Research On The Formation Mechanism Of Large Landslides Triggered By Earthquakes In Tianshui Area

Tianshui city, located in the loess hilly region in the middle of the North-south seismotectonic belt, where Ordos block, Qilian orogenic belt and Qinling fold belt joint. The neotectonic movement is quite active, the geologic structure is complex and late quaternary active fault belts are well developed. Several strong earthquakes struck the study area in history. These earthquakes triggered large numbers of landslides. For instance, the area of landslides triggered by Tongwei earthquake in1718 is about 665 km~2 which accounts for 90% of the epicentral area. The losses of secondary geo-hazards are even more than those caused by the earthquake itself, for example, earthquake-induced landslides could possess huge devastating impact and result in catastrophic disasters,. This work carries out extensive study on the distribution of earthquake-induced landslides and other geo-disasters, and discusses the formation mechanism of targeted typical earthquake-triggered landslides in the study area, through field investigation, experimental analysis and numerical simulation.Field investigation showed that the topography affected distribution of landslides significantly: the north bank of Weihe River belongs to loess hilly region and landslides were intensely developed; the south bank of Weihe River is close to mountainous terrain, landslides were relatively less and avalanches and debris flows were well developed.In order to study the formation mechanism of typical earthquake-induced landslides, this work conducted several loess kinetic experiments based on dynamic triaxial test system. Then instability mode and failure mechanism of typical earthquake-triggered landslides were studied and primary research result wereobtained, by virtue of FLAC3 D numerical simulation technology. The results showed that the loess-mudstone complex landslides firstly incline to unstably slide along loess uprightness joints and weak structural surfaces of the underlay mudstone at the foot of slopes. Then, under continuing seism action, shear failure may occur in middle-upper parts of the slope because of free face. The slip surface cuts through the underlay mudstone and the landslide is the typical insequent landslide with high speed and long runout. And the results of numerical simulation match those of engineering geologic drilling.In addition, the present stability and the possible instability mode and failure mechanism of the typical earthquake-induced landslides in the study area were studied in this work. Analysis showed that loess-mudstone complex landslides with smooth slope after high-speed and long-runout sliding are basically stable in natural state, but they have high possibility to unstably slide and develop to loess-mudstone interface landslides.