The Mechanism Of Large Earthquake-Triggered Landslides In Area Of High Intensity And Deep Canyon Of Northwest Sichuan

The large earthquake-triggered landslide(LETL)is referred to the landslide that triggered by earthquakes whose scale is larger than 100×10~4m~3.Due to its large scale,the LETL usual could block rivers,bury villages and cause a large number of casualties and property losses.Northwest Sichuan Province of China is the famous LETL-prone area in the world.The field investigations,long-term seismic monitoring and physical/numerical tests are carried out to study the causes and mechanism of LETLs in Northwest Sichuan Province supported by NSFC project(41072231),the fund of China Geological Survey,Open fund of SLGP(SKLGP2015Z001)and National Key Research and Development Program of China(Grant No.2017YFC1501000).The main conclusions could be summarized as follows:(1)Revealing distribution features of LETLs systematically.There are at least847 LETLs in study region according to field investigations.All the LETLs are concentrated in earthquake concentration area with intensity is not less than VII(PGA?0.10g),and mainly concentrated in deep canyon whose landforms belong to mid-alpine landscape.The landslide scale distribution follows the exponential equation:Y=4.0+400.0×0.999~x(Y-landslide number,x-LETL scale),the scale distributions in every basin both follow relative exponential equations.Distribution and activatiy of faults in study region affect the LETL distribution obviously:with increased distance to fault,the number of LETL increase and follows the exponential equation:Y=18.6+660.0×0.9992~x,and 96%LETLs are concentrated in the range of10 km from the fault.The LETL concentration present increases first and then decrease and 80.43%landslide are focused within 400-1200m that distance to river.The relative landslide height mainly focuses 200-800 m which occupies 80.43%,and the relationship between landslide scale and slope heigh meets Y=270×log(0.06x+1.67).89.61%landslides are occurred in layered slope,the residuals are in non-layered slope.There exit 8 landslide concentration areas which occupy 54.87%landslides(464),while whose affected areas only occupy 5.8%compared Chuanxibei region.The non-uniformed distribution indicates the active plate,closer to active fault,intersection and dislocation of faults,high crustal stress region and rapid reducetion region of regional GPS are LETL-prone factors based on the distribution characteristics of landslide concentration area.The typical feature of source area mainly include more than 76%landslides concentrated in the upper part of hills,and about 49.3%landslides are located in top part;the isolate ridge,slope break and multi-free surface slope are LETL-prone area,which occupy 34.34%,30.29%and14.57%respectively.The hard and soft alternated rock is the LETL-prone rock(44.77%);612 LETLs,which occupy 72.26%,are distributed in 14 rock groups such as Zhiliu group,Zhuwo group and etc.About 73%LETLs blocked rivers.(2)Revealing movement characteristics of LETL.Based statistics,the peak velocity both>15m/s,the landslides heigh(H)and landslide length(L)meets the relationship of L=1.85×H+95.More than 80%landslides are long-runout landslides(H/L<0.6);the reslationship between landslide scale and runout lengh meets Y=510+590×(log(x)-2)~2.The average H/L is 0.485,56%landslides character the high-speed and long-runout.76%landslides character debric flow in movement,33%landslides character sturzstrom in movement.High motility landslides are mainly high toe of rupture surface landslides;the low motility landslides are low toe of rupture surface landslides.(3)Concluding the deformation-failure geological modes.The slope failure modes in study region could be divided into rock slope failure modes and soil slope failure and mass ejection landslides.Soil slope failures are mainly concentrated in northern region of study regio whose geological mode is seismic liquefaction-plastic flow.Rock slope failure modes could be divided into tension-slide(bedding slope and transverse slope),tension-topple,tension-shear and tension-diffuse.Mass ejection landslides is a special co-seismic landslide whose failure mode is tension-ejection.(4)Revealing the mechanism of LETL.6 forming conditions,that is free space,fissures,slope structures,seismic fault,rock group and slope seismic response,are proposed.For mechanism,the source area,located in upper/top part of canyons that close to seismic fault,is cut by huge fissures induced by unloading and tectonics,the intense river erosion and frequent earthquakes that couple site effect developed these fissures continuously,a later strong earthquake pierces fissures and the slope failed finally.in addition,when the dynamic condition is strong enough,the failed rock mass could be ejected out to form mass ejection landslide.The geological modes of LETLs are“seismic tension+others”.