Study On The Characteristics Of Active Behavior On The Northern Yumushan Fault Based On High-Resolution Aerial Survey Data

The northern Yumushan fault,located in the middle of the Hexi Corridor,is the boundary fault between the Yumu Shan uplift and the Hexi Corridor.Clarifying the geometric distribution,coseismic displacement,and paleoseismic elapsed time of this fault is not only important for understanding the attribution of historical earthquakes but also helpful in understanding the structural deformation characteristics and active behavior of the northern Yumushan fault,which is of great significance for its future earthquake hazard assessment.This thesis takes the northern Yunushan fault as subjects and mainly discussed the geometric structure,faulted landform characteristics,displacement spatial distribution characteristics,paleoseismic elapsed time,and recurrence characteristics of the fault.Based on high-precision terrain data,using remote sensing image interpretation,micro-topographic analysis,faulted landform measurement,cumulative offset probability distribution,and chronology dating technology,the quantitative parameters of late Quaternary activity,such as rupture length,coseismic displacement,and paleoseismic event periods of this fault were obtained.The main conclusions of this thesis are as follows:(1)The northern Yumushan fault starts from Liyuan River in the east and extends westward along NW-SE direction,passing through Diaochaigou,Fengshankou,Dadoukou,Huaerchaikou,Mutougou,Wutongquan,Xunzigou,and ends at Yuanshanzi,with a strike of 275°-335° and a dip angle of 43°-61°.The fault is consisting of multi-level faults with a total length of approximately 61 km.The fault offsets the alluvial fan at different stages,forming discontinuous and different heights fault scarp.The older the geomorphic surface,the higher the fault scarps,reflecting the long-term continuous activity of the fault.According to the faulted landform characteristics and active behavior,it can be divided into three segments: eastern segment(from Liyuan River to Xidakou),middle segment(from Xidakou to Dahongshangou),and western segment(from Dahongshangou to Yuanshanzi).(2)According to the distribution characteristics of the fault,the cumulative displacement exhibits-approximately symmetrical distribution along the fault.The maximum vertical displacement of the northern Yumushan fault appears in the Mutougou and Shitankou areas in the middle a part,and gradually decreasing towards the both sides,reflecting the differences in the activity of each secondary segment.This indicates that the fault activity is the strongest in the middle segment of the northern Yumushan fault,while the activity of the fault gradually weakens towards the surrounding areas.Additionally,from the distribution characteristics of the slip cluster,there are peak values of vertical slip along the fault at 1.7 m,3.2 m,5.3 m,and 7.6 m,revealing that there have been at least four or more earthquake events in the northern Yumushan fault and resulting in different cumulative slip rates and obvious multiples of increase on the fault scarps of different periods.This is in accordance with the characteristic slip patterns,and in conjunction with paleoseismic studies and dating results of fault scarps,it reflects the conformity of the northern Yumushan fault to a characteristic seismic model.(3)Based on micro-geomorphic analysis,slip measurement,and chronostratigraphic dating at three typical sites of the northern Yumushan fault(Huaerchaikou,Dadoukou,and Fengshankou),the latest earthquake event was constrained.The earliest Holocene alluvial fan deposits that have not been faulted on the northern Yumushan fault was dated to 2775-2940 cal.a B.P.,indicating that the latest earthquake activity occurred before 2775-2940 cal.a B.P..Combining previous studies,the latest earthquake event on he northern Yumushan faul is constrained to have occurred between approximately 2775-2940 cal.a B.P.and 3940-4155 cal.a B.P..According to the results,it is inferred that the Biaoshi(Gaotai)Mw 71/2 earthquake in180 AD did not occur on the northern Yumushan fault.(4)Based on the rupture length and coseismic displacement of the fault,combined with empirical formulas,it is estimated that the potential earthquake magnitude of the northern Yumushan fault is around Mw 7.2.In addition,considering the seismic recurrence characteristics,earthquake recurrence intervals,and b-value scanning results,it is believed that the elapsed time on the northern Yumushan fault is close to or exceeds the earthquake recurrence interval,and the stress accumulation level is high,indicating that there is a high seismic risk.