Long Term Activity And Disaster Effect Of Earthquake Induced Landslides In Qionghai Lake Basin

In recent years, seismic activity occurred continualy throughtout the world, and the magnitude of earthquake and associating disaster area also increased.One strong earthquake in the complicated terrain mountain not only triggered large number of co-seismic landslides and ground fissures, but also generated strong influence on the regional geological environment, further induceing large amounts of unstable slopes with comprehensive fissures and cracks.As a result, the disaster phenomena of debris flow and sediment deposition in the mountain rivers would increase or intensify.but people had no idea about how long the severe disaster activity would sustain to recover gradually to the state before the earthquake, so many researche scholars were concerned about this question. Therefore, After one major earthquake, it is meaningful to study about how long co-seismic landslides affected a basin and associated disaster effect, and furether exploring the dominating factors and genetic mechanism for the prolonged activity of co-seismic landslides have theoretical and practical significance.Based on the Qionghai Lake Basin in southeastern Tibetan Plateau, by field investigation and aerial image the landslide characteristics and distributing rule in Qionghai Lake Basin were studied, and history data collection, statistical analysis of data and historic seismic analog together revealed the cause of earthquake triggering of landslides and its historical earthquake event. Futher employing jointly the statistical analysis, theoretical calculation, experimental analysis and typical case study, the controlling fators and reactivation mechanism about long term activity of co-seismic landslides was presented. In the end, by establishing and applying inversion model about historical earthquake induced landslides,to explore the feedback of co-seismic landslides for surface erosion and dynamic uplift processes, further to reveal the contribution of historical earthquke induced landslides to the sediment yield in Qionghai Lake Basin, and the prolonged impact on the mountain disasters. Main works and conclusions of this dissertation are shown as following:(1)The paper first indicated that extensive landslides in Qionghai Lake Basin were originated from 1850 Xichang earthquake (M7.5), and its activity at least lasted for a period of about 160 years. The developing regularity of large number of landslides in Qionghai Lake Basin were analyzed roundly, and eight criterions were summed up to judge for the seismic triggering characteristics and formation time about these landslides, such as the seismogenic environment, the feature of cluster distribution, the type and scale of landslides, high-potential energy sliding and fast-moving collision, slide belt, the effect of landslide dam,the new tectonic stress field, and historical earthquake induced landslide event, thereby it was inferred that landslides in Qionghai Lake Basin were originated from 1850 Xichang M7.5 earthquake. Meanwhile, based on the investigation of typical landslide activity and comparative analysis of different soil grain composition, it was found that these landslides in Qionghai Lake Basin were still active at present, and some new slides generally occurred in ancient slope failure zones in recent years. So we inferred that since 1850 these earthquake induced landslides in Qionghai Lake Basin at least had been activating for about 160 years.(2)the paper preliminarily presented that coupling of earth’s endogenic and exogenic geological processes significantly controlled the long term activity of earthquake induced landslides, such as frequent earthquke activities and extreme rainfall events near Qionghai Lake Basin, which dominated the reactivation and persistent activity. ①based on statistical analysis of the data about regional earthquakes,climate and geological disasters, it indicated that the occurrence of geological disasters had evident coupling relationship with frequent earthquake activities and extreme rainfall events near Qionghai Lake Basin; ②t was found that one strong earthquake could trigger far field occurrence of seismically induced landslides, and far field earthquake could induce the weaken slope to generate cumulative displacement deformation by calculating with Newmark method and earthquake acceleration data, which was controlled together by stratigrafic amplification effect, topographic amplification effect and fault amplification effect at the basin edge; further by the dynamic triaxis test it was reflected that frequent earthquakes near Qionghai Lake might accelerate substantially the soil to generate cummulative deformation and reduce soil strength, so this results revealed that frequent earthquake activities played an promoting role in remotivating the original co-seismic landsldies; ③Through the permeability test and the GDS instrument to carry out soil penetration test and consolidation test with different fine particles, it was found that the soil permeability coefficient was monotonically increasing with the decrease of fine particle content before and after consolidation, more significant changes would happen with the saturation degree decrease, but this trend was decreased through the consolidation process; so it refected that after the soil was shocked frequently the heavy rainfall would infiltrate into the soil with cracks and fissures, and erode and transport the fine particles, further the soil sructure and propertier changed with large porosity and high permeability, thus, the disturbed soil may accelerate seepage failure, however, during the effect of gravity and seepage consolidation the soil seepage stability would gradually be enhanced. ④after analyzing the material supply condition and rainfall triggering process of typical landslide and debris flow event, it was father substantiated that the space-time coupling relation between frequent seismic disturbance and rain infiltrating motivation controlled significantly the occurrence of this debris flow.(3)The paper preliminarily revealed that the co-seismic landslides signifigcantly contributed to the sediment yield in Qionghai Lake Basin, and kept prolonged impact on the mountain disasters, even an earthquake cycle. Based on the complete landlsides inventory throughout Wenchuan seismic area, a three parameters inverse gamma probability distribution model was proposed, and the model was applied to calculate the relative parameters about 1850 earthquake induced landslides, then it was inferred that the region erosion rate associated with these landslides was 1.88 (0.88-4.02) mm/a. Comparing with the erosion rate of 0.82mm/a during the Holocene studied by other researchers, the different erosion rates of two time periods reflected that the 1850 earthquke induced landslides signifigcantly contributed to the sediment yield in Qionghai Lake Basin. Further based on the investigation and analysis about the frequent debris flows and serious sediment deposion in Qionghai Lake Basin, this paper also revealed that the co-seismic landslides may still keep prolonged impact on the mountain disasters, even an earthquake cycle of about 300 years.