Spatial-temporal Evolution Of Post-seismic Geological Disaster And Susceptibility Assessment After Wenchuan Earthquake

On May 12,2008,an 8.0 magnitude earthquake occurred in Wenchuan.The earthquake released huge energy and caused strong ground shaking,triggering nearly 200,000 co-seismic geological disasters,which is the largest number of geological disasters caused by a single earthquake in mountainous areas recorded so far.Thousands of coseismic geological disasters(collapse,landslide,debris flow,etc.)caused immeasurable losses to lives and property.However,the strong destructivity of wenchuan earthquake is not only reflected in the casualties,the destruction of kinds of infrastructure,the induced co-seismic geological disasters directly caused by the earthquake,but also in a series of post-earthquake geological disasters.For example,on September 24,2008,rain-triggered debris flows in beichuan killed 42 people and severely damaged roads and other infrastructure.On August 13,2010,mass debris flow occurred in wenjiagou,qingping township,hongchungou,yingxiu town,and longchi,dujiangyan.On July 10,2013,52 people were killed and 109 others were missing in debris flow in wulipo,sanxi village,zhongxing town,dujiangyan city.Many geological disasters after the earthquake caused a huge impact to the people in the disaster areas and the post-disaster reconstruction work,the damage caused by the Wenchuan earthquake was far more than expected.On August 20,2019,debris flows broke out in the minjiang river basin from yingxiu to wenchuan under the continuous heavy rainfall.A large amount of material from the gully rushed out into minjiang river and destroyed the roads and houses along the river.It can be seen that the insufficient understanding of the evolution law of geological disasters after a strong earthquake has seriously endangered the safety of people's life and property,and the study of the temporal and spatial evolution of post-earthquake geological disasters is imminent.This article selects the epicentral area be yingxiu to husi 42 basin along the minjiang river as the study area,Based on a large amount of pre-and post-earthquake satellite images,we established a detailed landslide mapping method and prodeced a multi-temporal polygon-based inventory.The mapping inventory as data source,we quantify changes of active area,size frequency and types of movement.Mainly post-earthquake geological disasters in wenchuan are reactivation of co-seismic landslide,new landslide and debris flows.By studying the number and area of different types of geological disasters,it is shown that post-earthquake geological disasters are dominated by resurrected landslides at the beginning,and with the time evolution,the proportion of reactive landslides gradually decreases,while the proportion of new landslides even drops to 0,the proportion of post-earthquake debris flow gradually increases,the proportion of post-earthquake geological disasters is dominated by debris flow.The Arc GIS spatial analysis module is used to analyze the evolution of the spatial distribution of geological disasters over time after earthquake,including the evolution characteristics of the spatial distribution of post-earthquake landslide disasters in elevation,slope,curvature,topographic wetness index and runoff intensity index.The evolution characteristics of spatial distribution of debris flow disaster in catchment area,interval relief,average slope degree,length of main gully and channel density after earthquake.In addition,divided the post-earthquake disaster into hillslope deposits and channel deposits according the location of disasters,and calculated the volumes of the two types to show their evolution over time.The study found that the most of reduction of hillslope deposits and increment of channel deposits from 2008 to 2011,then hillslope deposits reduce extremely obvious,the channel deposits change was small,only 2019 years increased slightly.The susceptibility of post-earthquake geological disaster can provide theoretical basis for reconstruction and disaster prevention and reduction after earthquake.We select factors as catchment area,interval relief,average slope degree,length of main gully and channel density in 42 catchments as evolution factors.The evaluation model of post-earthquake debris flows susceptibility was established based on grey relational degree method.Ultimately the post-earthquake debris flows susceptibility is divided into 5 categories,which include very high,high,moderate,low,and very low susceptibility.The prediction ability of the model was verified by the actual debris flows in 2019.2019 debris flows occurred in 16 catchments which belong to very high,high,and the moderate susceptibility catchment,accounting for 88.89% of the total number of debris flows,indicating that the vast majority of debris flows occurred in the moderate and above vulnerable catchments,and the accuracy of the evaluation model is good.