Late quaternary slip rate and paleoseismic sequence of the Cuopuhu section of the Litang-Yidun fault, western Sichuan, China
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摘要:
理塘-义敦断裂是川滇菱形块体内部一条延伸约130 km的全新世活动的左旋走滑断裂带，是川西理塘地区地震活动的重要控震构造，目前其北段的措普湖段研究程度相对较低。将理塘-义敦断裂措普湖段作为研究对象，运用野外勘察、高精度测绘、探槽与14C测年等方法对措普湖段进行滑动速率和古地震探究。开挖的2处探槽位于冬欧山坡麓处，通过识别探槽内断裂与地层的切割关系、地层沉积特征、断层运动性质等标志；共识别出4次古地震事件：事件Ⅰ发生于BC 3382± 60 a之前；事件Ⅱ发生于BC 3382±60 a~BC 1094±51 a之间；事件Ⅲ与事件Ⅳ均发生于AD 1330±44 a之后。可以推断理塘-义敦断裂措普湖段具有大概率的古地震复发间隔为2.4 ka左右，不排除有小概率复发间隔0.4±0.3 ka的可能。理塘-义敦断裂措普湖段古地震事件与大毛垭坝段和理塘段古地震事件之间存在差异，但是不同分段断裂的地震活动性在全新世以来均表现出持续增强趋势。根据测绘断错地貌和末次冰期冰碛垄推算出晚更新世以来措普湖段平均滑动速率为4.15±0.5 mm/a，与理塘-义敦断裂第四纪晚期不同分支滑动速率处于同一量级水平。文章完善了理塘-义敦断裂的构造特征全貌和古地震、滑动速率等信息，有助于更好地理解该断裂及该地区地震活动史和构造变形模式，为今后地震的中长期预测提供更多的数据，同时也有助于川藏铁路沿线相关工程的地震风险评估。
Abstract:Objective The Litang-Yidun fault is a left-lateral strike-slip active fault zone extending approximately 130 km in the Sichuan-Yunnan rhombic block in the Holocene. As a significant seismogenic structure controlling seismic activity in the Litang area of western Sichuan, research on both paleoseismicity and surface ruptures primarily focuses on the Litang and Damaoyaba sections, with relatively limited study on the Cuopuhu section in the northern part. Detailed investigation of the Cuopuhu section can provide fundamental information on its activity characteristics, paleoseismic events, and slip rates.
Methods The Cuopuhu section of the Litang-Yidun fault was investigated using field surveys, high-precision mapping, trenching, and 14C dating methods to explore its slip rate and paleoseismic events. Two trench sites were excavated at the foothills of Dongou Mountain to identify the relationships between faulting and strata, sedimentary characteristics, and fault motion.
Conclusion Four paleoseismic events were identified: Event Ⅰ occurred before BC 3382±60 a; Event Ⅱ occurred between BC 3382±60 a and BC 1094±51 a; Events Ⅲ and Ⅳ occurred after AD 1330±44 a. The recurrence intervals of the four earthquakes are approximately 0.4±0.3 ka, 2.42±0.1 ka, and 2.40±0.1 ka, respectively. Based on the calculated intervals, Events Ⅰ and Ⅱ, and Events Ⅱ and Ⅲ, Ⅳ, have recurrence intervals of about 2.4 ka. Events Ⅲ and Ⅳ occurred after AD 1330±44 a, making it difficult to determine their sequence and exact timing. It can be inferred that the Cuopuhu section of the Litang-Yidun fault likely has a recurrence interval of about 2.4 ka for paleoseismic events, with a possibility of seismic events with recurrence intervals of 0.4±0.3 ka. By comparing the research data between the Cuopuhu section and the Litang and Damaoyaba segments, differences in paleoseismic events between the Cuopuhu section and the other sections are evident. However, the seismic activity of different fault sections has shown a sustained strengthening trend since the Holocene. Based on mapped fault scarps and moraine ridges from the last glacial period, the average slip rate of the Cuopuhu section since the Late Pleistocene is estimated to be 4.15±0.5 mm/a, similar to the slip rates of different branches of the Litang-Yidun fault in the late Quaternary period.
Significance This study provides information on the tectonic features, paleoseismicity, and slip rates of the Litang-Yidun fault, aiding in a better understanding of the seismic history and structural deformation patterns in the area and giving more data for medium- and long-term earthquake prediction in the future. It also contributes to the seismic risk assessment of relevant projects along the Sichuan-Tibet Railway.
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Key words:
- active fault /
- Litang-Yidun fault /
- paleoearthquake /
- slip rate /
- Cuopuhu /
- eastern Tibetan Plateau
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表 1 理塘-义敦断裂(措普湖段)主要断错地貌位错量一览表
Table 1. Overview of the main fault scarps and displacement amounts along the Litang-Yidun fault (Cuopuhu section)
序号 位移点位置/(°) 位移量/m 被断错的地质体 经度 纬度 水平 垂直 1 99.6085 30.4607 21±3 0.4 拉隆公玛沟口东冲洪积扇侧缘 2 99.5886 30.4749 6.8 0.7 章德北东山前冲洪积扇上一纹沟 3 99.5878 30.4745 8 - 章德北东山前冲洪积扇上一纹沟 4 99.5235 30.5022 8.8 1.3 措普湖东边冰碛台地上一冲沟 5 99.5533 30.4824 53±4 48±4 - 措普湖南东冰碛垄和冰水垄岗侧缘 6 99.5279 30.5014 76±10 措普湖西山前冰碛扇侧缘 7 99.4435 30.5422 - 2 别宗隆洼沟西阿冬山前冲洪积扇 8 99.4424 30.5421 - 2.5 别宗隆洼沟西阿冬山前冲洪积扇 9 99.4410 30.5424 - 2~3 别宗隆洼沟西阿冬山前冲洪积扇上断层陡坎高约2~3 m 10 99.4405 30.5430 - 3 别宗隆洼沟西阿冬山前冲洪积扇 11 99.4379 30.5430 - 3~4 别宗隆洼沟西阿冬山前冲洪积扇 12 99.4368 30.5434 - 2 别宗隆洼沟西阿冬山前冲洪积扇 13 99.4365 30.5434 - 3 别宗隆洼沟西阿冬山前冲洪积扇 14 99.4357 30.5434 - 2.5 别宗隆洼沟西阿冬山前冲洪积扇 15 99.4351 30.5436 - 1 别宗隆洼沟西阿冬山前冲洪积扇 16 99.4926 30.5308 - 2~3 章柯河北岸陡石山前冲洪积扇 17 99.4943 30.5300 - 3 章柯河北岸陡石山前冲洪积扇 18 99.4966 30.5290 - 2 章柯河北岸陡石山前冲洪积扇 
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表 2 措普湖段(F3)探槽内及拉隆公玛沟口东冲洪积扇侧缘壁14C样品数据
Table 2. 14C sample data from trenches within the Cuopuhu section (F3) and the sidewall of alluvial fan at the east margin of the Lalonggongma gully
实验室编号 样品编号 样品描述 物质 碳峰值/ % 常规年龄/ a B.P. 95.4%(2σ)标准年龄/ Cal a B.P. 68.2%(2σ)标准年龄/ Cal a B.P. LUG12-153 CPTC1-1 黄褐色古土壤 散装有机物 89.84±0.57 861±44 689~907 700~898 LUG12-154 CPTC1-2 黄褐色古土壤 散装有机物 92.69±0.63 610±47 539~662 553~650 LUG12-155 CPTC1-3 黄褐色古土壤 散装有机物 92.57±0.59 620±44 543~663 556~653 LUG12-156 CPTC1-4 黄褐色古土壤 散装有机物 68.46±0.51 3044±51 3079~3371 3172~3343 LUG12-157 CPTC2-1 黄褐色古土壤 散装有机物 69.78±0.57 2891±69 2852~3245 2927~3256 LUG12-158 CPTC2-2 黄褐色古土壤 散装有机物 51.50±0.45 5332±60 5948~6227 6002~6192 LUG12-160 CPLL-1 灰褐色古土壤 散装有机物 75.40±0.35 8120±70 7860~8356 8532~8786
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