Discovery of the surface rupture zone along the southern branch of the Longshoushan Fault Zone, NW China and its significance to the deep structures of the 1954 Shandan MS 7¼ earthquake
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摘要: 基于详细的遥感解译和野外调查，发现龙首山南缘断裂发育有较新的地震地表破裂遗迹，包括断层坎、地震鼓包、河道的系统位错等断层地貌标志，破裂带总长度超过20 km，沿断裂走向其垂向位移介于0.35~4 m，水平位移介于0.3~1.9 m，龙首山南缘断裂主体表现为逆冲性质，仅在西端表现为局部左旋走滑的性质。通过剖面和探槽揭示，龙首山南麓地区全新世以来发生多次断层活动，最新的一次在约3.96 ka以来。经过与区域内的强震记录比对，认为此次新发现的地震地表破裂带可能是1954年山丹MS 7¼地震所致。1954年山丹MS 7¼地震在浅表沿两条断裂同时发生了地表破裂，表现为正花状构造的变形样式。这种同震位移分配现象以往多发现于走滑型地震中，此次在逆冲型地震中发现。龙首山南缘断裂地表破裂带的发现为揭示1954年山丹地震的震源过程和破裂样式提供了新的证据和思路。
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关键词:
- 青藏高原东北缘 /
- 1954山丹MS 7¼地震 /
- 龙首山断裂带 /
- 同震地表破裂 /
- 花状构造
Abstract: A detailed field-based remote sensing interpretation in this study revealed abundant recent seismic surface ruptures on the southern margin of the Longshoushan Fault, including fault scarps, mole tracks and displacements of river channels. The total length of the surface rupture zone is over 20 km. The most recent vertical and horizontal displacements of the late Quaternary geomorphic markers fall into the range of 0.35~4 m and 0.3~1.9 m, respectively. The southern Longshoushan Fault is a high-angle thrust fault, and it only exhibits left-lateral strike-slip on the west end. Revealed by profile and trench along the surface rupture, the southern branch of the Longshoushan Fault Zone has gone through several earthquake events since the Holocene, and the latest one happened around 3.96 ka. By comparing with the strongest earthquake recorded in this region, it is suggested that the 1954 MS 7¼ Shandan earthquake may have caused the newly discovered surface ruptures, which developed along both the southern and northern branches of the Longshoushan fault zone, presenting a positive flower structure at the shallow surface. This coseismic displacement distribution mostly is found in strike-slip earthquakes in previous studies, but it is found in a thrust earthquake this time. The discovery of coseismic surface rupture on the southern branch of the Longshoushan Fault Zone will throw light on the focal mechanism and rupture pattern of the 1954 Shandan MS 7¼ earthquake. -
图 1 龙首山地区及其邻区DEM解译图
F1—东昆仑断裂；F2—柴北缘断裂；F3—阿尔金断裂；F4—北祁连断裂；F5—海原断裂；F6—龙首山南缘断裂；F7—龙首山北缘断裂；F8—阿右旗断裂；F9—合黎山断裂
a—研究区大地构造位置图；b—河西走廊地区历史地震记录及青藏高原东北缘主要断裂分布(据Xu et al., 2010; Wang et al., 2020修改)Figure 1. DEM interpretation of the Longshoushan and its adjacent areas
(a) Geotectonic location of the study area; (b) Historical seismic records in the Hexi Corridor and distribution of main faults in the northeastern margin of the Tibet Plateau (Modified from Xu et al., 2010; Wang et al., 2020)
F1-Eastern Kunlun Fault; F2-Northern Qaidam Fault; F3-Altyn Tagh Fault; F4-Northern Qilian Fault; F5-Haiyuan Fault; F6-Southern Longshoushan Fault; F7-Northern Longshoushan Fault; F8-Ayouqi Fault; F9-Helishan Fault
图 2 研究区地质简图(据李佳昱等，2020修改; 北缘破裂带位置和烈度等值线来源于謝毓寿和郭履灿(1957)、董治平等(2000)、郑文俊等(2013))
Figure 2. Geological map of study area (Modified from Li et al., 2020; Location of the northern Longshoushan surface rupture and seismic intensity sourced from Xie and Guo, 1957; Dong et al., 2000; Zheng et al., 2013)
图 3 桃花山附近断层地貌特征
a—桃花拉山破裂带卫星图像；b—断层谷地；c—位错的山咀
Figure 3. Faulted landform surrounding the Taohuashan site
(a) Satellite image showing the topographic features in the Taohuashan fault zone; (b) Fault valley along the southern branch fault of the Longshoushan fault zone; (c) Dislocated ridge
图 4 独峰顶西段破裂带照片解译
a—独峰顶西段地表破裂带卫星图像；b—d—具有不同位移大小的断层坎；e—g—冲沟改道
Figure 4. Surface rupture characteristics at the western section of Dufengding
(a) Satellite image showing the topographic features at the western section of Dufengding; (b-d) Fault scarps with varied vertical displacements; (e-g) River migration due to the relative movement of strike-slip fault
图 5 独峰顶中段地表破裂特征
a—独峰顶中段卫星图像；b—断层坎；c—地震鼓包；d、e—断层坎及坡中谷中生长的茂盛的芦苇，指示下部有地下水活动
Figure 5. Surface raptures at the central section of Dufengding
(a) Satellite image showing the surface raptures at the central setion of Dufengding; (b) Fault scarp; (c) Mole track; (d-e) Fault scarps facing south with reeds around, indicating underground activity
图 6 独峰顶东段地表破裂特征
a—独峰顶东段卫星图像；b—冲沟位错现象，断层左旋走滑位错约1.9 m；c—断层坎；d—反向断层坎；e—断层坎(垂直位错约2.65~2.75 m)
Figure 6. Surface ruptures at the eastern section of Dufengding
(a) Satellite image showing the locations of outcrops at the eastern section of Dufengding; (b) Left-lateral strike-slip displacement of the river channel with a offset amount of~1.9 m; (c) Fault scarp; (d) Fault scarp facing the ridge; (e) Fault scarp with a vertical displacement of 2.65~2.75 m
图 7 河道天然剖面照片及素描图(位置见图 4a)
U1—含中砾松散黏土；U2—细粒黄土沉积；U3—深灰色河流相中—粗粒砾岩；U4—浅灰色河流相中—粗粒砾岩
Figure 7. Photo and sketch of the natural outcrop (see Fig. 4a for the location)
U1-Loose clay with medium pebbly; U2-Fine-grained loess; U3-Dark grey fluvial medium-coarse conglomerate; U4-Light grey fluvial medium-coarse conglomerate
图 9 龙首山南缘断裂和北缘断裂深部模式图
Figure 9. Simplified model showing the deep structures of the northern and southern branch faults of the Longshoushan Fault Zone
表 1 光释光样品年龄结果
Table 1. Results of the OSL samples
样品编号 U/×10-6 Th/×10-6 K/% 含水率/% 剂量率/(Ga/ka) 等效剂量/Gy 年龄/ka 2020920-1 3.74±0.03 14.05±0.20 3.22±0.08 10±5 5.89±0.32 23.98±0.82 4.07±0.26 2020920-2 5.07±0.06 17.66±0.19 2.25±0.04 10±5 4.65±0.14 89.45±10.91 19.25±2.42 2020920-3 3.72±0.03 15.38±0.14 2.00±0.05 10±5 4.85±0.29 46.10±1.20 9.50±0.62 2020920-4 3.24±0.04 15.37±0.18 2.87±0.07 10±5 5.53±0.30 44.53±1.97 8.05±0.56 2020920-5 3.25±0.03 12.63±0.13 1.99±0.02 10±5 4.47±0.25 17.73±0.55 3.96±0.26 2020920-6 3.43±0.10 12.54±0.45 1.97±0.01 10±5 4.47±0.26 30.33±0.53 6.79±0.41 
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