang-Dewu fault zone in the region.28 ¹⁴C samples and 1 OSL sample were taken on site,and tested and analyzed.It is determined that there are five periods of obvious seismically active episodes in 5000 years ago.Secondly,by collecting and sorting out the dating data of Luanshibao landslide,the trigger age of Luanshibao landslide is determined again,and two time periods（3635±387 a BP and 2100-2200a BP）are determined.Through the analysis of the age spatial correlation between the paleoseismic episode and landslide age,it is found that the two time periods of the landslide age are completely consistent with the paleoseismic active episodes,which indicates that the landslide may unstable twice,and they are both very likely to be triggered by the earthquake.The first seismic magnitude Mw≥7.43±0.09.（4）By dating and analyzing the lithologic samples near the landslide deposit and the original surface（drilling sampling）,the deposition rate of Quaternary alluvium in Maoyaba basin（0.5mm/yr）is determined,and the entrainment depth at different positions is determined.（5）Analysis of formation characteristics,genetic mechanism,movement and mechanism of complex micro terrain.Firstly,by systematically analyzing the geological background of Luanshibao landslide,the formation conditions of Luanshibao landslide are summarized,include Litang river erosion and the formation of tributaries,micro terrain of thinned ridge;the long termactivities of Litang fault makes fractured rocks in hanging wall area,and develop multi structure planes forming huge wedge rock masses.The large tense stress,produced by strong seismic shaking coupled with site amplification（seismic magnitude Mw≥7.43±0.09）,develops the structure planes quickly,shear out the locked section,and have a high speed and long runout movement.There also exist phenomenon of ejection,entrainment and inverse sequence.