Significance Of Historical Landslides In Qishan Longxian Area To The Focal Parameters Of BC780 Qishan Earthquake

Because there are no instrument records for historical strong earthquakes before AD1900,the determination of their source parameters is often based on historical seismic records,but the limited documentation alone cannot eliminate the uncertainty of the source parameters,especially for the long history.In terms of few historically difficult earthquakes.For some strong earthquakes and large earthquakes that trigger secondary disasters,its polar earthquake zone/heavy destruction zone range can be obtained by combining the description of historical earthquake damage and the spatial distribution and characteristics of secondary disasters with high-resolution remote sensing interpretation.The extension of the relatively heavily damaged area of ??earthquakes and major earthquakes along the seismogenic faults can approximately represent the extension of the rupture,which is of great significance for determining the scope of the macroseismic area,macroepicenter,intensity of the polar earthquake area and the structure of the earthquake.The southwestern margin of the Ordos block is a region where historical strong earthquakes occur frequently.Due to the long history and the unknown or missing historical data,the magnitude,macro epicenter,polar earthquake area range and seismic structure of some historical strong earthquakes in the area recorded by historical materials have been there is controversy.The Qishan earthquake in BC780 was the earliest destructive earthquake recorded in the history of China.The "Sanchuan exhaustion,Qishan collapse","Hundreds of rivers boiled,and the mountain tomb fell.High shore turns into valley,deep valley turns into hill" recorded in the "Book of Songs" and "Historical Records" shows that this earthquake triggered a large number of co-seismic landslides,so this The earthquake should be a strong earthquake event,but there is a lack of comprehensive systematic research.There are many controversies about this earthquake.Trying to use high-resolution remote sensing images to interpret the seismic landslides related to the earthquake will be a research worth exploring.In this paper,based on the basic principle of "considering the present and the ancient",referring to the interpretation method of landslide investigation after theoccurrence of strong earthquakes,combined with the historical document sorting and field investigation,the high-resolution remote sensing image is used to interpret the historical landslide in Qishan Longxian area in detail.The historical landslide distribution maps in this area were obtained.All the interpreted landslides were manually assigned parameters and a co-seismic landslide database was established.By comparing the rainfall landslide database with the AD1718 Tongwei earthquake landslide database,the geometric characteristics and distribution of historical landslides in the study area were analyzed.According to the methods of drone photogrammetry of single landslide,drilling of barren lake sediments,accurate calculation of landslide and barrage volume,and collection and determination of the age of single landslide,some typical landslides in the region were carefully studied.Finally,with reference to the results of modern earthquakes-2008 Wenchuan earthquake-triggered landslide research,the possible magnitude,epicenter,and structure of the BC780 Qishan earthquake were discussed in conjunction with the distribution of landslides in dense areas.The results showed that:(1)As a reference comparative study of the Wenchuan earthquake landslide,A large number of small landslides(single landslide area <10,000 m2)contribute very little to the total area and volume of the landslide triggered by a strong earthquake,and nearly 80% of the landslides are distributed within the ? degree zone.Therefore,large-scale landslides in dense areas that can be identified today can represent the main body of historical earthquake landslides,and can also reflect the scale of a historical earthquake landslide.(2)There are 6,876 landslides in Qishan Longxian landslide dense area,with a total area of 643 km2.The landslide-intensive area is mainly distributed unilaterally along the Longxian-Qishan-Mazhao fault in the Huangtugou valley area on the north side of the fault.There are few dense landslides distributed in the I,II,III loess plateau areas,Weihe terraces and floodplains under the terrain conditions.At the same time,there is no direct interpretation of dense seismic landslides in the surface bedrock exposed areas.(3)Comparing the historical landslides in the dense area with the 2010 Tianshuirainfall landslide and the 1718 Tongwei earthquake landslide in terms of area,length,width,aspect ratio and other parameters,it can be shown that the main body of the historical landslide in the dense area should be triggered by an earthquake event rather than caused by rainfall.(4)The AMS-14 C age of the organic carbon at the bottom of the borehole in the Qiuzigou barrier lake is 2,890±30 BP,which coincides with the time of the Qishan earthquake in BC780.It is speculated that the two landslides in Qiuzigou were caused by the Qishan earthquake.Using high-precision unmanned aerial vehicle DEM,the volume of the landslide body on the right bank and left bank of Qiuzigou is calculated to be 235×104 m3 and 229×104 m3,respectively.It is 573×104 m3;the volume of the two dams in Sanlangtan is about 363×104 m3 and 50.76×104 m3,respectively.According to the deposition rate and the area of the respective basin,the deposition time is estimated to exceed about 2,760 and 2,669 years,respectively.(5)Combined with the comprehensive analysis of the age of single landslides and the distribution characteristics of landslides,it is considered that the main body of dense historical landslides explained in this paper is the most likely to be caused by the BC780 Qishan earthquake.Based on this,the BC780 Qishan earthquake with a magnitude of 7.6(Mw7.3),the epicenter is located in the northwest direction of Qishan County,near Fengxiang,and the earthquake structure is the Longxian-Qishan section of the Longxian-Qishan-Mazhao fault."Sanchuan exhaustion,Qishan landslide" can be explained as: The BC780 Qishan earthquake triggered a large number of landslides in the Loess Valley area of Qishan Longxian.The landslide material blocked the river valley and formed a dammed lake.The dammed lake caused some tributaries of the Jinghe River and Weihe River to be blocked for a long time after the earthquake.As a result,the amount of water in the main stream of the Jinghe River and Weihe River decreased significantly.Instead of causing the three rivers of Jinghe river,Weihe river and Luohe river to be cut off.