汶川地震白沙河段最大地表水平位移量的成因分析
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摘要
最大同震位移量是活动构造研究、断层活动特性判定和地震危险性分析的一个重要参数,它关系到地震危险区最大震级、地震复发间隔的确定等定量评估。地震地表破裂的同震位移分布十分复杂,影响因素也多种多样。汶川8.0级地震是一个以逆冲型为主的破裂事件,同震位移多表现为垂直位错,同时也存在右旋走滑位移,这与震源机制解的结果以及破裂面上擦痕所显示的运动方式基本一致。此次地震的最大同震水平位移量为4.5m,附近还有高达5m的垂直位移量。破裂的几何学和运动学分析表明,最大水平位移量可能是逆冲量沿不同方向破裂带分解以及断裂倾角变化的结果。
Maximum co-seismic displacement is an important parameter for research of active tectonics,estimation of fault's activity characteristic and analysis of earthquake hazard.It relates to the quantitative research of determining the upper limit of maximum magnitude of earthquake in potential seismic source area and recurrence interval of large earthquake.Co-seismic displacement distribution of earthquake surface rupture is complex,and the influencing factors are various.The MS8.0 Wenchuan earthquake is a typical thrust event,whose co-seismic displacement is the vertical displacement,with dextral strike-slip component,which is identical to the results from focal mechanism solutions and scratches on the fault plane.The maximum co-seismic dextral horizontal displacement is 4.5m,and near the site of the maximum displacement,the co-seismic vertical displacement is 5m.The distance between the site of maximum vertical displacement of 5m(observation point 1)and the horizontal displacement of 4.5m(observation point 2)is only 50m;The earthquake made the ground move to SE.The strike between observation sites always changes,arcing towards NW.Generally speaking,it is impossible for a thrust event changing to a dextral event in 50m.The stratum in the hanging wall is shortened under the effect of earthquake.Because of the change of the strike of fault,the thrust direction is perpendicular to the strike in some part of the fault,and in other part they are oblique.When thrust direction is oblique to the strike,thrust motion can be decomposed in three directions:extrusion perpendicular to the fault(SE),uplift perpendicular to the surface and shear parallel to the fault.We suppose that the fault dip is γ in observation point 1,α in point 2,the angle between fault's movement and fault strike in observation point 1 is β,and the angle difference between strike in observation point 1 and point 2 is θ.According to space geometry,we can obtain formulas among these parameters,and calculate them.Geometric and kinematical analyses of the ruptures show that the maximum co-seismic horizontal displacement may result from thrust motion decomposed in different directions and the change of fault dip.
Maximum co-seismic displacement is an important parameter for research of active tectonics,estimation of fault's activity characteristic and analysis of earthquake hazard.It relates to the quantitative research of determining the upper limit of maximum magnitude of earthquake in potential seismic source area and recurrence interval of large earthquake.Co-seismic displacement distribution of earthquake surface rupture is complex,and the influencing factors are various.The MS8.0 Wenchuan earthquake is a typical thrust event,whose co-seismic displacement is the vertical displacement,with dextral strike-slip component,which is identical to the results from focal mechanism solutions and scratches on the fault plane.The maximum co-seismic dextral horizontal displacement is 4.5m,and near the site of the maximum displacement,the co-seismic vertical displacement is 5m.The distance between the site of maximum vertical displacement of 5m(observation point 1)and the horizontal displacement of 4.5m(observation point 2)is only 50m;The earthquake made the ground move to SE.The strike between observation sites always changes,arcing towards NW.Generally speaking,it is impossible for a thrust event changing to a dextral event in 50m.The stratum in the hanging wall is shortened under the effect of earthquake.Because of the change of the strike of fault,the thrust direction is perpendicular to the strike in some part of the fault,and in other part they are oblique.When thrust direction is oblique to the strike,thrust motion can be decomposed in three directions:extrusion perpendicular to the fault(SE),uplift perpendicular to the surface and shear parallel to the fault.We suppose that the fault dip is γ in observation point 1,α in point 2,the angle between fault's movement and fault strike in observation point 1 is β,and the angle difference between strike in observation point 1 and point 2 is θ.According to space geometry,we can obtain formulas among these parameters,and calculate them.Geometric and kinematical analyses of the ruptures show that the maximum co-seismic horizontal displacement may result from thrust motion decomposed in different directions and the change of fault dip.
引文
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邓起东,陈社发,赵小麟,等.1994.龙门山及其邻区的构造和地震活动及动力学[J].地震地质,16(4):389—402.DENG Qi-dong,CHEN She-fa,ZHAO Xiao-lin,et al.1994.Tectonics,seismicity and dynamics of LongmenshanMountains and its adjacent regions[J].Seismology and Geology,16(4):389—402(in Chinese).
何宏林,孙昭民,王世元,等.2008.汶川MS8.0地震地表破裂带[J].地震地质,30(2):359—362.HE Hong-lin,SUN Zhao-min,WANG Shi-yuan,et al.2008.Rupture of theMS8.0Wenchuan Earthquake[J].Seis-mology and Geology,30(2):359—362(in Chinese).
唐荣昌,韩渭宾主编.1993.四川活动断裂与地震[M].北京:地震出版社.123—132.TANG Rong-chang,HAN Wei-bin(eds).1993.Active Faults and Earthquakes in Sichuan Province[M].SeismologicalPress,Beijing.123—132(in Chinese).
中国地震台网中心.2008.http:∥219.143.71.221/manage/fastReport.jsp?id=8a85532219b131b40119dba5e2c7011b.China Earthquake Networks Center.2008.http:∥219.143.71.221/manage/fastReport.jsp?id=8a85532219b131b40119dba5e2c7011b(in Chinese).
Densmore A L,LI Y,Ellis M A,et al.2005.Active tectonics and erosional unloading at the eastern margin of the Ti-betan Plateau[J].Journal of Mountain Science,12(2):146—154.
USGS Earthquake Center.2008.http:∥earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2008ryan.php.
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