对2008年汶川M_S8.0地震沿龙门山后山出现地表破裂现象的讨论
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摘要
介绍了汶川地震沿龙门山后山汶川—茂县断裂带11个点位出现的地表破裂现象.这些地表破裂点位南起汶川卧龙乡鱼丝洞,向北经耿达乡牛坪、草坡乡金波、漳排、足湾、绵虒乡高东山、玉龙乡岭岗、威州镇七盘沟到茂县壳壳寨、凤仪镇马良沟,长度达100km.尽管这些地表破裂点位多数断续展布在高山山顶或山体一侧,与山体走向一致,但仍有少数地表破裂点位位于河谷地带,与河流流向垂直或斜交分布.本文介绍的11个点位中的多数点位为NW侧抬升,垂直位移20—40cm.其中,位于汶川县威州镇走马岭的地表陡坎的方向与七盘沟的方向垂直,NW盘抬升的地表陡坎与岷江河谷位置相反.该处断面出现平直的摩擦镜面和侧伏角57°的斜向擦痕,显示断面存在右旋走滑位移.为此,不得不质疑这些地表破裂是否完全由于山体的重力滑塌形成,还是有可能包含了汶川地震沿汶川—茂县断裂产生的地表位移.值得关注的是,在本文介绍的11个调查点位中,有6个点位存在断错地貌现象,其中2个地表破裂点位于山顶出现的断层沟槽内,由此也佐证了汶川地震出现在山顶的地表破裂也可以由构造活动形成.此外,文中还介绍了5个点位见到晚第四纪断错剖面.本文作者认为,晚第四纪时期龙门山3条主要活动断裂带各自清晰的断错地貌,显示了该时期龙门山3条断裂带的活动并不遵循中生代的前展式活动方式,而是不同程度地同步活动.在汶川地震中,沿后山出现的小位移量的地表破裂现象有可能是这种同步活动的反映.本文资料的展示有助于重视对龙门山后山开展汶川地震地表破裂调查,推进对龙门山构造带汶川地震活动状况的总体评估.
The information of surface fractures at eleven investigated sites along the Wenchuan--Maoxian fault zone in the rear-range of Longmengshan mountain are presented in this paper.The investigation sites include,from south to north,Yusidong village of Wolong town in Wenchuan county,Niuping of Gengda town,Jinbo village,Zhangpai village and Zuwan village of Caopo town,Gaodongshan village of Miansi town,Linggang village of Yulong town,Qipangou village of Weizhou town,Qiaoqiaozhai village of Maoxian county,to Malianggou gully of Fengyi town,with a total length of 100 km.Although most surface fractures are located at the top or on the slope of the mountain,with the same strike as the mountain trend,there are also a few surface fractures stretching on terraces of rivers,with the fracture trends perpendicular to or obliquely intersecting the rivers.The northwestern side of most fracrures among the 11 sites are uplifted.Vertical displacements are mostly 20—40 centimeters.Among them,the strike of surface scarp on the top of Zoumaling range is perpendicular to the trend of Qipangou gully in the Weizhou town of Wenchuan county,with the scarp on north-western hanging wall being just opposite to the Minjiang river.On the wall of this fault scarp straight smooth frictional mirror surface and striations with 57° plunge-angle can be seen,indicating a displacement with dextral strike-slip component.Here we would query whether these fracture surfaces were formed solely by gravitational slide or probably caused by co-seismic displacement along Wenchuan--Maoxian fault during the Wenchuan earthquake.Note that,among 11 investigated sites,there are 6 sites with land scarps,among which two surface fractures are located in fault trough on mountain tops.This shows that the surface fractures on mountain tops during Wenchuan earthquake could have been formed by tectonic activity.Besides,surface dislocation profiles on 5 sites are described in the paper.The three main fault belts in Longmenshan mountain have distinct dislocated geomorphology during Late Quaternary.We think it shows that the three fault belts did not move following the frontward style of Mesozoic tectonic motion,but were synchronal activity during the same events of Late Quaternary with different active intensity.The distribution of small-displacement of surface fractures along the Longmengshan rear-range formed by Wenchuan earthquake is probably the reflection of such synchronal activity.Investigation in this paper would be helpful in calling for more attention to surface fracture survey along the rear-range of Longmenshan mountain during Wenchuan earthquake,facilitating an overall assessment of the Longmengshan tectonic zone activity during the Wenchuan earthquake.
The information of surface fractures at eleven investigated sites along the Wenchuan--Maoxian fault zone in the rear-range of Longmengshan mountain are presented in this paper.The investigation sites include,from south to north,Yusidong village of Wolong town in Wenchuan county,Niuping of Gengda town,Jinbo village,Zhangpai village and Zuwan village of Caopo town,Gaodongshan village of Miansi town,Linggang village of Yulong town,Qipangou village of Weizhou town,Qiaoqiaozhai village of Maoxian county,to Malianggou gully of Fengyi town,with a total length of 100 km.Although most surface fractures are located at the top or on the slope of the mountain,with the same strike as the mountain trend,there are also a few surface fractures stretching on terraces of rivers,with the fracture trends perpendicular to or obliquely intersecting the rivers.The northwestern side of most fracrures among the 11 sites are uplifted.Vertical displacements are mostly 20—40 centimeters.Among them,the strike of surface scarp on the top of Zoumaling range is perpendicular to the trend of Qipangou gully in the Weizhou town of Wenchuan county,with the scarp on north-western hanging wall being just opposite to the Minjiang river.On the wall of this fault scarp straight smooth frictional mirror surface and striations with 57° plunge-angle can be seen,indicating a displacement with dextral strike-slip component.Here we would query whether these fracture surfaces were formed solely by gravitational slide or probably caused by co-seismic displacement along Wenchuan--Maoxian fault during the Wenchuan earthquake.Note that,among 11 investigated sites,there are 6 sites with land scarps,among which two surface fractures are located in fault trough on mountain tops.This shows that the surface fractures on mountain tops during Wenchuan earthquake could have been formed by tectonic activity.Besides,surface dislocation profiles on 5 sites are described in the paper.The three main fault belts in Longmenshan mountain have distinct dislocated geomorphology during Late Quaternary.We think it shows that the three fault belts did not move following the frontward style of Mesozoic tectonic motion,but were synchronal activity during the same events of Late Quaternary with different active intensity.The distribution of small-displacement of surface fractures along the Longmengshan rear-range formed by Wenchuan earthquake is probably the reflection of such synchronal activity.Investigation in this paper would be helpful in calling for more attention to surface fracture survey along the rear-range of Longmenshan mountain during Wenchuan earthquake,facilitating an overall assessment of the Longmengshan tectonic zone activity during the Wenchuan earthquake.
引文
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李传友,魏占玉.2009.汶川MS8.0地震地表破裂带北端位置的修定[J].地震地质,31(1):1--8.
李海兵,付小方,Van der Woerd J,司家亮,王宗秀,侯立玮,邱祝礼,李宁,吴富晓,许志琴,Tapponnier P.2008.汶川地震(MS8.0)地表破裂及其同震右旋斜向逆冲作用[J].地质学报,82(12):1623--1643.
李海兵,司家亮,付小方,邱祝礼,李宁,Van der Woerd J,裴军令,王宗秀,侯立玮,吴富晓.2009.2008年汶川地震同震滑移特征、最大滑移量及构造意义[J].第四纪研究,29(3):387--402.
刘静,张智慧,文力,孙杰,邢秀臣,胡古月,许强,Tapponnier P,曾令森,丁林,刘延乐.2008.汶川地震8.0级大地震同震破裂的特殊性及构造意义[J].地质学报,82(12):1707--1722.
刘静,孙杰,张智慧,文力,邢秀臣,胡古月,许强.2010.汶川地震映秀—北川地表破裂带虹口乡段精细填图、位移特征和地震构造分析[J].第四纪研究,30(1):1--29.
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