Late Quaternary Activity Characteristics And Risk Analysis Of Large Earthquakes Of The Zhuozishan West Piedmont Fault

The Wuhai basin is located in a crutial site of tectonic deformation and transformation in the northwestern margin of Ordos.The quantitative research on the characteristics and regularity of the current tectonic activity of the active faults in this area not only helps to predict the potential of regional earthquakes,but also of great significance to understand the tectonic deformation model and mechanism in the northwestern margin of Ordos.This thesis chooses the Zhuozishan west piedmont Fault which lies in the western margin of the Wuhai Basin as the research object,takes geometric characteristics,slip rate,and recurrence characteristics of paleoearthquake as research content.Using interpretation of aerial satellite images,measurement of typical topographical surface displacements,paleoseismic trenching,and chronology,we derive the quantitative parameters of late Quaternary tectonic activity of the fault and some new insights.(1)The Zhuozishan west piedmont fault starts from the southeast of Dengkou County,extending southward by Dalugai,Qianligou,Dongshanbei Community,Huangbaici,terminates at Erdaokan along the NE-SW direction from north to south,with a length of 90 km.The fault trends in NNE,dipping NW at angles of 60-80°.The traces of the Zhuozishan west piedmont fault are zigzag and discontinuous.Based on the geometry,the fault can be roughly divided four segments,i.e.Erdaokan-Wuhai,Wuhai – Qianligou,Qianligou,Dalugai from south to north.The Zhuozishan west piedmont fault is characterized by normal component and offsets different geomorphic surfaces such as river terrace and alluvial fans along the fault with clear marks.Based on the comprehensive analysis of the changes of the vertical displacement of the fault terraces,the results of optically stimulated luminescence dating to the corresponding stratum in paleoseismic trenches and typical fault landforms,it is believed that the fault is still active since the late Quaternary,which belongs to a late Pleistocene fault.The latest event of the fault is constrained closely to(11.4 ± 1.3)ka.(2)According to the displacements measurement and chronological test results of four typical faulted geomorphological survey sites,i.e.Dalugai,Qiangxiao Mausoleum,Dongshanbei Community,and Erdaokan,the Zhuozishan west piedmont fault shows a stable vertical slip rate during different periods in a range of0.07±0.01-0.16±0.05 mm/a.(3)Seven earthquake events have been revealed by three paleoseismic trenches at the Dalugai site,with corresponding age at(73.4 ± 12.9)ka,(64.4 ± 8.2 – 63.7 ±7.4)ka,(58.7 ± 6.5-53.2 ± 7.0)ka,(45.7 ± 5.1-42.6 ± 6.3)ka,(31.3 ± 3.4-31.0 ±3.4)ka,(28.1 ± 3.8-25.7 ± 3.0)ka and(21.0 ± 2.5-15.8 ± 1.9)ka,respectively.The latest event suggests a longer elapsed time of(20.2 ± 2.6-15.8 ± 1.9)ka.Considering the dating errors,we think the earthquake event has a certain recurrence pattern with a recurrence interval of 8-13 ka.(4)According to the relationship between magnitude and surface rupture length and co-seismic average displacement,we infer a magnitude range of 7.4-7.7 for the Zhuozishan west piedmont fault.Based on the seismic gap recognition of historical strong earthquakes,historical earthquake catalogs,and time series maps and recurrence characteristics of paleoearthquakes,we suggest the Zhuozishan west piedmont fault lies in a seismic gap segment and is still in seismic active periods.Theelapsed time of paleoearthquake has approached or exceeded its recurrence interval.Taking consideration of the tectonic background and ability of potential earthquake occurrence,we believe that the potential for strong earthquakes on the Zhuozishan west piedmont fault is high.