| Earthquake is one of the main triggering factors of landslides.A strong earthquake(M>6.0)in mountainous area can induce hundreds to tens of thousands of landslides.The direct loss of life and property caused by co-seismic landslides can account for one third or more of the total earthquake losses.Besides,many mountains were shattered by earthquake shaking and a large number of loose materials accumulate in valleys and slopes,resulting in a significant increase in the scale and frequency of geological hazards such as landslides and debris flows.The strong activity of landslides post-earthquake can last for several years or even hundreds of years.Over the past 10years since the 2008 Wenchuan Ms 8.0 earthquake,several strong earthquakes have occurred in China mainland,such as Yushu Ms 7.1 in 2010,Lushan Ms 7.0 earthquake in 2013,Ludian Ms 6.5 earthquake in 2014,Jiuzhaigou earthquake Ms 7.0 and Milin Ms 6.9 earthquake in 2017.Strong earthquakes show a trend of frequent occurrence,which is seriously threatening to the safety of life and property of people in mountainous areas.After the 2008 Wenchuan Earthquake,the distribution of co-seismic landslides,the chain effect and long-term effect of post-earthquake landslides have become a hot topic for scholars at home and abroad,and a lot of research results have been achieved.However,most of the studies only focus on a single earthquake in a certain research area,while comparative studies on the development and distribution of co-seismic landslides under different seismogenic mechanisms and geological environments are relatively few.Most of the studies on the dynamic evolution mechanism of post-earthquake landslides,disaster chain effect and long-term effect are still in qualitative evaluation,while quantitative studies on long-term time series are still very scarce.In the present paper,coseismic landslides of 8 strong earthquakes(Ms 8.5Haiyuan earthquake in 1920,Ms 8.0 Wenchuan earthquake in 2008,Ms 7.0 Lushan earthquake in 2013,Ms 6.5 Ludian earthquake in 2014,Ms 7.0 Jiuzhaigou earthquake in 2017,Ms 6.9 Milin earthquake in 2017,Mw 6.6 Hokkaido(Japan)earthquake and Mw 7.5 Palu(Indonesia)in 2018)were inventoried by using multi-source remote sensing images.The coseismic landslides distribution of the 8 earthquakes was detailed analyzed.On this basis,the rapid prediction of the spatial distribution of coseismic landslides for different types of seismogenic faults was studied.Finally,with Mianyuan River in Wenchuan earthquake area as the main research area,the characteristics and evolution laws of landslides and debris flows after the Wenchuan earthquake were tracked and studied through comparative interpretation of long-time series remote sensing images and on-site monitoring.Through the above research,the following main results can be drawn:(1)The inventory databases of 8 strong earthquake co-seismic landslides with different magnitudes and types of seismogenic faults have been established.Statistical results show that the total area and distribution range of co-seismic landslides are in accordance with the power function relationship with earthquake magnitude.Besides,the faulting mechanism of the seismic events also has an impact on the distribute intensity of co-seismic landslides.Under the same magnitude condition,the number of co-seismic landslides of thrusting fault type earthquakes is more than that of strike-slip fault type earthquakes,and the larger the fault dip angle,the more co-seismic landslides occur.(2)The distribution pattern of co-seismic landslides is mainly affected by three factors:seismic factors,topographic factors and geological factors.Seismic intensity,peak ground acceleration,distance from seismogenic faults and other seismic factors mainly control the macro-distribution pattern of co-seismic landslides.Topographic factors such as slope gradient,slope aspect and distance from rivers mainly control the micro-distribution pattern of co-seismic landslides,while lithology mainly control the types of co-seismic landslides.The detailed distribution patterns are as follows:(1)The distribution density of coseismic landslides is positively correlated with seismic intensity.Coseismic landslides mainly occur in areas with seismic intensity>VII,and there are few landslides in areas with seismic intensity<VII.Large-scale co-seismic landslides are mainly distributed in areas with seismic intensity?VIII.In the regions with seismic intensity>IX,the co-seismic landslides show the characteristics of the concentrated and high-density distribution.(2)The distribution density of co-seismic landslides is also positively correlated with the peak ground acceleration(PGA).A large number of co-seismic landslides can be triggered when PGA is>0.1g and the distribution density of co-seismic landslides in the regions with PGA>0.2g increases significantly.(3)Generally,co-seismic landslides are distributed in zones along seismogenic faults.For earthquakes of different magnitudes,the bandwidth is different.For earthquakes with magnitude>Mw7.5,the bandwidth can reach 10 km.The influence range of seismogenic faults for earthquakes with magnitude>Mw6.5 is generally 5km.The influence range of seismogenic faults for earthquakes with magnitude<Mw6.5 is generally 2 km.The spatial distribution of co-seismic landslides of thrusting earthquakes has significant"hanging wall"effect.(4)Co-seismic landslides are centralized distributed in a specific elevation range.The elevation range of centralized distributed landslides corresponds to the area where the main river transits from the lower gorge to the upper wider valley.These topographic transitions and turning parts have been subjected to long-term gravity unloading and the rock mass is relatively fragmented and has good air-facing conditions.Therefore,co-seismic landslides are more likely to occur under the shaking of earthquakes.(5)When the co-seismic landslides were mainly rocky landslides,the distribution density increases with the increase of slope gradient,and mainly distributes in the slopes of 25-45°.When co-seismic landslides are mainly soil landslides caused by earthquake liquefaction,co-seismic landslides mainly distribute in the areas with slope<20°.(6)Co-seismic landslides show significant“aspect effect”in specific areas.The significant area of"aspect effect"has the following typical characteristics:the strike of slopes is nearly vertical or parallel to seismogenic faults,and generally,they are steep and thin mountains.(7)Co-seismic landslides show not only zonal distribution along seismogenic faults,but also linear distribution along main rivers.The influence of rivers on the spatial distribution of co-seismic landslides ranges from 0 to 1 000 m from the main rivers.(8)Lithology mainly controls the types of co-seismic landslides.Rockfalls mainly occur in hard rocks such as magmatic rocks,limestone and dolomite.Shallow landslides mainly occur in soft rocks such as phyllite,slate and metamorphic sandstone,and giant rocky landslides mainly occur in"upper hard and lower soft"rocks.(9)Strong earthquakes can result in the liquefaction of sand and trigger a low-angle fluidized landslide.Preliminary studies show that the spatial distribution of fluidized landslide is mainly affected by the thickness of the soil layer,groundwater level,soil material composition and saturation degree of soil.(3)In this study,PGA,distance from the seismogenic fault,slope gradient,elevation(slope height),lithology and distance from rivers are selected as evaluation parameters.Based on the logistic regression method,the susceptibility assessment model has built for co-seismic landslides in Wenchuan earthquake area.And the rapid prediction of co-seismic landslides in the Lushan earthquake area was realized using the model of Wenchuan earthquake.Based on analytic hierarchy analysis,the susceptibility assessment model of co-seismic landslides for thrust earthquake was established and the rapid prediction of co-seismic landslides of the Ludian earthquake was finished.We published the rapid assessment results to the society through the official website of our laboratory within 2 days after the two earthquakes.They have provided important support for the delineation of the seriously damaged areas,the determination of rescue areas,the selection of rescue channels and the safe location of temporary and transitional settlements.(4)In the Mianyuan river basin,loose materials of about 4.0×10~8 m~3 were produced by the co-seismic landslides of the Wenchuan earthquake,which provided abundant material sources for the occurrence of large-scale debris flows post the earthquake.According to the source supply and initiate mode,the post-earthquake debris flow can be divided into 4 types:large-scale landslide accumulation initiate type,branch-ditch source initiate type,dispersed source initiate type,and seismic fractured slope initiate type.Five years after the earthquake,the total amount of sediment transport in the main river of Mianyuan river is?1.31×10~7 m~3.Assuming that the transport rate remains unchanged,it will take 23 to 46 years to transport all removable solid materials to the main river.(5)The Wenchuan earthquake destroyed vegetation seriously in Mianyuan river basin,and the vegetation coverage decreased by?20%after the earthquake.In the past 10 years post-earthquake,the vegetation in the basin has been recovering year by year.The highest vegetation coverage rate is 95.1%,which is slightly lower than that before the earthquake(98.7%).It will take many years for vegetation to recover to the level of pre-earthquake coverage in the study area. |
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