Jianganlin surface fracture induced by 2013 Lushan MS7.0 earthquake
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摘要: 中国地震局地球物理勘探中心芦山MS7.0地震现场工作队在地震应急考察期间发现了WNW向的箭杆林地表破裂带, 并详细考察了该地表破裂带的产状、 破裂面上的擦痕及断错特征， 讨论了对该地表破裂带的不同认识以及对发震构造研究的意义. 箭杆林地表破裂带位于雅安市上里镇箭杆林村二组西侧海拔1529—1578 m、 坡度30°—40°的山坡上. 在长约80 m的范围内， 该地表破裂带分3段近直线展布， 每段长6—10 m， 最宽约40 cm. 破裂带总体走向120°， 破裂面平整， 倾向SW， 倾角62°—85°. 破裂通过的岩性多为黏土， 破裂面上有两组明显的新鲜擦痕， 倾伏角较大的一组擦痕一般仅保留在距地面10—20 cm以上的范围内， 其下倾伏角较小的一组擦痕则覆盖倾伏角较大的一组擦痕. 破裂经过处见一直径约4 cm的树根被明显错断， 上盘上升15 cm， 并向SE方向平移10 cm； 另见上盘地层发生翻转现象. 经探槽揭示， 破裂带上盘黑色腐植土层相对下盘上升约15 cm. 综合分析表明， 箭杆林地表破裂带在芦山MS7.0地震期间经历了先逆冲、 后左旋平移的运动过程. 震源机制解表明， 芦山MS7.0地震是错断面为NE走向的逆冲型地震. WNW向箭杆林地表破裂带可能是芦山地震产生的次级破裂带， 是否存在NW-WNW向的发震构造值得研究. 芦山MS7.0地震箭杆林地表破裂带的发现为发震构造和震害研究提供了重要的基础资料.Abstract: A WNW-trending Jianganlin surface fracture induced by the Lushan MS7.0 earthquake was found by the fieldwork team of Geophysical Exploration Center, China Earthquake Administration, during the period of emergency investigation into the earthquake. The geometry, striation and slip features of the surface fracture were inspected in detail，and different understandings of the fracture and its significance to seismogenic structure research are discussed. The Jianganlin surface fracture is located at the mountain slope having an elevation of 1529—1578 m and dipping 30°—40° to the west of Group 2, Jianganlin village of Shangli town region, Ya’an City, Sichuan Province of China. The surface fracture with a strike of 120° consists of three segments stretching near linearly in the range of about 80 m long, each with a length of 6—10 m, and their maximum width being 40 cm. The fracture surface is smooth, dipping 62°—85° southwestward. Most part of the fracture passes through clay. There are two sets of fresh striations on the fracture surfaces, on which the striations with larger slip-angle are usually seen above the range of 10—20 cm below ground surface, while in deeper part the large slip angle striations are covered by smaller slip-angle ones. A tree root with a diameter of about 4 cm was cut and offset due to the rupture passing through. The hanging wall of the fracture rose 15 cm and shifted 10 cm toward SE. Besides, some strata were inversed on the hanging wall. The trench exposes that the black humus layer on the hanging wall rose about 15 cm relative to the black humus layer on the footwall. A comprehensive analysis shows that during the Lushan earthquake the Jianganlin surface fracture experienced thrust motion firstly, and then sinistral strike-slip. The focal mechanism solution shows that the earthquake is a thrust event with a NE-striking fault plane. The WNW-trending Jianganlin surface fracture is possibly a secondary one of the event, and it is worth studying whether there exist seismogenic structures striking NW-WNW. The find of Jianganlin surface fracture associated with the Lushan MS7.0 earthquake provides important information for studying the seismogenic structures and earthquake damage.
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Keywords:
- Lushan MS7.0 earthquake /
- surface fracture /
- striation /
- thrust /
- sini-stral strike-slip
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图 1 芦山MS7.0地震的地震构造简图
F1: 江油—灌县断裂; F2: 北川—映秀断裂; F3: 茂汶—汶川断裂； F4: 绵竹断裂； F5: 彭州断裂; F6: 大邑断裂； F7: 新开店断裂； F8: 鲜水河断裂
Figure 1. Seismotectonic sketch of the Lushan MS7.0 earthquake
F1: Jiangyou--Guanxian fault; F2: Beichuan--Yingxiu fault; F3: Maowen--Wenchuan fault; F4: Mianzhu fault; F5: Pengzhou fault; F6: Dayi fault; F7: Xinkaidian fault; F8: Xianshuihe fault
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图 3 箭杆林A段地表破裂带(镜向NW)
Figure 3. Segment A of the Jianganlin surface rupture (View to the NW)
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图 4 A段地表破裂带SW盘(镜向W)
Figure 4. The SW wall of segment A of the Jianganlin surface rupture (View to the W)
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图 5 A段地表破裂带SW盘两组擦痕(镜向W)
Figure 5. Two groups of striations in the SW wall of segment A (View to the W)
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图 6 箭杆林B段地表破裂带(镜向NW)
Figure 6. Segment B of the Jianganlin surface rupture (View to the NW)
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图 7 B段地表破裂带NW端SW盘断面错动擦痕(镜向W)
Figure 7. Shift striation in the SW wall at the NW end of segment B (View to the W)
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图 8 B段地表破裂带探槽(镜向NW)
Figure 8. The trench at segment B of the Jianganlin surface rupture (View to the NW)
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图 10 树根被错断，显示破裂为逆冲兼左旋走滑特征(镜向NW)
Figure 10. The faulted root showing that the rupture is thrusting with sinistral strike-slip(View to the NW)
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图 11 直径约10cm的树根被错断显示破裂为左旋走滑特征(镜向NW)
Figure 11. The faulted root with a diameter of about 10 cm, displaying that the rupture has sinistral strike-slip characteristics (View to the NW)
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