Semiquantitative Evaluation Of Paleoearthquake Identification From Trenches And Vertical Slip Rates On Normal Faults

Accurate slip rate and long paleoearthquake series are crucial for understanding the recurrence patterns of active faults and evaluating the seismic hazards of faults.Traditional methods using fault scarps or trenches may produce inaccurate estimates of a fault’s vertical slip rate and obtain incomplete paleoearthquake series.In this paper,we take the three faults in the northern margin of Hetao Basin as a case,using the terraces in footwalls and boreholes in hanging wall to calculate the slip rates.We found that the slip rates of the three active faults since the late Pleistocene and Holocene increase from west to east.Based on the findings and a dynamic model of the formation and evolution of the Ordos block,it is concluded that the depositional centre of the Hetao Basin has tended to migrate from west to east.The vertical force generated by deep material movement is the dominant factor leading to a greater vertical slip rate in the eastern portion of the northern margin of theHetao Basin.The modern stress field in the Hetao Basin results in an increase in the vertical slip rate of active faults from west to east along the northern margin of the basin.In addition,in this study,72 paleoearthquake trench sites on normal faults around the world were analyzed,and the main indicators for identifying paleoearthquakes on normal faults include the presence of vertical offset(VO),collapse wedge(CW),fissure(FIS),buried paleosol(BP),angular unconformity(AU),upward termination(UT),and sand liquefaction(LF)features.Applying semiquantitative evaluation of paleoearthquake identification from trenches to normal faults for the first time and proposing classification criteria for identifying paleoearthquakes in trenches on the normal faults.33 trenches for paleoearthquakes on a fault system in the northern margin of the Hetao Basin were comprehensively analyzed to determine Holocene paleoearthquake events on the Sertengshan,Wulashan,and Daqingshan piedmont faults,and the reliability of paleoearthquake events is discussed.The integrity of the paleoearthquake events obtained was tested by the displacement limit method.The Holocene paleoearthquake recurrence on the three faults was quasiperiodic;the coefficients of variation(COVs)were 0.44,0.58,and 0.4.Based on the above study,on the one hand,this paper took the northern marginal faults as a whole and studied the characteristics of slip rate and paleoearthquake recurrence comprehensively.It also provided the data support for assessing the regional seismic hazard.On the other hand,we tried to extend the calculation method of slip rate constrained by both fault walls and semiquantitative evaluation of paleoearthquake identification from trenches to normal faults and verified the accuracy of slip rate and feasibility of semiquantitative evaluation of paleoearthquake events on normal faults.