The Holocene Activity and Its Evidence from Paleoearthquake of the Middle Segment of Wudaoliang-Changshagongma Fault Inside the Bayan Har Block
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摘要:
为揭示巴颜喀拉块体内部构造变形特征，基于地震地质调查和新年代学方法研究了五道梁-长沙贡玛中段的最新活动性.地质地貌和探槽揭露的证据显示，五道梁-长沙贡玛断裂中段为全新世活动的左旋走滑断裂，也表明其作为巴颜喀拉块体内部的主干断裂之一，具有孕育和发生强震的构造条件.该断裂中段的最晚一次古地震事件发生在（4 409~4 225）a BP之后，距今可能超过千年；而且近两年来断裂中东段小震活动频繁，2020年4月1日发生了5.6级中强地震，其未来的地震危险性需引起关注.
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关键词:
- 青藏高原 /
- 五道梁-长沙贡玛断裂 /
- 巴颜喀拉块体 /
- 地震危险性 /
- 地震学
Abstract:To reveal the internal tectonic deformation characteristics of the Bayan Har block, this paper studies the middle segment of the Wudaoliang-Changshagongma fault activity of the Late Quaternary based on seismic geological survey and new chronological methods. The geological and geomorphological evidences show that the middle segment of the Wudaoliang-Changshagongma fault is a Holocene left-lateral strike-slip fault, indicating that the fault has tectonic condition for generating strong earthquakes as one of the main faults inside the Bayan Har block. The latest paleoearthquake event on the middle segment of the fault occurred after (4 409-4 225) a BP. The elapsed time may be more than a thousand years. Moreover, small earthquakes have been active on this segment in the past two years. And a M5.6 earthquake occurred on it on April 1st, 2020. So, we should pay attention to the strong earthquake risk of the fault in the future.
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图 1 巴颜喀拉块体中东部主要活动断裂及地震活动分布
断裂数据修改自徐锡伟等（2016）；GPS数据引自Wang and Shen（2020）；紫色实线框为研究断裂段范围
Fig. 1. Distribution of main active faults and seismic activity in the middle-eastern part of the Bayan Har block
图 2 五道梁‒长沙贡玛断裂中段展布
底图数据采用ALOS 12.5 m的DEM数据
Fig. 2. Distribution of the middle segment of the Wudaoliang-Changshagongma fault
图 3 五道梁‒长沙贡玛断裂新活动行迹
a.为哈曲巴玛沟以西，长沙贡玛盆地北缘的五道梁‒长沙断裂中段构造地貌；b.哈曲尕玛沟附近一带的断裂中段构造地貌；底图为Google Earth影像
Fig. 3. New active trails of the Wudaoliang-Changshagongma fault
图 4 阿日扎镇约达村一带断裂剖面与Ⅱ级阶地的砂砾石层变形
a.阿日扎镇北俄溪河东岸，五道梁‒长沙贡玛断裂发育于砖红色古近系砾岩与早‒中更新统砂砾岩之间；b.断层活动导致俄溪河东岸Ⅱ级阶地的砂砾石层倾斜变形
Fig. 4. The fault profile of the Wudaoliang-Changshagongma fault and the deformation of the gravel layer of the Ⅱ-level terrace near Yueda Village, Arizha Town
图 5 火然村北部的断层槽谷与断层陡坎地貌
a.俄布绒河出山口西侧，断裂发育于古近系砂砾岩与三叠系板岩、变质砂岩之间；b. 俄布绒河出山口东，山前洪积扇上形成的断层陡坎及坎下发育沼泽带
Fig. 5. Fault landforms near the north of the Huoran Village
图 6 掀日玛沟口一带的断层陡坎地貌
a.掀日玛沟口附近洪积扇上形成高约0.3~0.6 m的反向陡坎；b.照片远处见断裂发育于砖红色古近系砂砾岩与三叠系变质砂岩之间，近处断裂在洪积扇上形成高约1.6 m的断层陡坎
Fig. 6. The fault scarp landforms near the mouth of Xianrimagou river
图 7 哈曲巴玛沟口西山前洪积扇上的断层陡坎地貌
a.断裂在哈曲巴玛沟口西的山前冲洪积扇分为多支次级断层；b.断裂在最新一期冲洪积扇上形成高约0.6 m的断层陡坎
Fig. 7. The fault scarp landforms on the alluvial fan in the east of the Haqubama river
图 8 哈曲巴玛沟口西山前洪积扇的挤压脊与断塞塘
Fig. 8. The contrusion ridge and sag pond developed on the alluvial fan near the mouth of the Haqubama river
图 9 哈曲巴玛沟东山前洪积扇上的断层陡坎
a.哈曲巴玛沟东的宽断层槽谷中发育有高约0.2~0.4 m的反向小陡坎，可能为最新一次地震活动所形成；b.洪积扇上形成的断层线性陡坎
Fig. 9. The fault scarps developed on the alluvial fan near the mouth of the Haqubama river
图 10 冲沟被断裂水平位错约10.5 m
a. 哈曲巴玛沟口东山前洪积扇上见一冲沟被断裂左旋位错约10.5 m；b.洪积扇的取样照片和地层剖面
Fig. 10. A gully was dislated by the fault with a horizontal displacement of about 10.5 m
图 11 火然村北山前洪积扇跨断层陡坎开挖2个探槽
a.探槽TC1开挖的照片；b.探槽TC2开挖的照片
Fig. 11. Two trenches were excavated across the fault scarp on the alluvial fan in the northern of the Huoran Village
图 12 探槽TC1南东壁照片及解释剖面
Fig. 12. Photograph and interpretation section of the south-eastern wall of trench TC1
图 13 探槽TC1南东壁断层F2的局部特征
Fig. 13. Photograph showing the characteristic of the F2 that revealed in the south-eastern wall of trench TC1
图 14 探槽TC2北西壁照片及解释剖面
Fig. 14. Photograph and interpretation section of the north-western wall of trench TC2
图 15 探槽TC2揭露的标志层被断错的局部照片
Fig. 15. The partial photo showing the mark layer U3-1 were dislocated by the F2 that reveal in the trench TC2
表 1 年代样品测试结果
Table 1. The results of the chronological samples
取样编号 实验室编号 样品描述 δ13C（‰） 测试年龄（a BP） 校正年龄（a BP） WC-C14-01 Beta-607296 炭屑 ‒24.4 3 840±30 4 405~4 220 WCTC-C14-01 Beta-607291 炭屑 ‒20.4 12 430±40 15 052~14 782 WCTC-C14-06 Beta-607292 炭屑 ‒23.4 3 830±30 4 409~4 225 
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