塔藏断裂位于东昆仑断裂带东段,属于"玛曲空区"范围,汶川MS8.0级地震对该区域造成了应力的加载,但是本区尚未开展古地震方面研究,地震危险性认识不足。通过断错地貌实测、古地震探槽、 14 C定年等方法,在坡中谷、断塞塘附近位置开挖两处探槽,获得了一些古地震方面的认识。探槽附近微地貌测量结果表明,断错量主要分布在两个范围区间72m和71m,以及27m,25和22m,两区间之间近倍数关系,推测由多次地震事件造成。两处探槽采集 14 C样品共计19个,探槽剖面分别记录了2次古地震事件,事件发生的层位与年龄具有一定的可比性,经逐次限定后得到2次古地震事件,时间分别为 320±30~500±30aB.P.和 2730±30~2840±30aB.P.; 最近一次古地震离逝时间为 320±30~500±30aB.P.; 实际重复间隔约为2300a,与东昆仑断裂玛曲段的重复间隔较为接近。
The East Kunlun active fault zone in northern Tibetan is typically characterized by sinistral strike-lip fault.Tazang Fault is situated in the east segment of the East Kunlun active fault as well as in the scope of "Maqu seismic gap".The coulomb stress on the region around Tazang fault was increased due to the Wenchuan MS8.0 earthquake.However,there is little quake-proof research work about paleoearthquakes and the seismic risk of the region has not been profoundly recognized.Based on field investigations,the interpretation of remote sensing images,the measurement of earthquake-related landforms by difference global positioning system,trench excavating and radiocarbon dating of 14 C by Accelerator Mass Spectrometry(AMS),the paper tries to find out some evidences and knowledge about the characteristics of paleoearthquakes of the fault in the Late Quaternary.Two trenches were excavated near the sag-ponds,the lower edge of reverse scar and slop-hanging-ditch structure,which is perpendicular to the fault-strike.High-precision surveying work was carried out by the method of difference global positioning system around the two trenches in order to carefully study the characteristic of geomorphology near the trenches and define the positions of trenches.According to the statistics of 1364 difference global positioning system measurement data around Trench 1(33°45'45.50″N,103°15'25.70″E),the left-slip displacements of three nearly north-south trending gullies perpendicular to the fault-strike are respectively as follows: 72m,27m,and 25m.On the other hand,based on the 3566 difference global positioning system measurement data around Trench 2(33°47'37.00″N,103°10'8.50″E),which is 10km to the west of Trench 1,the left-slip displacement of a nearly north-south trending terrace ridge on the west side of the trench nearly perpendicular to the fault-strike is about 71m and the left-slip displacement of a nearly north-south trending terrace ridge on the east side of the trench is about 22m. All above displacements are mainly distributed in the two ranges: 72~71m and 27~22m,which are close to multiple relation,probably caused by paleoearthquakes.7 dating samples of 14 C from the west geological log of Trench 1 and 12 dating samples of 14 C from the east geological log of Trench 2 were taken,and 19 final infallible results of radiocarbon dating came out,which were tested by the Beta Analytic Limited utilized Accelerator Mass Spectrometry and chronological calibration.2 paleo-earthquake events since the Late Quaternary are uncovered by the two trenches.Based on the comparison diagram of the two trenches’stratigraphic columns,dating results of 14 C and multiple trench constraining method,a conclusion can be made that the occurrence time of the 2 constrained events are respectively as follows: 320±30~500±30aB.P.,2730±30~2840±30 aB.P.,and the departure time of the most recent earthquake is 320±30~500±30aB.P.,and the recurrence interval of earthquake near the fault is about 2300a.Given above discussion,it shows consistency between the Tazang fault and Maqu fault in the recurrence interval.Further,Tazang Fault remains not only encircled in a growth stage of coulomb stress but also situated in the seismic gap,and has certain seismic risk.The future seismic risk of this region deserves more attention.This paper firstly concludes that 2 paleoearthquake events occurred along the Tazang Fault,and enriches the information of seismicity of the East Kunlun active fault zone.It will perform a role in seismic hazard assessment in this region.