Evidence of the Holocene Seismic Activity of the Wenchuan Segment of the Maoxian-Wenchuan Fault
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摘要: 茂县-汶川断裂是龙门山推覆构造带的重要组成部分，又称龙门山后山断裂，是龙门山构造带形成的重要断裂，对解释青藏高原动力学和南北地震带的变形特征具有重要意义。已有学者对茂县-汶川断裂活动性采取不同手段展开调查研究，使用ESR、TL等多种方法表明断裂在晚更新世有过活动，但断裂在晚第四纪特别是全新世有无活动缺乏可靠的直接证据。为此，在汶川县草坡一带，综合地震地貌、探槽和年代分析等手段，揭露了2次古地震事件，分别距今约（345±20）a、（2015±30）a，证明了茂县-汶川断裂汶川段在全新世以来有过活动。Abstract: The Maoxian-Wenchuan Fault, also called Longmenshan Back—Mountain Fault, is an important component of the Longmenshan Nappe structure belt, which is the important fault of the Longmenshan tectonic belt. It has great significance to the interpretation of the transformation characteristics of the Qinghai—Tibet plateau dynamics and its seismic belts in the south and north. Scientists from China and abroad have carried out survey and research works on activity of the Maoxian—Wenchuan Fault using various means and methods, such as ESR and TL. The results indicate that the fault had been active during the Late Pleistocene. However, the activity of the fault during the Late Quaternary, especially the Holocene, is in lack of reliable and direct evidence. In the Caopo region of Wenchuan County, two paleo—seismic events are identified by methods of seismic geomorphology, exploratory trench and chronologic analysis, which are aged (345±20) a, (2015±30) a, respectively. The events prove that there had been seismic activities in Wenchuan segment of the Maoxian—Wenchuan Fault since the Holocene. Further works need to do on whether the 345±20 a seismic event was related to the 6½ magnitude earthquake in 1657 in Wenchuan County in Wenchuan County, and on the paleo—seismic implication the colluvial wedge represents in the trench Tc1.
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Key words:
- Paleoearthquake events /
- Holocene /
- Maoxian-Wenchuan fault /
- Wenchuan county /
- Caopo township
1)1 2 四川省地质调查院，2010. 绵阳市1∶25万区域地质调查.2)2 3 四川省地质局区域地质测量队，1975. 1∶20万茂汶幅、灌县幅区域地质调查报告（地质部分）.3)3 4 四川省地质矿产勘查开发局川西北地质队，2013. 1∶25万绵阳市幅区域地质调查报告. -
图 5 茂县-汶川断裂带汶川七盘沟口剖面（1∶25万绵阳幅修编）
Figure 5. Qipan ditch section of the Maoxian-Wenchuan fault zone （According to 1∶250 000 Mianyang revision）
图 7 草坡乡金波村槽探分布遥感图
Figure 7. Remote Sensing map of trough exploration distribution in Jinbo Village
图 8 金波村Tc1探槽拼接图及解译结果
Figure 8. Tc1 probe slot mosaics and interpretation results in Jinbo village
表 1 探槽揭露地层与样品年代
Table 1. Form of trenching exposes strata and sample age
层号 地层描述 样品描述 Tc1—U1 灰色含黏土砾石层，砾石呈棱角状或次棱角状，分选差，砾径2～40 cm。岩石成分以玄武岩、安山岩等为主，物源来源于探槽东侧山区元古界黄水河群，因此，该层应是古河谷的沉积物。 Tc1—U2 灰白色，层中顺坡堆积大小不一、碎块状灰绿色千枚岩，应为西侧志留系千枚岩崩塌，在坡脚处受流水改造堆积而成。 Tc1—U3 浅灰黄色土层，局部夹风化的小砾石，该层厚度整体较稳定，在东侧受断层作用突然下掉加深。 炭屑样14C：(2 015±30 ) a Tc1—U4 灰黑色含砾黏土层。砾石砾径约1 cm，磨圆好；局部发育磨圆较差、砾径约5 cm砾石。该层呈上凹型，地层在剖面东侧较厚。 炭屑样14C：(260±25) a Tc1—U5 棕黄色含砾黏土层，整体呈上凹型。 炭屑样14C：(345±20) a Tc1—U6 灰黑色含砾黏土层，较薄，往两端尖灭。 Tc1—U7 浅棕黄色含砾黏土层，局部含砾径约2 cm的小砾石。厚度上具有向南侧变厚的趋势，终止于断层附近。 炭屑样14C：(155±20) a Tc1—U8 灰黑色地表耕植土，在剖面北侧较厚，向东减薄。 Tc1—U9 灰色～灰黑色砾石层，砾石松散，呈点、线接触，砾石间黏土少，充杂植物根系，疑似人工堆砌形成 Tc2—U1 浅棕红色含砾粉砂、黏土层，砾石粒径1～2 cm，磨圆较好。 Tc2—U2 灰黄色砾石层，砾石分选差，大的砾径可达50 cm左右，磨圆较好，砾石成分主要为玄武岩或安山岩。 Tc2—U3 灰色含砾石黏土层，局部夹有砾径较小的砾石，磨圆较好，具中间薄、两侧厚特征。 炭屑样14C：(315±20) a Tc2—U4 地表根植层，层中局部有现代植物腐木等。 炭屑样14C：(85±30) a 
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