Period Division And Trigger Exploration Of The Giant Lianhuasi Bedrock Collapses And Landslides On The North Slope Of Huashan Mountains In The Southeast Margin Of Weihe Basin

Collapsing landslide is an important process of landscape evolution.Study on paleo-collapsing landslides induced by earthquakes is of great significance to understand the interaction among landscape evolution,tectonics and the potential risk of seismic events.In the east of Huazhou district of Weinan City,the mountain peak around the Lianhuasi area to the SE Weihe Basin is named as Zhangjia Shan,and it is located on the north slope of Hua Shan(Shan means Mountain in Chinese)and to the south of Hua Shan piedmont fault.Near this mountain peak,landslide-induced or relevant multi-level armchair-like terrain can be clearly observed.Correspondingly,a large area of unconsolidated breccia can be identified to the north of Hua Shan piedmont fault.These two aspects collectively indicate that the site is a giant paleo-bedrock landslide remanent,which is called"Lianhuasi giant bedrock collapsing landslides"(hereinafter referred to as"Lianhuasi landslide").Lianhuasi landslide has attracted many scholars'attention.However,existing discussions on the specific accumulation scope of this massive landslide remanant are still insufficient;there is particularly a lack of an overall understanding of its subsurface morphology and accumulation characteristics,and existing understanding of the triggers,sliding periods and ages are also ambiguous.These limits prevent us from producing the overall picture of Lianhuasi landslide itself,further restraint/obstacle the in-depth understanding of the relevant landscape evolution,active tectonic deformation and future seismic and geological disaster risks.The present study targets Lianhuasi giant bedrock landslide in particular.On the basis of fully understanding existing relevant researches and using methods such as remote sensing,field detailed geological survey,Ground Penetrating Radar(GPR)and historical literature review,the overall morphology,material composition,coverage,accumulation thickness and volume of the giant landslide are exmined and determined firstly in the study.By analyzing the relationship between morphology and accumulation characteristics,along with the historical document review and associated deposition rate-base estimation,the sliding periods and the occurrence time or time range of each sliding period are then discussed and estimated.Finally,the trigger of the giant landslide is explored.The main understandings are as follows:(1)The overall morphology and accumulation characteristics of Lianhuasi landslide.The overall sliding direction of Lianhuasi landslide is about 340°;the south sliding boundary is near the top of Zhangjia Shan,while the north accumulation boundary is near the line from current Lianshepu to Dongxinzhuang villages,with a total length of about 6700m from north to south,a maximum width approximating 1940m from east to west,and a total accumulation area of about8.9km~2.The maximum burial depth of its front part is about 28m.The volume of the accumulated material of Lianhuasi slide is at least 110 million m~3 in total(Among them,the volume of the first period bedrock collapse and slide is at least 52 million m~3,and that of the second period is at least 58 million m~3.Due to long-time impacts and destruction from various human activities,the volume of the third period collapse and slide is difficult to estimate;for specific information regarding the period division of Lianhuasi landslide,please see the second understanding point below)indicating that it belongs to giant landslide(the largest catogery)in current China's landslide ranking scale.In the accumulation area of this paleo-landslide remains with wide accumulation coverage and huge accumulation volume,the current population is dense and several national level key east-west traffic lines travel through in busyness.These overlaps show that if similar landslides happen again on the north slope of Hua Shan in the southern margin of Weihe Basin in the future,the social and economic impacts will be very far-reaching,indicating an important aspect of seismic and geological risk analysis and risk prevention and mitigation for the local area.(2)Periods of Lianhuasi landslide.Based on the comprehensive analysis of the morphology and accumulation characteristics of the landslide relics,especially after examining(1)the cross-cutting relationship between the higher and lower armchairs in the sliding source area,(2)the slope angle difference of the sliding beds,and(3)the overlapping relationship among different accumulation bodies(and other sedimentary layers),and(4)relevant historical literature records,the sliding periods of Lianhuasi landslide is explored in this study.It is concluded that Lianhuasi landslide remnant was formed by three periods of successive collapse and landslides,of which the first two are Zhangjia Shan bedrock collapses and landslides and the latest is Fuchengshang(Banjie Shan)accumulated breccia collapse and landslide in1072 A.D.(3)The ages of Lianhuasi Landslide.According to the burial depth of 1072landslide accumulation observed from the surface,the deposition rate of"normal"deposit(i.e.relative to landslide accumulation)at the front part of Lianhuasi landslide accumulation area since 1072 is calculated.Combining this result together with(1)the burial depth of the accumulating ground of the first period bedrock collapse and landslide event detected by GPR in the front part of Lianhuasi landslide accumulation area and(2)the deposition rate of the related geological units provided by other studies,the age of Lianhuasi landsilde's first period bedrock collapse and landslide is estimated.The results show that Lianhuasi landslide's first period of collapsing and sliding occurred in about 23.1ka B.P.It is clearly recorded in relevant historical documents that the third period of collapse and landslide occurred in 1072A.D.With these,it is clear that the second period of bedrock collapse and landslide should occur between the above two time points.(4)Triggers of Lianhuasi landslide.The rock mass stability coefficients before the first and the second periods of bedrock collapses and landslides were estimated respectively using limit equilibrium method;the maximum velocities of the two times of bedrock collapses and landslides were also approximated through landslide characteristic parameter calculation method.Combining these two aspects of estimation together with the examination of Lanhussi landslide's shape and accumulation characteristics itself and existing trigger understanding of other researchers,this study suggests that the first and the second period Zhangjia Shan bedrock collapses and landslides of Lianhuasi landslide are high-speed and long-distance collapse/landslide-debris flow triggered by earthquakes.The third period of collapse and landslide of breccia occurred in Fucheng Shan was induced either by earthquake or by precipitation,and two trigger possibilities are both high.The above understanding could enrich and deepen the overall understanding of Lianhuasi bedrock landslide itself.Secondly,it can also provide a reliable reference for collecting the dating samples of each period of landslide events,so as to more substantially analyze the relationship between this giant paleo-landslide remains and seismic activities.This kind of enhanced landslide-earthquake relationship study may provide opportunities for better understanding of the activities of Hua Shan piedmont fault,and further,better understanding of the landscape evolution and related tectonic events in the whole southeast margin of Weihe Basin.Thirdly,from the technical point of view,it is still rare to apply GPR to the study of paleo-landslides in China.This study can accumulate certain technical experience on this aspect for future similar work.Finally,because Lianhuasi paleo-landslide has a vast accumulation coverage and a giant volume,especially because it exactly lies in a densely populated area with intense socio-economic activities;the analysis of its morphology and accumulation characteristics(esp.its accumulation coverage/scope),sliding types,and triggers is also an important aspect to understanding of the seismic and geological disaster risks of the area.In the future,Lianhuasi giant landslide can be used as a typical analogy analysis reference for the study of"seismic and geological disaster risks in areas with similar geological and geomorphic background".With such analogy analysis,the depth and pertinence of the associated risk analysis work,seismic and geological disaster scenario building for example,can expect to be futher strengthened or sharpend,which benefits significantly both better understanding of seismic and geological disaster risks and better serving of disaster prevention and mitigation.