中国大陆古地震研究的关键技术与案例解析(1)——走滑活动断裂的探槽地点、布设与事件识别标志
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摘要
古地震研究的主要目标是识别或揭露地质地貌记录的大地震变形遗迹,确定大地震发生的时间、复发特征、同震位移量等参数。要实现这样的目标,探槽开挖地点能否完整记录晚第四纪发生的古地震事件、能否获取大量的测年样品以控制事件发生的年代、能否正确地识别这些事件等是关键,并直接关系到未来大地震危险性评价的可靠性。由于走滑断裂位移发生的特殊性,好的探槽研究地点并不普遍。文中在综合分析走滑断裂同震地表变形特征、影响因素的基础上,通过案例分析,总结出可能成为走滑断裂古地震研究和探槽开挖的候选地点,如洼地、盆地、槽谷、断塞塘、被同步位移连续错开的冲沟床、连续的坎前堆积地层和多级地貌面连续变形等。组合探槽或三维探槽应该是走滑断裂探槽布设的首选。跨断层微地貌位错、断错地层以及上覆更新地层、局部坎前堆积和裂缝充填堆积、不同地层单元沿断层面位移量的突然增加或降低、不同程度的弯曲变形、不同期次的古断塞塘(坑)堆积等,可以作为重要的事件识别依据。要降低古地震识别的不确定性,需要理顺研究程序,明确技术方案,逐一精心实施。认识和结论的得出需要反复推敲,并广泛讨论,同时需要注意细节以及各种信息的相互补充和印证。
The main purpose of paleoseismic study is to distinguish or reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features,and obtain corresponding seismic parameters such as timing,recurrence behavior,and coseismic displacement of large earthquakes. To achieve the aforementioned scientific aim,whether a trenching site preserves evidence of a complete paleoseismic sequence since late Quaternary and contains multiple measurable samples or not,and whether it can accurately identify paleoseismic events and collect well-constrained samples on events or not,all of these problems are directly responsible for reliabilities on assessment of future large earthquake hazard. Due to special displacement styles on strike-slip faults,good trenching sites are not widespread. Through comprehensive analysis on characteristics of coseismic surface ruptures and influencing factors on several trenching cases,we suggest micro-landforms such as depressions,basins,troughs,sag ponds,successive-offset channels,continuous scarp-derived deposit and multiple geomorphic surfaces are likely to be good trenching sites for paleoseismic studies on strike-slip faults. Multiple trenching or three-dimensional trenching should be the primary layout on strike-slip faults. Offsets of micro-landform across a fault,young stratigraphic units overlying on faulted units,locally distributed scarp-derived colluvial deposits,filled fissures,abrupt increases or decreases in displacement of different stratigraphic units on a fault,warping in different degrees,and multiple periodic paleo-sag ponds accumulation,all of these deformation evidences are good indicators for identifying paleoseismic events. To narrow uncertainties of paleoseismic studies,we should base on an organized research process and make a technical proposal and sophisticatedly conduct trenching work. Conclusions need to be repeatedly checked and widespread discussed,and also we should pay much attention on details and use more evidence to support or supplement analysis.
The main purpose of paleoseismic study is to distinguish or reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features,and obtain corresponding seismic parameters such as timing,recurrence behavior,and coseismic displacement of large earthquakes. To achieve the aforementioned scientific aim,whether a trenching site preserves evidence of a complete paleoseismic sequence since late Quaternary and contains multiple measurable samples or not,and whether it can accurately identify paleoseismic events and collect well-constrained samples on events or not,all of these problems are directly responsible for reliabilities on assessment of future large earthquake hazard. Due to special displacement styles on strike-slip faults,good trenching sites are not widespread. Through comprehensive analysis on characteristics of coseismic surface ruptures and influencing factors on several trenching cases,we suggest micro-landforms such as depressions,basins,troughs,sag ponds,successive-offset channels,continuous scarp-derived deposit and multiple geomorphic surfaces are likely to be good trenching sites for paleoseismic studies on strike-slip faults. Multiple trenching or three-dimensional trenching should be the primary layout on strike-slip faults. Offsets of micro-landform across a fault,young stratigraphic units overlying on faulted units,locally distributed scarp-derived colluvial deposits,filled fissures,abrupt increases or decreases in displacement of different stratigraphic units on a fault,warping in different degrees,and multiple periodic paleo-sag ponds accumulation,all of these deformation evidences are good indicators for identifying paleoseismic events. To narrow uncertainties of paleoseismic studies,we should base on an organized research process and make a technical proposal and sophisticatedly conduct trenching work. Conclusions need to be repeatedly checked and widespread discussed,and also we should pay much attention on details and use more evidence to support or supplement analysis.
引文
陈立春,王虎,冉勇康,等.2010.玉树MS7.1级地震地表破裂与历史大地震[J].科学通报,55(13):1200—1205.CHEN Li-chun,WANG Hu,RAN Yong-kang,et al.2010.The MS7.1Yushu earthquake surface rupture and large historical earthquakes on the Ganze-Yushu Fault[J].Chinese Science Bulletin,55(31):3504—3509.
邓起东,陈立春,冉勇康.2004.活动构造定量研究与应用[J].地学前缘,11(4):383—392.DENG Qi-dong,CHEN Li-chun,RAN Yong-kang.2004.Quantitative studies and applications of active tectonics[J].Earth Science Frontiers,11(4):383—392(in Chinese).
邓起东,冯先岳,杨晓平,等.1994.利用大型探槽研究新疆北天山玛纳斯和吐谷鲁逆断裂-褶皱带全新世古地震[A].见:国家地震局地质研究所编.活动断裂研究(3).1—18.DENG Qi-dong,FENG Xian-yue,YANG Xiao-ping,et al.1994.Using large-scale trenching method to study the Manasiand Tugulu reverse fault along the Tianshan,in Xinjiang-Holocene paleoseismology of the folding zone[A].In:Institute of Geology,SSB(ed).Active Fault Research(3).1—18(in Chinese).
国家地震局地质研究所,宁夏回族自治区地震局.1990.海原活动断裂带[M].北京:地震出版社.Institute of Geology,SSB,Seismological Bureau of Ningxia Hui Autonomous Region.1990.Haiyuan Active Fault[M].Seismological Press,Beijing(in Chinese).
冉勇康,陈立春,程建武,等.2008.安宁河断裂冕宁以北晚第四纪地表变形与强震破裂行为[J].中国科学(D辑),38(5):543—554.RAN Yong-kang,CHEN Li-chun,CHENG Jian-wu,et al.2008.Late Quaternary surface deformation and rupture behavior of strong earthquake on the segment north of Mianning of the Anninghe Fault[J].Science in China(Ser D),51(9):1224—1237.
冉勇康,段瑞涛,邓起东,等.1997.海原断裂高湾子地点三维探槽的开挖和古地震研究[J].地震地质,19(2):97—107.RAN Yong-kang,DUAN Rui-tao,DENG Qi-dong,et al.1997.3-D trench excavation and paleoseismology at Gaowanziof the Haiyuan Fault[J].Seismology and Geology,19(2):97—107(in Chinese).
冉勇康,邓起东.1999.古地震学研究的历史、现状和发展趋势[J].科学通报,44(10):880—889.RAN Yong-kang,DENG Qi-dong.1999.History,status and trend about the research of paleoseismology[J].ChineseScience Bulletin,44(10):880—889(in Chinese).
王虎,冉勇康,李彦宝.2011.小型拉分盆地的生长与走滑断层的位移速率:以青藏高原东南缘则木河断裂带为例[J].地震地质,33(4):818—827.WANG Hu,RAN Yong-kang,LI Yan-bao.2011.Growth of a small pull-apart basin and slip rate of strike-slip fault:With the example of Zemuhe Fault on the southeastern margin of the Tibetan Plateau[J].Seismology and Geology,33(4):818—827(in Chinese).
王虎,冉勇康,李彦宝,等.2012.则木河断裂上古地震破裂与小型三角状拉分盆地演化[J].中国科学(D辑),出版中.WANG Hu,RAN Yong-kang,LI Yan-bao.2012.Paleoseismic ruptures and evolution of a small triangular pull-apart basin on the Zemuhe Fault[J].Science in China(Ser D)(in press).
新疆维吾尔自治区地震局.1985.富蕴地震断裂带[M].北京:地震出版社.Seismological Bureau of Xinjiang Uygur Autonomous Region.1990.Fuyun Active Fault[M].Seismological Press,Beijing(in Chinese).
徐锡伟,陈文彬,于贵华,等.2002.2001年11月14日昆仑山库赛湖地震(MS8.1)地表破裂带的基本特征[J].地震地质,24(1):1—13.XU Xi-wei,CHEN Wen-bin,YU Gui-hua,et al.2002.Characteristic features of the surface ruptures of the Hoh Sai Hu(Kunlunshan)earthquake(MS8.1),northern Tibetan Plateau,China[J].Seismology and Geology,24(1):1—13(in Chinese).
Carver G A.1987.Geologic criteria for recognition of individual paleoseismic events in compressional tectonic environments[R].U S Geological Survey Open File Report,87-683:115—128.
Crone A J.1987.Introduction to Directions in Paleoseismology[R].U S Geological Survey Open File Report,87-683:1—6.
Fu Bihong,Awata Y,Du Jianguo,et al.2005.Late Quaternary systematic stream offsets caused by repeated large seismicevents along the Kunlun Fault,northern Tibet[J].Geomorphology,71(3-4):278—292.
Klinger Y,Etchebes M,Tapponnier P,et al.2011.Characteristic slip for five great earthquakes along the Fuyun Fault inChina[J].Nature Geoscience,4:389—392.
Liu-zeng Jing,Klinger Y,Sieh K,et al.2006.Serial ruptures of the San Andreas Fault,Carrizo Plain,California,revealedby three-dimensional excavations[J].Journal of Geophysical Research,111(B2).doi:10.1029/2004jb003601.
McCalpin J.1987.Geologic criteria for recognition of individual paleoseismic events in extensional environments[A].In:Crone A J et al.(eds).Directions in Paleoseismology.U S Geological Survey Open File Report,87-683:102—114.
McCalpin J.2009.Paleoseismology[M].Academic Press,New York.1—802.
Nelson A R.1987.A facies model of colluvial sedimentation adjacent to a single-event normal-fault scarp,Basin andRange Province,western United States[R].U S Geological Survey Open File Report,87-683:136—145.
Rockwell T.1987.Recognition of individual paleoseismic events in strike-slip environments[A].In:Crone A J et al.(eds).Directions in Paleoseismology.U S Geological Survey Open File Report,87-683:129—135.
Sieh K E.1978.Pre-historic large earthquakes produced by slip on the San Andreas Fault at Pallett Creek,California[J].Journal of Geophysical Research,83(B8):3907—3939.
Sieh K E.1984.Lateral offsets and revised dates of large earthquakes at Pallett Creek,Southern California[J].Journalof Geophysical Research,89(B9):7641—7670.
Sieh K E,Stuiver M,Brillinger D.1989.A more precise chronology of earthquakes produced by the San Andreas Faultin southern California[J].Journal of Geophysical Research,94(B1):603—623.
Swan F H,Schwartz D P,Cluff L S,et al.1980.Recurrence of moderate-to-large magnitude earthquakes produced bysurface faulting on the Wasatch Fault zone,Utah[J].Bulletin of the Seismological Society of America,70(5):1431—1462.
Wallace R E.1977.Profiles and ages of young fault scarps,north-central Nevada[J].The Geological Society of America Bulletin,88(9):1267—1278.
Wallace R E.1981.Active faults,paleoseismology,and earthquake hazards in the western United States[A].In:Simpson D W et al.(eds).Earthquake Prediction-An International Review.Washington D C,American Geophysical Union,Maurice Ewing Series,4:209—216.
Yeats R S.1996.Introduction to special section:Paleoseismology[J].Journal of Geophysical Research,101(B3):5847—5853.
邓起东,陈立春,冉勇康.2004.活动构造定量研究与应用[J].地学前缘,11(4):383—392.DENG Qi-dong,CHEN Li-chun,RAN Yong-kang.2004.Quantitative studies and applications of active tectonics[J].Earth Science Frontiers,11(4):383—392(in Chinese).
邓起东,冯先岳,杨晓平,等.1994.利用大型探槽研究新疆北天山玛纳斯和吐谷鲁逆断裂-褶皱带全新世古地震[A].见:国家地震局地质研究所编.活动断裂研究(3).1—18.DENG Qi-dong,FENG Xian-yue,YANG Xiao-ping,et al.1994.Using large-scale trenching method to study the Manasiand Tugulu reverse fault along the Tianshan,in Xinjiang-Holocene paleoseismology of the folding zone[A].In:Institute of Geology,SSB(ed).Active Fault Research(3).1—18(in Chinese).
国家地震局地质研究所,宁夏回族自治区地震局.1990.海原活动断裂带[M].北京:地震出版社.Institute of Geology,SSB,Seismological Bureau of Ningxia Hui Autonomous Region.1990.Haiyuan Active Fault[M].Seismological Press,Beijing(in Chinese).
冉勇康,陈立春,程建武,等.2008.安宁河断裂冕宁以北晚第四纪地表变形与强震破裂行为[J].中国科学(D辑),38(5):543—554.RAN Yong-kang,CHEN Li-chun,CHENG Jian-wu,et al.2008.Late Quaternary surface deformation and rupture behavior of strong earthquake on the segment north of Mianning of the Anninghe Fault[J].Science in China(Ser D),51(9):1224—1237.
冉勇康,段瑞涛,邓起东,等.1997.海原断裂高湾子地点三维探槽的开挖和古地震研究[J].地震地质,19(2):97—107.RAN Yong-kang,DUAN Rui-tao,DENG Qi-dong,et al.1997.3-D trench excavation and paleoseismology at Gaowanziof the Haiyuan Fault[J].Seismology and Geology,19(2):97—107(in Chinese).
冉勇康,邓起东.1999.古地震学研究的历史、现状和发展趋势[J].科学通报,44(10):880—889.RAN Yong-kang,DENG Qi-dong.1999.History,status and trend about the research of paleoseismology[J].ChineseScience Bulletin,44(10):880—889(in Chinese).
王虎,冉勇康,李彦宝.2011.小型拉分盆地的生长与走滑断层的位移速率:以青藏高原东南缘则木河断裂带为例[J].地震地质,33(4):818—827.WANG Hu,RAN Yong-kang,LI Yan-bao.2011.Growth of a small pull-apart basin and slip rate of strike-slip fault:With the example of Zemuhe Fault on the southeastern margin of the Tibetan Plateau[J].Seismology and Geology,33(4):818—827(in Chinese).
王虎,冉勇康,李彦宝,等.2012.则木河断裂上古地震破裂与小型三角状拉分盆地演化[J].中国科学(D辑),出版中.WANG Hu,RAN Yong-kang,LI Yan-bao.2012.Paleoseismic ruptures and evolution of a small triangular pull-apart basin on the Zemuhe Fault[J].Science in China(Ser D)(in press).
新疆维吾尔自治区地震局.1985.富蕴地震断裂带[M].北京:地震出版社.Seismological Bureau of Xinjiang Uygur Autonomous Region.1990.Fuyun Active Fault[M].Seismological Press,Beijing(in Chinese).
徐锡伟,陈文彬,于贵华,等.2002.2001年11月14日昆仑山库赛湖地震(MS8.1)地表破裂带的基本特征[J].地震地质,24(1):1—13.XU Xi-wei,CHEN Wen-bin,YU Gui-hua,et al.2002.Characteristic features of the surface ruptures of the Hoh Sai Hu(Kunlunshan)earthquake(MS8.1),northern Tibetan Plateau,China[J].Seismology and Geology,24(1):1—13(in Chinese).
Carver G A.1987.Geologic criteria for recognition of individual paleoseismic events in compressional tectonic environments[R].U S Geological Survey Open File Report,87-683:115—128.
Crone A J.1987.Introduction to Directions in Paleoseismology[R].U S Geological Survey Open File Report,87-683:1—6.
Fu Bihong,Awata Y,Du Jianguo,et al.2005.Late Quaternary systematic stream offsets caused by repeated large seismicevents along the Kunlun Fault,northern Tibet[J].Geomorphology,71(3-4):278—292.
Klinger Y,Etchebes M,Tapponnier P,et al.2011.Characteristic slip for five great earthquakes along the Fuyun Fault inChina[J].Nature Geoscience,4:389—392.
Liu-zeng Jing,Klinger Y,Sieh K,et al.2006.Serial ruptures of the San Andreas Fault,Carrizo Plain,California,revealedby three-dimensional excavations[J].Journal of Geophysical Research,111(B2).doi:10.1029/2004jb003601.
McCalpin J.1987.Geologic criteria for recognition of individual paleoseismic events in extensional environments[A].In:Crone A J et al.(eds).Directions in Paleoseismology.U S Geological Survey Open File Report,87-683:102—114.
McCalpin J.2009.Paleoseismology[M].Academic Press,New York.1—802.
Nelson A R.1987.A facies model of colluvial sedimentation adjacent to a single-event normal-fault scarp,Basin andRange Province,western United States[R].U S Geological Survey Open File Report,87-683:136—145.
Rockwell T.1987.Recognition of individual paleoseismic events in strike-slip environments[A].In:Crone A J et al.(eds).Directions in Paleoseismology.U S Geological Survey Open File Report,87-683:129—135.
Sieh K E.1978.Pre-historic large earthquakes produced by slip on the San Andreas Fault at Pallett Creek,California[J].Journal of Geophysical Research,83(B8):3907—3939.
Sieh K E.1984.Lateral offsets and revised dates of large earthquakes at Pallett Creek,Southern California[J].Journalof Geophysical Research,89(B9):7641—7670.
Sieh K E,Stuiver M,Brillinger D.1989.A more precise chronology of earthquakes produced by the San Andreas Faultin southern California[J].Journal of Geophysical Research,94(B1):603—623.
Swan F H,Schwartz D P,Cluff L S,et al.1980.Recurrence of moderate-to-large magnitude earthquakes produced bysurface faulting on the Wasatch Fault zone,Utah[J].Bulletin of the Seismological Society of America,70(5):1431—1462.
Wallace R E.1977.Profiles and ages of young fault scarps,north-central Nevada[J].The Geological Society of America Bulletin,88(9):1267—1278.
Wallace R E.1981.Active faults,paleoseismology,and earthquake hazards in the western United States[A].In:Simpson D W et al.(eds).Earthquake Prediction-An International Review.Washington D C,American Geophysical Union,Maurice Ewing Series,4:209—216.
Yeats R S.1996.Introduction to special section:Paleoseismology[J].Journal of Geophysical Research,101(B3):5847—5853.
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