关键词: 汶川地震, 地表破裂带, 最大水平位移, 分解

Abstract: 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 M_S 8.0 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.

Key words: Wenchuan earthquake, surface rupture zone, maximum horizontal displacement, decomposition