西藏安多最新史前大地震的地质证据及其年龄
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摘要
在西藏安多地区进行活动断裂研究过程中,进行地表调查和探槽开挖,证实错那-安多地堑北缘主边界断裂上的最新地表断层陡坎实际是该区史前大地震的地表遗迹。根据地表观察和实地测量结果可知,该区最新古地震断坎的延伸范围可达26～36km,平均垂直位移量达1.0～1.4m。相关地层-地貌体的年代学测试结果和古地震破裂参数表明,最新的古地震发生在距今约10.0～8.1ka期间,估计当时的古地震震级介于6.9～7.3级之间,当时的极震区烈度可能≥Ⅸ度。此古地震破裂的发现表明,青藏高原中部正断层型大地震之后的沉寂时间可以长达近万年,明显长于藏南裂谷带上的正断层型古地震活动间隔。由于安多地区最新大地震之后的离逝时间已足以积累类似强度的大地震,因此,该区未来的大地震危险性较高。
Based on surface surveying and excavated trench investigation, we discovered evidences for the most recent palaeo-earthquake event at Tsona - Amdo Basin. The NE normal fault scarps along the normal faults at the north margin of the NE Tsona - Amdo Graben was likely generated by a paleo-earthquake. The surface measuring results indicate that the maximum and minimum vertical displacements of the fault scarps are 1.4 ± 0.1 m and 0.6 ± 0.2m respectively. The small fault scarp intermittent extends southwestward from the north of Amdo Town to around the Zhasha Village along the north margin of the Tsona -Amdo Basin,and is about 26km or 36km long. The trenching profile crossing the surface rupture reveals that there is only a set of colluviums along the small fault scarp, and demonstrates the deduction that the surface normal faulting expressed as small fault scarps on surface was resulted from a severe palaeo-earthquake. The optically stimulated luminescence (OSL) dating results of the youngest terrace ( Tt ) deposits offset by the surface rupture indicate that the most recent palaeo-earthquake occurred after about 13.8 ～ 17.8ka before present,and the OSL age of the soil layer overlying the youngest terrace deposits infers that the most recent palaeo-earthquake occurred before about 6.2ka before present. In trench profile,three OSL ages(including one terrace deposits and two fault colluviums) suggest that the recent paleo-earthquake occurred during 10.0 ～ 8. lka before present. Based on empirical formula between earthquake magnitude and surface rupture length and displacementse, the estimated magnitude and maximum intensity of the most recent palaeo-earthquake occurred at Amdo is probable of Ms6.9 ～ 7.3 and ≥IX degree. Regional research shows that similar surface ruptures of palaeo-earthquake are also distributed along the normal faults at the eastern margin of the Peng Co-Dung Co Graben that constitutes the southern segment of the Peng Co -Amdo Graben system. The oldest surface rupture is located at the southeastern edge of the Dung Co Graben,and occurred probably at about 28.2ka before present, but the youngest surface normal faulting runs along the northeastern margin of the Dung Co Graben,occurring at about 1320± 60A. D. The new discovers show that the quiescence period between two severe earthquake events resulted from normal faulting may last for ten thousands years or even longer along the Peng Co -Amdo Graben system in Central Tibet. The quiescence period after severe earthquake is obviously longer than that along the SN trending rift in Southern Tibet. It implies that the magnitude of extension deformation in Southern Tibet should be greater than that in Central Tibet. The displacements of T_2 formed during 39.9 ～ 42. 3kaB. P. indicate an average vertical slip rate of 0. 14 ± 0.03mm/a for the normal fault at the northern margin of the Amdo Basin. Based on the slip rate,the quiescence period has been long enough to cumulate a similar magnitude severe earthquake along the northern margin fault of the Tsona-Amdo Basin.
Based on surface surveying and excavated trench investigation, we discovered evidences for the most recent palaeo-earthquake event at Tsona - Amdo Basin. The NE normal fault scarps along the normal faults at the north margin of the NE Tsona - Amdo Graben was likely generated by a paleo-earthquake. The surface measuring results indicate that the maximum and minimum vertical displacements of the fault scarps are 1.4 ± 0.1 m and 0.6 ± 0.2m respectively. The small fault scarp intermittent extends southwestward from the north of Amdo Town to around the Zhasha Village along the north margin of the Tsona -Amdo Basin,and is about 26km or 36km long. The trenching profile crossing the surface rupture reveals that there is only a set of colluviums along the small fault scarp, and demonstrates the deduction that the surface normal faulting expressed as small fault scarps on surface was resulted from a severe palaeo-earthquake. The optically stimulated luminescence (OSL) dating results of the youngest terrace ( Tt ) deposits offset by the surface rupture indicate that the most recent palaeo-earthquake occurred after about 13.8 ～ 17.8ka before present,and the OSL age of the soil layer overlying the youngest terrace deposits infers that the most recent palaeo-earthquake occurred before about 6.2ka before present. In trench profile,three OSL ages(including one terrace deposits and two fault colluviums) suggest that the recent paleo-earthquake occurred during 10.0 ～ 8. lka before present. Based on empirical formula between earthquake magnitude and surface rupture length and displacementse, the estimated magnitude and maximum intensity of the most recent palaeo-earthquake occurred at Amdo is probable of Ms6.9 ～ 7.3 and ≥IX degree. Regional research shows that similar surface ruptures of palaeo-earthquake are also distributed along the normal faults at the eastern margin of the Peng Co-Dung Co Graben that constitutes the southern segment of the Peng Co -Amdo Graben system. The oldest surface rupture is located at the southeastern edge of the Dung Co Graben,and occurred probably at about 28.2ka before present, but the youngest surface normal faulting runs along the northeastern margin of the Dung Co Graben,occurring at about 1320± 60A. D. The new discovers show that the quiescence period between two severe earthquake events resulted from normal faulting may last for ten thousands years or even longer along the Peng Co -Amdo Graben system in Central Tibet. The quiescence period after severe earthquake is obviously longer than that along the SN trending rift in Southern Tibet. It implies that the magnitude of extension deformation in Southern Tibet should be greater than that in Central Tibet. The displacements of T_2 formed during 39.9 ～ 42. 3kaB. P. indicate an average vertical slip rate of 0. 14 ± 0.03mm/a for the normal fault at the northern margin of the Amdo Basin. Based on the slip rate,the quiescence period has been long enough to cumulate a similar magnitude severe earthquake along the northern margin fault of the Tsona-Amdo Basin.
引文
1 邓起东,刘百篪,张培震等.活动断层工程安全评价和位错量的定量评估.见:邓起东,汪一鹏主编.活动断裂研究(2) .北京:地震出版社,1992． 236-246 Deng Qidong,Liu Baichi,Zhang Peizhen et al.Researcher of active fault in evaluating engineering safety and assessing amount of displacement.In:Deng Qidong,Wang Yipeng eds.Researcher on Active Fault(2) .Beijing:Seismological Press,1992． 236-246
2 冉勇康,邓起东.古地震学研究的历史、现状和发展趋势.科学通报,1999,44(1) :12-20 Ran Yongkang,Deng Qidong.History,status and trend about the research of paleoseismology.Chinese Science Bulletin,1999,44(10) :880-889
3 Caputo R,Pavlides S B.Earthquake geology:Methods and applications.Tectonophysics,2008,453:1-6
4 Caputo R,Helly B.The use of distinct disciplines to investigate past earthquakes.Tectonophysics,2008 ,453:7-19
5 Molnar P,lapponnier P.Active tectonics of Tibet.Journal of Geophysical Research,1978,83:5361-5376
6 Armijo R,Tapponnier P,Mercier J L et al.Quaternary extension in Southern Tibet:Field observation and tectonic implication.Journal of Geophysical Research,1986,91(B14) :13803-13872
7 张培震,郑德文,尹功明等.有关青藏高原东北缘晚新生代的扩展与隆升的讨.论.第四纪研究,2006,26(1) :5-13 Zhang Peizhen,Zheng Dewen,Yin Gongming et al.Discussion on Late Cenozoic growth and rise of northeastern margin of the Tibetan Plateau.Quaternary Sciences,2006,26(1) :5-13
8 周荣军,叶友清,李 勇等.理塘断裂带沙湾段的晚第四纪活动性.第四纪研究,2007,27(1) :45-53 Zhou Rongjun,Ye Youging,Li Yong et al.Late-Quaternary activity of the Shawan segment of the Litang Faults.Quaternary Sciences,2007,27(1) :45-53
9 李勇,周荣军,Densmore A L等.青藏高原东缘龙门山晚新生代走滑-逆冲作用的地貌标志.第四纪研究,2006,26(1) :40-51 Li Yong,Zhou Rongjun,Densmore A L et al.Geomorphic evidence for Late Cenozoic strike-slipping and thrusting in the Longmen Shan,Eastern margin of the Tibetan Plateau.Quaternary Sciences,2006,26(1) :40-51
10 田勤俭,丁国瑜.青藏高原北部第四纪早期断裂活动的新生性变化初步研究.第四纪研究,2006,26(1) :32-39 Tian Qinjian,Ding Guoyu.Preliminary analysis on an Early Quaternary new fault trace in north of Tibetan Plateau.Quaternary Sciences,2006 ,26(1) :32-39
11 付碧宏,张松林,谢小平等.阿尔金断裂系西段--康西瓦断裂的晚第四纪构造地貌特征研究.第四纪研究,2006,26(2) :228-235 Fu Bihong,Zhang Songlin,Xie Xiaoping et al.Late Quaternary tectono-geomorphic features along the Kangxiwa Fault,Altyn Tagh Fault system,Northern Tibet.Quaternary Sciences,2006,26(2) :228-235
12 李海兵,Woerd Van der J,孙知明等.阿尔金断裂带康西瓦段晚第四纪以来的左旋滑移速率及其大地震复发周期的探讨.第四纪研究,2008,28(2) :197-213 Li Haibing,Woerd Van der J,Sun Zhiming et al.Late Quaternary left-slip rates and large earthquake recurrence time along the Kangxiwar segment of the Altyn Tagh fault zone,Northern Tibet.Quaternary Sciences,2008,28(2) :197-213
13 石许华,王二七,王 刚等.青藏高原东南缘玉龙雪山(5596m)晚新生代隆升的侵蚀与构造控制作用.第四纪研究,2008,28(2) :222-231 Shi Xuhua,Wang Erchie,Wang Gang et af.Late Cenozoic uplift of the Yulong Snow Mountain(5596m),SE Tibetan Plateau,caused by erosion and tectonic forcing.Quaternary Sciences,2008,28(2) :222-231
14 汪一鹏.青藏高原活动构造基本特征.见:《活动断裂研究》编委会编.活动断裂研究(6) .北京:地震出版社,1998． 135-144 Wang Yipeng.Principal features of the active tectonics in Qinghai-Xizang Plateau.In:Editorial Committee of“Research on Active Fault” ed.Research on Active Fault(6) .Beijing:Seismological Press.1998． 135-144
15 吴中海,张永双,胡道功等.西藏错那-拿日雍错地堑的第四纪正断层作用及其形成机制探讨.第四纪研究,2008,28(2) :232-242 Wu Zhonghai,Zhang Yongshuang,Hu Daogong et al.Quaternary normal faulting and its dynamic mechanism of the Cona-Nariyong Co graben in south-Eastern Tibet.Quaternary Sciences,2008,28(2) ::232-242
16 胡海涛,许贵森.青南-藏北高原的构造体系及其对地下水的控制.见:地质矿产部《青藏高原地质文集》编委会.青藏高原地质文集(5) .北京:地质出版社,1982． 1-40 Hu Haitao,Xu Guisen.The structural system of the Southern Qinghai-Northern Xizang Plateau and its control over ground-water.In:Editorial Committee of“Contribution to the Geology of the Qinghai-Xizang Plateau” ,Ministry of Geology and Mineral Resources ed.Contribution to the Geology of the Qinghai-Xizang(Tibetan)Plateau(5) .Beijing:Geological Publishing House,1982． 1-40
17 胡海涛,阎树彬.青藏公路沿线(格尔木-安多)的区域工程地质特征.见:地质矿产部《青藏高原地质文集》编委会.青藏高原地质文集(5) .北京:地质出版社,1982． 130-144 Hu Haitao,Yan Shubin.Regional engineering geology along the Qinghai-Xizang highway(Golmud to Amdo).In:Editorial Committee of“Contribution to the Geology of the Qinghai-Xizang Plateau”,Ministry of Geology and Mineral Resources.Contribution to the Geology of the Qinghai-Xizang(Tibetan)Plateau(5) .Beijing:Geological Publishing House,1982． 130-144
18 Kidd W S F,Molnar P.Quaternary and active faulting observed on the 1985 Academia Sinica--Royal society geotraverse of Tibet.Philosophical Transactions of the Royal Society of London(Series A),1988 ,327:337-363
19 汪一鹏,任金卫,叶建青等.怒江缝合带上活动断裂新知.地震地质,1995,17(1) :52-53 Wang Yipeng,Ren Jinwei,Ye Jianqing et al.New evidence for active fault along the Nujiang suture zone.Seismology and Geology,1995,17(1) :52-53
20 吴中海,赵希涛,吴珍汉等.西藏安多-错那湖地堑的第四纪地质、断裂活动及其运动学特征分析.第四纪研究,2005,25(4) :490-502 Wu Zhonghai,Zhao Xitao,Wu Zhenhan et al.Active faults and their kinematics features at the Amdo-Tsona graben,Central Xizang.Quaternary Sciences,2005,25(4) :490-502
21 吴中海,吴珍汉,胡道功等.2004年西藏懂错M_S5． 6地震的宏观烈度调查与控震构造分析.地震地质,2007,29(1) :63-76 Wu Zhonghai,Wu Zhenhan,Hu Daogong et al.The intensity and epicenter of the 2004 M_S5． 6 Dung Co earthquake and the relation with active faulting in Central Tibet.Seismology and Geology,2007,29(1) :63-76
22 Yeats R S,Prentice C S.Introducton to speccial section:Paleoseismology.Journal of Geophysical Research,1996,101(B3) :5847-5853
23 McCalpin J P ed.Paleoseismology.Toronto:Academic Press,1996． 588
24 邓起东,于贵华,叶文华.地震地表破裂参数与震级关系的研究.见:邓起东,汪一鹏主编.活动断裂研究(2) .北京:地震出版社,1992． 247-264 Deng Qidong,Yu Guihua,Ye Wenhua.Relationship between earthquake magnitude and parameters of surface ruptures associated with historical earthquakes.In:Deng Qidong,Wang Yipeng eds.Researcher on Active Fault(2) .Beijing:Seismological Press,1992． 247-264
25 Wells D L,Coppersmith K J.New empirical relationships among magnitude,rupture length,rupture width,rupture area,and surface displacement.Bulletin of the Seismological Society of America,1994,84:974-1002
26 吴中海,叶培盛,吴珍汉2008年10月06日西藏当雄M_S6． 6级强震的地震烈度、控震构造和发震机理.地质通报,2009(待刊) Wu Zhonghai,Ye Peisheng,Wu Zhenhan.The seismic intensity,seismogenic tectonics and mechanism of the October 6,2008 Ms6． 6 Damxung earthquake in Southern Tibet China.Geological Bulletin of China,2009(in press)
27 吴中海,吴珍汉,胡道功等.青藏铁路唐古拉山-拉萨段全新世控震断裂研究.地质通报,2006,25(12) :1387-1401 Wu Zhonghai,Wu Zhenhan,Ha Daogong et al.Holocene seismogenic faults along the Tanggula-Lhasa section of the Qinghai-Tibet Railway,China.Geological Bulletin of China,2006,25(12) :1387-1401
28 Blisniuk P M,Sharp W D.Rates of Late Quaternary normal faulting in Central Tibet from U-series dating of pedogenic carbonate in displaced fluvial gravel deposits.Earth and Planetary Science Letters.2003 ,215:169-186
2 冉勇康,邓起东.古地震学研究的历史、现状和发展趋势.科学通报,1999,44(1) :12-20 Ran Yongkang,Deng Qidong.History,status and trend about the research of paleoseismology.Chinese Science Bulletin,1999,44(10) :880-889
3 Caputo R,Pavlides S B.Earthquake geology:Methods and applications.Tectonophysics,2008,453:1-6
4 Caputo R,Helly B.The use of distinct disciplines to investigate past earthquakes.Tectonophysics,2008 ,453:7-19
5 Molnar P,lapponnier P.Active tectonics of Tibet.Journal of Geophysical Research,1978,83:5361-5376
6 Armijo R,Tapponnier P,Mercier J L et al.Quaternary extension in Southern Tibet:Field observation and tectonic implication.Journal of Geophysical Research,1986,91(B14) :13803-13872
7 张培震,郑德文,尹功明等.有关青藏高原东北缘晚新生代的扩展与隆升的讨.论.第四纪研究,2006,26(1) :5-13 Zhang Peizhen,Zheng Dewen,Yin Gongming et al.Discussion on Late Cenozoic growth and rise of northeastern margin of the Tibetan Plateau.Quaternary Sciences,2006,26(1) :5-13
8 周荣军,叶友清,李 勇等.理塘断裂带沙湾段的晚第四纪活动性.第四纪研究,2007,27(1) :45-53 Zhou Rongjun,Ye Youging,Li Yong et al.Late-Quaternary activity of the Shawan segment of the Litang Faults.Quaternary Sciences,2007,27(1) :45-53
9 李勇,周荣军,Densmore A L等.青藏高原东缘龙门山晚新生代走滑-逆冲作用的地貌标志.第四纪研究,2006,26(1) :40-51 Li Yong,Zhou Rongjun,Densmore A L et al.Geomorphic evidence for Late Cenozoic strike-slipping and thrusting in the Longmen Shan,Eastern margin of the Tibetan Plateau.Quaternary Sciences,2006,26(1) :40-51
10 田勤俭,丁国瑜.青藏高原北部第四纪早期断裂活动的新生性变化初步研究.第四纪研究,2006,26(1) :32-39 Tian Qinjian,Ding Guoyu.Preliminary analysis on an Early Quaternary new fault trace in north of Tibetan Plateau.Quaternary Sciences,2006 ,26(1) :32-39
11 付碧宏,张松林,谢小平等.阿尔金断裂系西段--康西瓦断裂的晚第四纪构造地貌特征研究.第四纪研究,2006,26(2) :228-235 Fu Bihong,Zhang Songlin,Xie Xiaoping et al.Late Quaternary tectono-geomorphic features along the Kangxiwa Fault,Altyn Tagh Fault system,Northern Tibet.Quaternary Sciences,2006,26(2) :228-235
12 李海兵,Woerd Van der J,孙知明等.阿尔金断裂带康西瓦段晚第四纪以来的左旋滑移速率及其大地震复发周期的探讨.第四纪研究,2008,28(2) :197-213 Li Haibing,Woerd Van der J,Sun Zhiming et al.Late Quaternary left-slip rates and large earthquake recurrence time along the Kangxiwar segment of the Altyn Tagh fault zone,Northern Tibet.Quaternary Sciences,2008,28(2) :197-213
13 石许华,王二七,王 刚等.青藏高原东南缘玉龙雪山(5596m)晚新生代隆升的侵蚀与构造控制作用.第四纪研究,2008,28(2) :222-231 Shi Xuhua,Wang Erchie,Wang Gang et af.Late Cenozoic uplift of the Yulong Snow Mountain(5596m),SE Tibetan Plateau,caused by erosion and tectonic forcing.Quaternary Sciences,2008,28(2) :222-231
14 汪一鹏.青藏高原活动构造基本特征.见:《活动断裂研究》编委会编.活动断裂研究(6) .北京:地震出版社,1998． 135-144 Wang Yipeng.Principal features of the active tectonics in Qinghai-Xizang Plateau.In:Editorial Committee of“Research on Active Fault” ed.Research on Active Fault(6) .Beijing:Seismological Press.1998． 135-144
15 吴中海,张永双,胡道功等.西藏错那-拿日雍错地堑的第四纪正断层作用及其形成机制探讨.第四纪研究,2008,28(2) :232-242 Wu Zhonghai,Zhang Yongshuang,Hu Daogong et al.Quaternary normal faulting and its dynamic mechanism of the Cona-Nariyong Co graben in south-Eastern Tibet.Quaternary Sciences,2008,28(2) ::232-242
16 胡海涛,许贵森.青南-藏北高原的构造体系及其对地下水的控制.见:地质矿产部《青藏高原地质文集》编委会.青藏高原地质文集(5) .北京:地质出版社,1982． 1-40 Hu Haitao,Xu Guisen.The structural system of the Southern Qinghai-Northern Xizang Plateau and its control over ground-water.In:Editorial Committee of“Contribution to the Geology of the Qinghai-Xizang Plateau” ,Ministry of Geology and Mineral Resources ed.Contribution to the Geology of the Qinghai-Xizang(Tibetan)Plateau(5) .Beijing:Geological Publishing House,1982． 1-40
17 胡海涛,阎树彬.青藏公路沿线(格尔木-安多)的区域工程地质特征.见:地质矿产部《青藏高原地质文集》编委会.青藏高原地质文集(5) .北京:地质出版社,1982． 130-144 Hu Haitao,Yan Shubin.Regional engineering geology along the Qinghai-Xizang highway(Golmud to Amdo).In:Editorial Committee of“Contribution to the Geology of the Qinghai-Xizang Plateau”,Ministry of Geology and Mineral Resources.Contribution to the Geology of the Qinghai-Xizang(Tibetan)Plateau(5) .Beijing:Geological Publishing House,1982． 130-144
18 Kidd W S F,Molnar P.Quaternary and active faulting observed on the 1985 Academia Sinica--Royal society geotraverse of Tibet.Philosophical Transactions of the Royal Society of London(Series A),1988 ,327:337-363
19 汪一鹏,任金卫,叶建青等.怒江缝合带上活动断裂新知.地震地质,1995,17(1) :52-53 Wang Yipeng,Ren Jinwei,Ye Jianqing et al.New evidence for active fault along the Nujiang suture zone.Seismology and Geology,1995,17(1) :52-53
20 吴中海,赵希涛,吴珍汉等.西藏安多-错那湖地堑的第四纪地质、断裂活动及其运动学特征分析.第四纪研究,2005,25(4) :490-502 Wu Zhonghai,Zhao Xitao,Wu Zhenhan et al.Active faults and their kinematics features at the Amdo-Tsona graben,Central Xizang.Quaternary Sciences,2005,25(4) :490-502
21 吴中海,吴珍汉,胡道功等.2004年西藏懂错M_S5． 6地震的宏观烈度调查与控震构造分析.地震地质,2007,29(1) :63-76 Wu Zhonghai,Wu Zhenhan,Hu Daogong et al.The intensity and epicenter of the 2004 M_S5． 6 Dung Co earthquake and the relation with active faulting in Central Tibet.Seismology and Geology,2007,29(1) :63-76
22 Yeats R S,Prentice C S.Introducton to speccial section:Paleoseismology.Journal of Geophysical Research,1996,101(B3) :5847-5853
23 McCalpin J P ed.Paleoseismology.Toronto:Academic Press,1996． 588
24 邓起东,于贵华,叶文华.地震地表破裂参数与震级关系的研究.见:邓起东,汪一鹏主编.活动断裂研究(2) .北京:地震出版社,1992． 247-264 Deng Qidong,Yu Guihua,Ye Wenhua.Relationship between earthquake magnitude and parameters of surface ruptures associated with historical earthquakes.In:Deng Qidong,Wang Yipeng eds.Researcher on Active Fault(2) .Beijing:Seismological Press,1992． 247-264
25 Wells D L,Coppersmith K J.New empirical relationships among magnitude,rupture length,rupture width,rupture area,and surface displacement.Bulletin of the Seismological Society of America,1994,84:974-1002
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