Investigation Of The Landslides Induced By The 1856 Qianjiang-Xianfeng(Daluba) Earthquake And Research On Their Generation Mechanism

History documented that an M614 earthquake occurred between Qianhjiang,Chongqing and Xianfeng,Hubei(also named the Daluba event)in 1856.It caused serious geological effects,including a lot of landslides at Xiaonanhai,Wangdahai,Zhangshangjie and other places.Remote sensing images show that some landslides are distributed roughly in NNE direction,consistent with the trend of the Qianhjiang fault,which is a major regional fracture considered as the seismogenic structure of this earthquake.Its epicenter is located in the middle reach of the Yangtze River where tectonic and seismic activities are relatively weak,and moderate-sized quakes like the 1856 event are rare.Why such a shock could generate remarkable landslides,blocking the river course to form a dammed lake? Even the scales of its individual landslides are comparable with the large-scale landslides triggered by the 2008 Wenchuan Mw7.9 earthquake at Donghekou of Qingchuan and Niujuangou of Yingxiu in Sichuan Province? What is their generation mechanism? These issues need in-depth studies.The 1856 Daluba event took place in the mountainous area of western Hubei Province,where M6 earthquakes are events of small probabilitiesas history recorded.But once such an event happens,slopes around the epicenter tend to suffer from failure,triggering landslides,to which the 1856 Daluba event is a typical case.This work made a detailed investigation to the landslides induced by this event,studied their generation processes exemplified by the Xiaonanhai landslide,and explored their mechanism.The research results are also helpful to understanding the genesis of the 1856 Daluba earthquake,and significant for the effort of earthquake prevention and disaster reduction in the middle reach of the Yangtze River(including the Three Gorge Reservoir area).1 Features of the earthquake-induced landslidesThrough field investigations and photography by unmanned aerial vehicles,this work has investigated the landslides at four places: Xiaonanhai,Zhangshangjie,Wangdahai,and Huolongping.Their common features are as follows:(1)The failed slopes dip gently,slightly tilting toward SE.(2)Overall,the slope material slid to SE over large distances,along the bed interfaces.(3)At Xiaonanhai,Zhangshangjie,and Wangdahai,the occurrences of back scarps are in accordance with those of two sets of steep joints in this area.As for the scale and weathering degree,the landslides at the four localities are different.Their scales follow such a order from large to small as Xiaonahai,Zhangshangjie,Wangdahai,and Huolongping.Comparison shows that the slumped rock at Zhangshangjie was most weathered,where the accumulation looks like “steamed bread” shape.Secondary is Wangdahai,where plate-like rock masses accumulated in the river course due to late erosion of water flow.Weak weathering characterizes the rocks around Xiaonanhai,of which the mudstone was weathered relatively stronger,forming concave rocky cavities,though with sharp edges of rock masses overall.At Huolongping far from Xiaonanhai,the landslides show a “mountain stripping” style,where late reform was serious,most rock blocks are covered by soil and vegetation,and also having sharp edges.2 Generation mechanism of the landslides Based on field investigations and dada analysis,this work has explored the formation mechanism of the landslides triggered by the 1856 Daluba earthquake,yielding the following perceptions:1)Under the recent tectonic stress field with a principle compressive stress in NW-SE,the hillslopes of the study area were subject to a shock of seismic acceleration from NW to SE.2)As strata dip direction is consistent with that of seismic shaking,slopes slid along bed interfaces.Due to gentle dip of strata,the slope material slid over long distances.3)There exist two sets of joints in rocks,which were weathered for a long time,with partial structural planes connected and destruction of rock integrity.4)The landslides occurred in a steep mountainous area,where likely exists the enlargement effect of topography to seismic vibration.3.Analysis of slope stability before landsliding at XiaonanhaiConsidering terrain around this place,this work reconstructed the shape of the slope before landsliding,and analyzed its stability by numerical modeling.The results demonstrate that:1)In the static state,the slope is stable.2)In rain seasons with much precipitation,small landslides or gravel falls can happen on the slope.3)Under the triggering of an earthquake,with increasing peak ground motion acceleration(PGA),the safety coefficient of the slope lowers.Particularly,when PGA grows from 0.25 g to 0.75 g,the least safety coefficient drops to 0.78,implying that the slope will fail leading to landsliding.