2008年汶川地震破裂的滑移向量

Slip vectors on the Longmenshan fault during the Wenchuan earthquake of 2008.

估计同震滑移向量对于认识和理解破裂方式和破裂过程具有重要意义。2008年汶川大地震在青藏高原东缘龙门山推覆构造带的中央断裂和前山断裂上各形成了一条长250 km和72 km的地表破裂带。地震发生后至今,已经发表了大量有关同震位错沿破裂带分布的论文和报告,但绝大部分都仅仅是破裂的走向位错和垂直位错,极少有同震滑移向量的报道。这不仅是因为野外难以直接测量到水平缩短量(或拉张量),而且还因为这些走滑位错实际上是视走滑位错,部分或全部来自水平缩短或拉张。因此,仅仅根据视走滑同震位错和垂直同震位错估计的同震总滑移量肯定包含了相当大的误差。尝试利用据不同走向参考线测量到的一组(两个以上)视走滑位错来计算水平滑移向量的这一新方法,获得了中央破裂带上的7个水平同震滑移向量,并结合垂直位错量进一步计算了走滑、倾滑和水平缩短三个同震滑移分量以及断层倾角和破裂面上的同震滑移向量,综合出露破裂面的擦痕所指示的滑移向量,并对比根据矩张量解获得的震源深度的滑移向量,得出以下认识:(1)破裂南段的地表滑移向量的方位角明显小于震源深度滑移向量的方位角,表明在破裂从震源向地表传播过程中破裂面上的滑移向量发生了逆时针旋转;(2)滑移方位角向北东方向逐渐增大,表明地平面上水平滑移向量表现出顺时针旋转的趋势,而且在破裂向北东方向传播过程中近地表的走滑分量逐渐减小而倾滑分量逐渐增大;(3)几乎在每一个观测点倾滑分量都大于走滑分量,表明汶川地震的破裂方式在任何地点都是以逆冲运动为主;(4)破裂面倾角在10.4°～64.7°,平均值为41°,与天然破裂露头和探槽揭示的结果基本一致;(5)滑移向量沿破裂带的分布显示,走滑分量中段大而两端小,倾滑分量则相反,中段小两端大。

It is very important to estimate the co-seismic slip vector on the source fault in order to understand the rupture style and process.Many studies on co-seismic displacement distribution of Wenchuan great earthquake have been made,the co-seismic slip vector and net slip,however,have not been reported because most strike slips measured in the field are only apparent ones,some of which are due to horizontal shortening（or extension） perpendicular to the surface break.The net slips estimated based only on these apparent strike offsets would include an appreciable amount of error.However,it is the apparent strike offset that makes us possible to estimate the horizontal slip vector.Based on apparent strike offset data measured on 7 sites from the central rupture zone,we calculated the horizontal slip vectors,and even strike slips,dip slips,fault-normal horizontal shortenings,fault dips,and net-slip vectors combined with the measured vertical displacements.Synthesizing the net slip vector measured directly from the exposure fault,and comparing the results with those deduced from the moment tensor solutions,the following main conclusions can be drawn.First,on the southwestern section of the rupture,the slip azimuth at surface is larger than that in the depth,showing an anti-clockwise rotation of slip vector on the rupture plane as the rupture propagated upward.Second,the slip azimuth increases northeastward,showing a clockwise rotation of slip vector on the surface and the decrease of strike slip component as the rupture propagated northeastward but with increase of dip slip components.Third,dip slip is larger than strike slip at almost all investigation sites,showing that the inverse faulting is the main rupture type everywhere in the rupture zone.Fourth,the dip angle ranges between 10.4° and 64.7°,average 41°,consistent with that in natural outcrop or revealed by trenches.Fifth,the strike-slip component decreases and the dip-slip component increases from the middle to the two ends of the rupture.