第四纪研究  2020, Vol.40 Issue (5): 1323-1333   PDF    
藏南多庆错地堑冲巴雍错段最新史前大地震遗迹及其年龄证据
左嘉梦1,2, 吴中海1, 盖海龙3, 哈广浩1,4, 周春景1     
(1 新构造运动与地质灾害重点实验室, 中国地质科学院地质力学研究所, 北京 100081;
2 中国地质大学(北京), 北京 100083;
3 青海省地震局, 青海 西宁 810001;
4 中国地震局地质研究所, 活动构造与火山重点实验室, 北京 100029)
摘要:西藏亚东-谷露裂谷是青藏高原南部规模最大的活动裂谷,但与其全新世强烈活动性不符的是该裂谷南段多庆错地堑历史上缺少MW ≥ 6地震记录,因此通过古地震研究来揭示该区大地震活动与断裂活动性的关系显得十分重要。本文根据在亚东-谷露裂谷南段多庆错地堑冲巴雍错至多庆错段(范围:28°21'~27°52'N和89°44'~89°17'E)发现的最新史前大地震的地表破裂遗迹,初步研究了地震的时间与震级。通过高分辨率卫星影像解译、利用小型无人机获得的数字高程模型分析和详细的地表调查,发现该区最新史前大地震的同震破裂带在冲巴雍错段错断了河流T2及同期泉华台地,形成了最高约4.5 m的断层陡坎。根据断层陡坎上探槽中古土壤样品的AMS14C测年结果,揭示被错断的最新地层年龄为6460~3260 cal.a B.P.。综合分析认为,西藏亚东-谷露裂谷南段的多庆错地堑在约3260 cal.a B.P.之后至少发生过一次矩震级为MW7.1~7.2的大地震,该地震同时造成冲巴雍错段和多庆错段断裂破裂,总的破裂带长度可能达到62 km,最大地震位移可能大于4.5 m。新发现的多庆错地堑全新世古地震填补了亚东-谷露裂谷南段古地震研究的不足,并且有助于更深入地认识西藏亚东-谷露裂谷南段大地震活动历史和区域强震危险性。
关键词亚东-谷露裂谷    多庆错地堑    地震地表破裂    古地震    正断层    
中图分类号     P694;P546                     文献标识码    A

0 引言

藏南近南北向裂谷带是青藏高原内部最显著的活动构造之一[1~5],也是高原南部活动最强烈的控震构造带[6~10]。亚东-谷露裂谷是藏南裂谷带中规模最大且历史强震最频繁的活动裂谷,其中包含了许多断续相接的第四纪期间持续活动的近南北向地堑和半地堑[6, 11~12](图 1),穿过或邻近西藏的拉萨、当雄、羊八井、尼木、康马、帕里和亚东等重要城镇区。历史地震资料显示,该裂谷自公元642年至今至少发生过14次M≥6.0级地震,但仅3次发生在雅鲁藏布江以南,而最南段的多庆错地堑更是长期处于闭锁状态,史料中无强震记录[13~15](图 1),这显然与该地堑的晚第四纪强烈伸展活动不符[16]。而且多庆错地堑是亚东-谷露裂谷南段重要地堑,其中分布或毗邻帕里镇和亚东县等人口密集县镇。因此,全面了解和认识该地堑的最新大地震活动与年代,对于深入理解亚东-谷露裂谷的大地震活动特征规律,以及该区的未来强地震危险性评价和区域防震减灾等都具有重要意义。

图 1 藏南亚东-谷露裂谷主要活动断裂与地震分布示意图[10] Fig. 1 Schematic map showing the distribution of main active faults and earthquakes in the Yadong-Gulu rift[10]

近期,本项目团队根据亚东-谷露裂谷南段主边界正断层的滑动速率时空变化[16],以及多庆错在2015年尼泊尔大地震后的湖水短暂异常消亡事件[17],不同程度地分析了多庆错地堑的未来强震活动危险性。并发现该地堑的冲巴雍错段发育有最新史前大地震遗迹[18]。本文在前期工作基础上,进一步通过采用高分辨率卫星影像解译、无人机测量、探槽揭露和年轻地质测年(14C)等方法,对新发现的多庆错地堑北段冲巴雍错段史前大地震遗迹的同震位移、破裂展布与特征、发震震级及时间等进行了厘定和分析。相关成果将有助于填补亚东-谷露裂谷南段的史前大地震活动记录,并对西藏亚东-康马地区的未来强地震危险性评价及防震减灾具有重要参考价值。

1 区域地质背景

亚东-谷露裂谷(范围:27°40′~28°42′N,89°05′~89°54′E)为藏南裂谷带中自东向西第二条裂谷,是藏南地区显著的活动构造带和强震活动带,其南端起于亚东县,向北可延伸至谷露镇以北约30 km处,整体呈N30°~35°E走向,全长约500 km。根据该裂谷主边界断裂带的结构特征,可分为北、中、南三段,共包含了7个地堑与地堑群[10](图 1)。其中,谷露地堑、当雄-羊八井地堑和格达地堑为北段;尼木地堑群为中段,包含了6个规模较小的地堑;南段包含了热龙地堑、涅如地堑和多庆错地堑这3个呈右阶斜列分布的半地堑。该裂谷的北-中段与南段以雅鲁藏布江为界。地震史料记载,该裂谷北-中段的历史强震活动强烈,至少发生过11次M≥6地震,其中1411年M8.0当雄地震也是藏南裂谷带上已知的最大地震[13~15]。与北段相比,该裂谷南段历史强震活动很少,仅发生过1909年M6.5浪卡子地震、1921年M6.2江孜地震、1935年M6.3江孜西南地震等3次M≥6地震。

多庆错地堑处于亚东-谷露裂谷的南端,该地堑总体呈NNE向,长约100 km,最宽约20 km,主边界正断层位于地堑的东缘。大地构造上,该地堑属特提斯喜马拉雅地块,处于主中央逆冲断裂带和东西向藏南片麻岩穹隆带之间,并向南穿切藏南拆离系[19~20]。按照主边界断层的特征,多庆错地堑自南向北可以分为三段:帕里盆地、多庆错盆地和冲巴雍错盆地[21](图 2),三者的主边界正断层空间上呈右阶斜列分布。冲巴雍错断裂段(28°21′~28°12′N和89°44′~89°39′E)恰好处于多庆错地堑与涅如地堑的衔接处,该段起自于冲巴雍错,整体走向NE,冲巴雍错与冲巴芒错均为冰川堰塞湖,位于断裂上升盘一侧的冲巴芒错被年楚河切穿,并共同构成了该河流的源头,河水流经断层处形成清晰的河流裂点,显示断裂晚第四纪活动强烈[22],断裂周围出露的基岩主要为奥陶系灰岩、三叠系灰岩和侏罗系-白垩系碎屑岩[21]

图 2 多庆错地堑地质图[21] ①帕里段(Pagri section);②多庆错段(Duoqing Co section);③冲巴雍错段(Chongba Yumtso section) Fig. 2 Geologic map of the Duoqing Co Graben[21]
2 史前大地震遗迹及其特征 2.1 同震地表破裂的空间展布

地震地表破裂是地壳弹性应变转化为永久性构造变形的表现形式[23~24],是强地震造成的地表岩土体错动,也是发震断层地震剪切位移错动延伸到地表的结果[25]。高分辨率的影像数据为研究地表破裂特征提供了一个高效精确的途径[26~29],本文使用购买自法国Pléiades商业遥感卫星的分辨率0.5 m级的Pléiades遥感影像来研究断层展布特征。影像上,在冲巴雍错段可以清晰地观察到断裂自冲巴雍错东岸向NNE方向延伸直至尖灭,沿线错动各级冰碛垄和阶地,形成长约20 km的地震地表破裂带(图 3),差分GPS测量可知陡坎垂直错动量最大可达15 m,表明该断裂在此处曾发生过数次地震。

图 3 冲巴雍错段Pléiades遥感影像图 Fig. 3 Pléiades remote sensing image of the Chongba Yumtso segment

值得注意的是,在Google Earth影像上,观察到多庆错段具有笔直的如刀切一般的延伸约42 km的地震地表破裂带,穿过了晚更新世和全新世的冰碛垄与冰水阶地、湖岸阶地等不同时代地貌体(图 4)。且在该地震地表破裂带南部,发现高4.1 m错动最新湖岸阶地的断层陡坎(图 4c)[16]

图 4 多庆错段Google Earth遥感影像图 (a)多庆错段整体地表破裂带行迹;(b,d,e)多庆错段分段地表破裂带行迹,充分展示地震破裂的空间分布;(c)在(b)处的近景照片 Fig. 4 Google Earth remote sensing image of Duoqing Co segment. (a)The trace of the whole fault fracture zone in Duoqing Co section; (b, d, e)Traces of segmented surface fracture zone in Duoqing Co section, fully show the spatial distribution of earthquake rupture; (c)Close-up photo at (b)
2.2 同震位移

冲巴雍错段,年楚河T2阶地上可见阶梯状断坎(图 5a),河流两侧T2阶地砾石层及地表土壤层均被错动(图 5f5g5h5i),河流北侧部分T2阶地受热泉活动而形成同期泉华台地(图 6a),该台地整体为小地堑(图 6b),东西两侧均有陡坎发育,河口处可见反向断层F5(图 6c),泉华层上清晰保存了最新同震地表破裂(图 6d6e),台地西北部可见古泉眼,断层与地震活动使热泉消失。

图 5 年楚河南侧Pléiades遥感影像、山体阴影图和野外观测 (a)年楚河南侧Pléiades遥感影像解译图;(b)断层F3与F4造成的最大地表错动量;(c)年楚河南侧山体阴影图;(d)F3造成的地表错动量与断层长度分布图,测线位置为(c)中蓝线,图中VSmax指最大地表垂直位移;(e)F4造成的地表错动量与断层长度分布图,测线位置为图(c)中紫线;(f)远景观测断层F3的陡坎;(g)远景观测断层F4的陡坎;(h,i)地震地表破裂的近景照片 Fig. 5 Remote sensing image, mountain shaded picture and field observation on the south side of the Nianchu River. (a)Interpretation Map of Pléiades remote Sensing Image on the south side of Nianchu River; (b)Maximum surface displacement caused by faults F3 and F4; (c)Mountain shaded picture on the south bank of Nianchu River; (d)The distribution map of surface dislocation momentum and fault length caused by F3, and the location of the survey line is the blue line of figure (c), VSmax in the figure refers to the maximum vertical displacement of the surface; (e)The distribution map of surface dislocation momentum and fault length caused by F4, and the location of the survey line is the purple line of figure (c); (f)Long-range photos of the steep slope of fault F3; (g)Long-range photos of the steep slope of fault F4; (h, i)Close-up photos of earthquake surface rupture

图 6 年楚河北侧遥感影像和野外观测 (a)年楚河南侧Pléiades遥感影像解译图;(b)远景观测年楚河北侧地堑;(c)远景观测年楚河北侧反向断层F5;(d,e)近景观测年楚河北侧同震地表破裂 Fig. 6 Remote sensing image and field observation of the north side of the Nianchu River. (a)Interpretation map of Pléiades remote sensing image on the south side of Nianchu River; (b)Long-range photos of the graben on the north side of the Nianchu River; (c)Long-range photos of the antithetic fault F5 on the north side of the Nianchu River; (d, e)Close-up photos of coseismic surface rupture on the north side of Nianchu River

为了获得该断裂段最新垂直错动量,笔者利用无人机(DJI Phantom 4)对此段进行了航拍,然后利用Agisoft Photoscan软件进行处理,生成分辨率可达14.2 cm/pix、点密度为49.5点/m2的数字高程模型(DEM)(图 5c)。因为当伸展变形量一致时,变形分解作用使得地堑的垂直位错量小于单一断层陡坎和阶梯状断坎。所以,在DEM的基础上,相较于泉华台地地堑,笔者选择年楚河T2阶地上位错现象清晰、后期改造较小、地貌面保留较为完好的阶梯状断坎,进行断层错动量的解译与恢复,共获得共18个T2阶地上F3、F4的错动量,其中F3错动量8个(图 5d),F4错动量10个(图 5e),分别对应于图 5c中的18条剖面测量线。经统计发现,最新一次地震中T2阶地分别被断层F3与F4最大错动垂直位移量(VSmax)为1.2 m与1.5 m(图 5b5d5e),而断层F2造成的地表错动量因修路而被严重破坏,无法基于DEM确定。

因研究区工作条件差,靠人工难以在泉华台地上布设探槽。因此,依据古地震探槽选址原则[30~31],笔者在冲巴雍错东岸年楚河南侧T2阶地多级断坎中的F2断层陡坎处布设探槽TC-1,并在T2阶地的地貌面采集了3个样品用于AMS14C测年(表 1)。该探槽长3.2 m,深2.0 m见图 7(位置见图 5a)。F2断层陡坎处地势较低,沉积速率较高,能够将地震地表形变快速的埋藏和保存,是开展古地震探槽的合适场所。探槽TC-1揭露产状为290°∠65°的正断层F2。断层上盘以胶结较好的灰黄色细砾-粗砾层为主,中间夹杂胶结松散的土黄色粗砾层,砾石层向东南倾斜且有序排列(图 7c);断层下盘自下往上分别为灰黄色细砾-粗砾层、古土壤层、崩积楔及现今地表土壤层(图 7)。崩积楔是古地震的标志,与地震断层陡坎具有准同生沉积关系,可以准确限定古地震事件层位[32~36]。因为破裂面穿过所有层面到达地表,所以崩积楔揭露的地震为研究区最近一次有地表破裂的地震,古土壤层顶部与现今T2地表土壤层表面之间的距离即为断层F2的错动量,野外测量得约为1.8 m。因短时间尺度内地层旋转量有限,故该断距忽略了地层的旋转。

表 1 研究区样品年龄 Table 1 Ages of samples in the study area

图 7 冲巴雍错段T2阶地上探槽TC-1照片及剖面结构示意图 (a)探槽位置照片;(b)探槽整体照片;(c)探槽砾石层特写照片;(d)探槽整体素描图 Fig. 7 Photos and profile structure diagram of trench TC-1 on terrace T2 of Chongba Yumtso section. (a)A photo of the location of the trench; (b)A photo of the whole trench; (c)A close-up picture of the gravel layer of the trench; (d)The sketch of the whole trench

结合断层F2、F3、F4造成的最大错动量1.8 m、1.2 m与1.5 m,可知该次古地震造成的T2阶地总错动量约为4.5 m。

3 史前大地震年龄与震级 3.1 大地震年龄

为了确定古地震发生的年代,笔者分别自探槽古土壤层的上部、中部和下部采集放射性碳定年样品,并在美国Beta Analytic Inc.实验室使用加速器质谱法(AMS14 C)进行分析,同时对所有测试结果使用2013 IntCal程序计算校正[37]。根据样品年龄及位置推测,古土壤层形成于6460~3260 cal.a B.P.,该次古地震发生时间应晚于3260±30 cal.a B.P.(表 1),具体发震时间将在今后工作中进一步研究。

3.2 震级估算

地表断层错动与地震破裂过程间存在内在联系,根据地震地表破裂带长度、破裂面积、同震位移量等参数可以估算古地震震级大小[38~40]。根据地表最大同震错动量4.5 m,结合Wells和Coppersmith[39]矩震级-地表最大同震错动量经验关系式MW=6.61+0.71logMD(其中MD为地表最大同震错动量),估算出该地区最新史前大地震的矩震级为MW7.1。

此外,多庆错断裂虽然分为三段,但是各段之间的阶区宽度很小,冲巴雍错段与多庆错段之间的阶区宽约4~5 km。Zhang等[41]认为一次正断层地震破裂中能够贯通的最大规模的阶区不超过5 km,Wesnousky[42]发现倾滑活动破裂可以穿过宽5~7 km的阶区。因此,冲巴雍错断裂一旦引发地震,震害区应该同时覆盖冲巴雍错段和多庆错段(范围:28°21′~27°52′N和89°44′~89°17′E)。此外,因为冲巴雍错段错动的最新的地层年龄为6460~3260 cal.a B.P.,多庆错段错动的最新湖岸阶地年龄为3~4 ka B.P.[16],所以,二者的地表破裂应属于同期地震破裂,则总的破裂带长度可达62 km。根据该地震地表破裂总长度62 km,与Wells和Coppersmith[39]矩震级-地表破裂长度经验关系式MW=4.86+1.32logL(其中L为地表破裂长度),估算认为沿该地表破裂发生了矩震级为MW7.2的大地震。

因此,多庆错地堑最新史前大地震矩震级为MW7.1~7.2,根据长度和位移估算的震级结果接近,说明结果相对可靠。

4 主要结论与认识

本文在前期工作基础上,对在亚东-谷露裂谷南段多庆错地堑冲巴雍错至多庆错段(范围:28°21′~27°52′N和89°44′~ 89°17′E)发现的最新史前大地震的地表破裂遗迹进行研究,并获得如下认识:

通过高分辨率卫星影像解译、利用小型无人机获得的数字高程模型分析和详细的地表调查,发现该史前大地震的最大地表错动量达4.5 m。

根据断层陡坎上探槽中古土壤样品的AMS 14 C测年结果,揭示了冲巴雍错地区被错断的最新地层年龄为6460~3260 cal.a B.P.,与多庆错地区最新湖岸阶地(年龄为3~4 ka B.P.)[16]地表破裂属于同期地震破裂,表明该史前大地震的地表破裂带总长度可达62 km。

该次古地震发生时间晚于3260±30 cal.a B.P.,根据地表最大错动量4.5 m与地表破裂带总长度62 km,可知该次古地震是一次矩震级为MW7.1~7.2的大地震。

致谢: 非常感谢中国地质科学院地质力学研究所胡萌萌博士研究生对英文摘要修改的帮助;感谢审稿专家和编辑部杨美芳老师建设性的修改意见。

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The latest prehistoric earthquake relics and its age evidence in Chongba Yumtso fault section of Duoqing Co Graben, Southern Tibet
Zuo Jiameng1,2, Wu Zhonghai1, Gai Hailong3, Ha Guanghao1,4, Zhou Chunjing1     
(1 Key Laboratory of Neotectonic Movement and Geohazard, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081;
2 China University of Geosciences(Beijing), Beijing 100083;
3 Qinghai Earthquake Agency, Xining 810001, Qinghai;
4 Key Laboratory of Active Tectonics and Volcanoes, Institute of Geology, China Earthquake Administration, Beijing 100029)

Abstract

The Yadong-Gulu Rift (27°40'~28°42'N, 89°05'~89°54'E) is the largest active rift in the southern Tibetan Plateau, but inconsistent with its strong Holocene activity there is a lack of MW ≥ 6 seismic records in the Duoqing Co Graben, the southern segment of the rift. Paleoseismology was considered a valid method to reveal the relationship between large seismicity and fault activity in this area. The latest surface rupture traces of a prehistorical earthquake found in the range from Chongba Yumtso to Duoqing Co (28°21'~27°52'N, 89°44'~89°17'E), preliminary study the time and magnitude of this earthquake. High-resolution satellite images, digital elevation model analysis obtained from the unmanned aerial vehicle (UAV) and detailed surface survey, were used to detect and analyze the coseismic displacement of a second stage terrace (T2) and the contemporaneous travertine platform, and a fault scarp was measured to around 4.5 m high. Further, the paleosol from this scarp was dated 6460 cal.a B. P. to 3260 cal.a B. P. using the AMS14C dating method. It is reasonable to think that at least an MW7.1~7.2 earthquake occurred after 3260 cal.a B. P. in Duoqing Co graben, forming the surface rupture between Chongba Yumtso and Duoqing Co, with a total length of up to 62 km, and a maximum coseismic displacement of 4.5 m. This found makes up for the deficiency of paleoearthquake research in the southern segment of the Yadong-Gulu Rift and contributes to deeply study the history of large earthquakes and regional strong earthquake risk to this area.
Key words: Yadong-Gulu rift    Duoqing Co graben    earthquake surface rupture    paleoearthquake    normal fault