SEISMOLOGY AND GEOLOGY ›› 2020, Vol. 42 ›› Issue (5): 1039-1057.DOI: 10.3969/j.issn.0253-4967.2020.05.002

• Research paper • Previous Articles     Next Articles

STUDY ON PALEO-SEISMIC EVENTS IN TRENCHES OF THE EASTERN QIULITAGE ANTICLINAL BELT

ZHANG Ling1), YANG Xiao-ping1), LI Sheng-qiang1,2), HUANG Wei-liang1,3), YANG Hai-bo1)   

  1. 1)State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;
    2)Hebei Earthquake Agency, Shijiazhuang 050021, China;
    3)Key Laboratory of Western China Mineral Resources and Geological Engineering, College of Engineering and Surveying of Chang'an University, Xi'an 710064, China
  • Received:2020-01-06 Revised:2020-03-16 Online:2020-10-20 Published:2021-01-06

秋里塔格褶皱带东段探槽的古地震事件

张玲1), 杨晓平1),*, 李胜强1,2), 黄伟亮1,3), 杨海波1)   

  1. 1)中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029;
    2)河北省地震局, 石家庄 050021;
    3)长安大学, 西部矿产资源与地质工程教育部重点实验室, 西安 710064
  • 通讯作者: *杨晓平, 男, 1962年生, 研究员, 现主要研究方向为活动构造及地震危险性评价, E-mail: yangxiaoping-1@163.com。
  • 作者简介:张玲, 女, 1986年生, 2016年于中国地震局地质研究所获活动构造专业博士学位, 现为中国地震局地质研究所国家动力学重点实验室博士后, 主要从事活动构造和现今地壳形变研究, 电话: 15117984244, E-mail: zhangling4255@126.com。
  • 基金资助:
    全国重点监视防御区活动断层地震危险性评价项目(1521044025)、 区划图预研 “逆冲型构造区潜在震源划分原则与三维模型构建方法”和西北潜在震源调整研究共同资助

Abstract: In response to the ongoing far-field effects of the India-Eurasia collision, the Tianshan Mountains experience rapid NS convergence, and most of the present N-S shortening is absorbed along the southern and northern edges. The resultant frequent large earthquakes have inspired many scientists to explore the neotectonic activity of the Tianshan Mountains. The eastern Qiulitage anticlinal belt located in the Kuqa depression, on the southern piedmont of the Tianshan Mountains, is a typical blind fault-related fold. The Kuqa M7$\frac{1}{4}$ earthquake in 1949 as typical folding earthquake once occurred on the northern limb of the eastern segment of the Qiulitage anticline, and the epicenter was near the Village of Kang which is sparsely populated. This earthquake is a typical folding earthquake whose dominant fault did not thrust onto the earth surface. Although many tectonic-induced scarps and deformed Quaternary strata have been reported, there are still no direct evidences for the surface ruptures and corresponding causative faults of this earthquake at present. And systematic understanding of paleoseismic events in Qiulitage area is also limited by the lack of relevant chronological researches.
We conducted 1︰50 000 scale geological mapping in the Qiulitage anticline area. The local surface geological characteristics are investigated based on interpretation of Google Earth image and confirmation in the field. Together with interpreted subsurface structure by petroleum seismic reflection profiles, the relationship between the active faults thrust on the surface, low-dip-angle decollement faults in deep, and fold deformation are subsequently qualitatively analyzed. In this study, the active faults which have thrust to the surface and generated fault scarps are focused on.
Totally five trenches were chosen and cleared up, two of which are located on the southern limb of the eastern Qiulitage anticline and the others are on its northern limb. And all excavation sites are situated on fresh fault scarps. We carefully interpreted different characteristics of tectonic deformation and sedimentary process which are correlated with paleo-seismic events from trenches. According to the OSL(Optically Stimulated Luminescence)and 14C dating results, a reliable chronological framework for the deformed stratigraphic sequences was established. Based on the classic successive limiting method, six paleoseismic events were finally constrained.
Some of these interpreted paleo-seismic events produced surface ruptures on the breakthrough faults simultaneously on the southern and northern limbs of the Qiulitage anticline, and others only caused local surface ruptures on its northern limb. In a broad sense, the surface ruptures caused by these paleoseismic events have similar characteristics to those which are popular among the low-dip-angle thrust faults on the southern piedmont of the Tianshan Mountains. And the two common phenomena are that multiple ruptures may occur a single fault and multiple faults may rupture simultaneously. We speculate that only when the displacement of master faults at depth is big enough, multiple shallow secondary faults can be triggered at the same time. Conversely, only one fault is active at one time. In other words, constrained by the length and displacement of dominant faults, not all paloseismic events can cause surface ruptures on the northern and southern limbs of the Qiulitage anticline at the same time.
The revealed paleoearthquakes may have a clustering feature since ~7.4ka. They behaved as follows: 1)Three events occurred during 5.7~7.4ka. 2)one event occurred during 3.3~4.7ka. 3)the latest cluster of events may be marked by the 1949 MW7$\frac{1}{4}$ Kuqa earthquake. Thus, the earthquake sequences have a recurrence period of about 2.5~4ka.
Significantly, the incompleteness of the paloseismic events recorded in trenches and the quality and intrinsic error of the OSL dating samples can mislead judgments. It is inevitable that the time of paloseismic event cannot be constrained strictly. In our research area, because of the lack of seismic events between event E5 and event E6(7.25~19.1ka), there is a gap in seismic event records for up to~11.85ka. However, our result offers a relatively systemic event sequence to fill the gap in studies on paleoseismicity in this area. Whether there will be a strong shock after the 1949 MW7$\frac{1}{4}$ Kuqa earthquake remains to be further studied in detail.

Key words: paleo-seismic event, Qiulitage anticline, thrust-fold belt, Kuqa earthquake, Tianshan

摘要: 位于天山南麓库车坳陷中的东秋里塔格背斜带是典型的活动逆断裂褶皱带, 1949年的库车M7$\frac{1}{4}$地震发生在该逆断裂褶皱带上。 目前, 仍然没有发现此次大地震的地表破裂带, 也没有确定其发震断层。 同时, 由于这一地区相关的年代学研究匮乏, 对于这一地区的古地震事件仍然缺乏系统性地认识。 文中基于野外调查获取的地表地质特征和通过石油地震反射剖面解读的深部构造, 定性地分析了研究区内的地表活动断层与深部低倾角滑脱断层、 褶皱变形的关系。 我们聚焦于冲破至地表的活动断层, 选择在库车塔吾背斜北翼和东秋里塔格背斜南翼的突破断层上开挖了5个探槽, 分析了探槽中与古地震事件相关的构造和地层沉积特征。 根据OSL(光释光)和14C测年结果, 利用逐次限定法识别出了6次古地震事件。 在这些古地震事件中, 有的地表破裂同时发生在褶皱带南、 北两翼的突破断层上, 有的仅在褶皱带北翼的突破断层上发生。 总体而言, 这些古地震事件引起的地表破裂具有南天山低角度逆断层古地震破裂变形普遍存在的2种现象, 即单条断层多次破裂和多条断层同时破裂。 已揭示的古地震事件具有一定的丛集性, 在距今7.4ka以来具体表现为: 距今5.7~7.4ka共发生了3次古地震事件, 在距今3.3~4.7ka发生了1次古地震事件, 最新的丛集事件可能以1949年库车M7$\frac{1}{4}$地震的发生为标志。 研究区地震丛集的发生近似有2.5~4ka的重复周期, 1949年库车M7$\frac{1}{4}$地震之后是否还会发生强震, 还有待进一步的观察和详细研究。

关键词: 古地震, 秋里塔格背斜带, 逆断裂褶皱带, 库车地震, 天山

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