Late Quaternary Activity Study Of The Piedmont Fault Zone

The Hetao fault belt located in the ordos block north rim.It is oriented E-W and controlled by the Langshan,Sertengshan,Wulashan and Daqingshan faults from west to east.It is about 600km.Earthquake magnitude and frequency are high,and there are happened many times 6 magnitude erathquack,including a magnitude 8.0 erathquake.So,the beasin is a major earthquake belt in North China and a key research area of the seismic activity.We study the Quatemary seismic activity on here,not only it is significant to recognize the characteristics of strong earthquake recurrence,but the earthquake disaster reduction work has important application value.Sertengshan piedmont fault is located in the south segment of Yingshan mountain,controlling the north boundary of Hetao basin with Wulashan piedmont fault and Daqingshan piedmont fault.The fault is from dongwugaigou in the west,to east,along Hulesitai,Wujiahe town and Wubulangkou,and it deflects to 120°in the direction of southeast,along Kuleubulong,Dashetai town to Tailiang town.It is about 180km.The Sertengshan piedmont developed typical tectonic landforms such as terraces and planation surfaces.The Quatemary activity of Sertengshan piedmont fault is researched in this pepar,including the segmentation of fault,the characteristics of structural landscape and displacements,slip rates of the boundary faults and paleoearthquakes.Through meticulous measurements and quantitative analysis,the main conclusions are drawn as follows:(1)Sertengshan piedmont fault belt is about 180km.According to the characteristics of geometrical distribution,structural landscape,kinematic of the fault and paleoearthquakes,we can be divided into characteristics of different 4 segments:Langshankou segment,Hongqicun segment,Kuluebulong segment and Dashetai segment.There is a T-shaped junction between Hongqicun segment and Kuluebulong segment.The mean heights of T1,T2 and T3 are 6.8 m,15.1 m and 24.1 m,respectively.Along the Sertengshan piedmont fault,three-level terraces are present west of Wayaotan,whereas two-level terraces are present east of Wayaotan.The Wujiahe and Hongqicun segments feature relatively low T1 heights but relatively high T3 heights.The Kuoluebulong segment has higher T1 heights but lower T3 heights.The Dashetai segment has lower T1 heights and lacks T3 terraces.Because the terrace levels and heights along different segments represent the different degrees of fault activity on the different segments,we conclude that the Wujiahe and Hongqicun segments have been highly active since the terraces formed,the Kuoluebulong segment has been less active,and the Dashetai segment has been the least active.(2)We selected nine sites uniformly along the fault to study the fault's vertical slip rates.The resule shows that the elevation of T3 terraces is about 1060m,the elevatioin of T1 terraces is about 1043m.The geological boreholes in the basin adjacent to the nine study sites reveal that,the bottom elevation of the Holocene series is 1017-1035m and the top elevation of the lacustrine strata is 925-1009m.We also know that the top of the lacustrine strata is about 65ka old according to the terraces and the boreholes.So we obtained the vertical slip rate which is 0.74-1.81mm/a since 65ka based on the 57-117m vertical displacements of the stratigraphic marcker strata,is 0.86-2.28mm/a since the Holocene based on the 10-27m vertical displacements of the bottom of the Holocene series.The slip rate is the highest along the Wujiahe segment and lower to the west and east,which is in accord with the number of terrace levels and terrace heights along the Sertengshan piedmont fault.(3)Assuming that the axis of the principal tensile stress in the Hetao graben basin has an orientation of 138.66°,we calculated the vertical component of the principal tensile stress considering the strike direction of each fault along the northern margin of the Hetao graben basin.The result shows that the vertical slip rates of the Daqingshan and Langshan faults are higher than those of the Sertengshan and Wulashan faults.The strike-slip rates of these faults are low because the strike-slip component of the principal tensile stress is small,and the right-stepping distribution limits stress propagation.The Wulashan mountains block the left-lateral strike-slip movement of the Sertengshan fault,whereas the Daqingshan fault has a higher rate of left-lateral strike-slip because it is not completely blocked.Based on the analysis of the historical and modern instrumental records of earthquake epicenter distributions and the seismogenic structure,we think that the Langshankou,Wujiahe,and Wubulangkou faults,as well as the eastern end of the Sertengshan fault and the Baotou area of the Daqingshan fault,are at a high risk of earthquakes in the future.Among these areas,the Baotou area has a high risk of devastating earthquakes.The whole fault activity is basically identical in the period of late Pleistocene,and the time of T3 terrrace formation of the 4 segments is same.The sedimentary strata of T3 terrace is yellow-green sand sediments.The time is same as the time of the highest water level of Jilantai Megalake.Since the Holocene,Sertengshan piedmont fault activity has obvious changes,the entire fracture activity increase gradually from east to west.(4)Comprehensive trenches revealed that,6 paleoseismic events have occurred on the Kuoluebulong segment since about 30ka BP,the occurrence times of which are successively 19.01-37.56ka,18.73ka,15.03-15.86ka,10.96ka,5.77-6.48ka and 2.32ka BP.Trenches also revealed that 6 paleoseismic events have occurred on the Dashetai segment since about 30ka BP,the occurrence times of which are successively 29.07ka,19.12-28.23ka,13.92-15.22ka,9.38-9.83ka,6.08-8.36ka and 3.59ka BP.The results indicate the quasi-periodically clustering recurrence behavior along the two segments with about 4000a mean recurrence interval.